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Chromosome

Chromosome

A chromosome is a structure that occurs within cells and that contains the cell's genetic material. That genetic material, which determines how an organism develops, is a molecule of deoxyribonucleic acid (DNA). A molecule of DNA is a very long, coiled structure that contains many identifiable subunits known as genes.

In prokaryotes, or cells without a nucleus, the chromosome is merely a circle of DNA. In eukaryotes, or cells with a distinct nucleus, chromosomes are much more complex in structure.

Historical background

The terms chromosome and gene were used long before biologists really understood what these structures were. When the Austrian monk and biologist Gregor Mendel (18221884) developed the basic ideas of heredity, he assumed that genetic traits were somehow transmitted from parents to offspring in some kind of tiny "package." That package was later given the name "gene." When the term was first suggested, no one had any idea as to what a gene might look like. The term was used simply to convey the idea that traits are transmitted from one generation to the next in certain discrete units.

The term "chromosome" was first suggested in 1888 by the German anatomist Heinrich Wilhelm Gottfried von Waldeyer-Hartz (18361921). Waldeyer-Hartz used the term to describe certain structures that form during the process of cell division (reproduction).

One of the greatest breakthroughs in the history of biology occurred in 1953 when American biologist James Watson (1928 ) and English chemist Francis Crick (1916 ) discovered the chemical structure of a class of compounds known as deoxyribonucleic acids (DNA). The Watson and Crick discovery made it possible to express biological concepts (such as the gene) and structures (such as the chromosome) in concrete chemical terms.

The structure of chromosomes and genes

Today we know that a chromosome contains a single molecule of DNA along with several kinds of proteins. A molecule of DNA, in turn, consists of thousands and thousands of subunits, known as nucleotides, joined to each other in very long chains. A single molecule of DNA within a chromosome may be as long as 8.5 centimeters (3.3 inches). To fit within a chromosome, the DNA molecule has to be twisted and folded into a very complex shape.

Words to Know

Deoxyribonucleic acid (DNA): The genetic material in the nucleus of cells that contains information for an organism's development.

Eukaryote: A cell with a distinct nucleus.

Nucleotide: The building blocks of nucleic acids.

Prokaryote: A cell without a nucleus.

Protein: Large molecules that are essential to the structure and functioning of all living cells.

Imagine that a DNA molecule is represented by a formula such as this:

-[-N1-N4-N2-N2-N2-N1-N3-N2-N3-N4-N1-N2-N3-N3-N1-N1-N2-N3-N4-]

In this formula, the abbreviations N1, N2, N3, and N4 stand for the four different nucleotides used in making DNA. The brackets at the beginning and end of the formula mean that the actual formula goes on and on. A typical molecule of DNA contains up to three billion nucleotides. The unit shown above, therefore, is no more than a small portion of the whole DNA molecule.

Each molecule of DNA can be subdivided into smaller segments consisting of a few thousand or a few tens of thousands of nucleotides. Each of these subunits is a gene. Another way to represent a DNA molecule, then, is as follows:

-[-G-D-N-E-Y-D-A-B-W-Q-X-C-R-K-S-]-

where each different letter stands for a different gene.

The function of genes and chromosomes

Each gene in a DNA molecule carries the instructions for making a single kind of protein. Proteins are very important molecules that perform many vital functions in living organisms. For example, they serve as hormones, carrying messages from one part of the body to another part; they act as enzymes, making possible chemical reactions that keep the cell alive; and they function as structural materials from which cells can be made.

Every cell has certain specific functions to perform. The purpose of a bone cell, for example, is to make more bone. The purpose of a pancreas cell, on the other hand, might be to make the compound insulin, which aids in the manufacture of glucose (blood sugar).

The job of genes in a DNA molecule, therefore, is to tell cells how to manufacture all the different chemical compounds (proteins) they need to make in order to function properly. The way in which they carry out this function is fairly straightforward. At one point in the cell's life, its chromosomes become untangled and open up to expose their genes. The genes act as a pattern from which proteins can be built. The proteins that are constructed in the cell are determined, as pointed out above, by the instructions built into the gene.

When the proteins are constructed, they are released into the cell itself or into the environment outside the cell. They are then able to carry out the functions for which they were made.

Chromosome numbers and Xs and Ys

Each species has a different number of chromosomes in their nuclei. The mosquito, for instance, has 6 chromosomes. Lilies have 24, earthworms 36, chimps 48, and horses 64. The largest number of chromosomes are found in the Adder's tongue fern, which has more than 1,000 chromosomes. Most species have, on average, 10 to 50 chromosomes. With 46 chromosomes, humans fall well within this average.

The 46 human chromosomes are arranged in 23 pairs. One pair of the 23 constitute the sex hormones, called the X and Y chromosomes. Males have both an X and a Y chromosome, while females have two X chromosomes. If a father passes on a Y chromosome, then his child will be male. If he passes on an X chromosome, then the child will be female.

The X chromosome is three times the size of the Y chromosome and carries 100 times the genetic information.

However, in 2000, scientists announced that the X and Y chromosomes were once a pair of identical twins. These identical chromosomes were found some 300 million years ago in reptiles, long before mammals arose. The genes in these creatures did not decide sex on their own. They responded to some environmental cue like temperature. That still goes on today in the eggs of turtles and crocodiles. But in a single animal at that time long ago, a mutation occurred on one of the pair of identical chromosomes, creating what scientists recognize today as the Y chromosomea gene that when present always produces a male.

[See also Genetic disorders; Genetic engineering; Genetics; Mendelian laws of inheritance; Molecular biology; Mutation; Nucleic acid; Protein ]

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Chromosome

Chromosome


A chromosome is a compactly folded complex of DNA and proteins containing many genes, found in the nuclei of eukaryotic organisms and in the nucleoids of prokaryotic organisms.

Each cell in an organism has a complete set of genetic information called a genome. Different organisms have different numbers of chromosomes in their genomes, ranging from a single chromosome in most bacteria to seventy-eight chromosomes in chickens. Humans cells have forty-six chromosomes, but these represent two sets of information as humans are diploid organisms; each cell has one information set inherited from the organism's mother and a second set inherited from its father. If an offspring inherits one X chromosome and one Y chromosome, he will be genetically male. If an offspring inherits X chromosomes from both parents, she will be genetically female.

Chromosomes consist of two kinds of molecules, deoxyribonucleic acid (DNA) strands and proteins. Chromosomes from eukaryotic organisms have linear DNA strands containing approximately fifty genes per millimeter, compared to 2,500 genes per millimeter in bacteria. Some of the noncoding DNA (DNA that does not code for proteins) is found in special structures at the ends of the chromosomes called telomeres. Much of the noncoding DNA in eukaryotic chromosomes may be involved in compacting the DNA into the highly organized chromosome structure. Some of this DNA has highly repetitive sequences and has been useful in forensic analysis.

Proteins help to compact DNA: this is important because the DNA in a chromosome could not fit inside its cell if it were not compacted. Histones are positively charged proteins that neutralize negative DNA strands when they wrap around and form complexes with the DNA. This wrapped structure, called "beads on a string," represents the first level of compaction. The "beads" are condensed to form fibers, fibers fold into loops, loops combine with nuclear scaffold proteins to form rosettes, and rosettes condense to form coils. Finally, a chromatid with ten or more coils is formed. Nonhistone proteins within chromosomes are also important. These proteins have varied functions, including assisting in the unwinding of DNA and in the repairing of DNA.

Chromosomes from prokaryotic organisms have DNA strands that loop and form circles. The DNA in prokaryotic chromosomes forms complexes with histone like proteins that help to compact the DNA, link it to the cell membrane, and localize it in the nucleoid region of the cell. Some bacteria have extra chromosomal DNAa mini-chromosome called a plasmid. Plasmids contain only a few genes but are rapidly exchanged among cells of a bacteria population. Plasmids have become useful tools in biotechnology and genetic engineering.

Recombination is a natural process of exchange of fragments of DNA strands between paired chromosomes, which happens occasionally during cell division. Genetic engineering techniques allow scientists to cut and paste DNA fragments from one source to another to produce recombinant chromosomes and transgenic organisms.

see also DNA Replication; Genes; Genome; Proteins.

David Speckhard

Bibliography

DeLange, Titia (1998). "Telomeres and Senescence." Science 279:334335.

Felsenfeld, Gary (1985). "DNA." Scientific American 253(4): 5867.

McCarty, MacLyn (1985). The Transforming Principle: Discovering That Genes Are Made of DNA. New York: Norton.

Miller, Robert (1998). "Bacterial Gene Swapping in Nature." Scientific American 278(1): 6771.

Neufeld, Peter, and Colman, Neville (1990). "When Science Takes the Witness Stand." Scientific American 262(5): 4653.

Watson, James D., and Crick, Francis H. (1953). Nature 171:737738.

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chromosome

chromosome (krō´məsōm´), structural carrier of hereditary characteristics, found in the nucleus of every cell and so named for its readiness to absorb dyes. The term chromosome is usually reserved for the structure when it is condensed and readily visible during cell division (see mitosis). At other times the chromosome appears as a fibrous structure, called the chromonema, consisting of accumulations (called chromomeres) of chromatin, the dye-absorbing material. During nuclear division, when each chromosome splits, each of the duplicate chromosomes is called a chromatid. A certain number of chromosomes is characteristic of each species of plant and animal; e.g., the human has 46 chromosomes, the potato has 48, and the fruit fly Drosophila has 8. Each of these chromosome numbers is the so-called diploid number, i.e., the number found in the somatic (body) cells and in the germ cells that give rise to the gametes, or reproductive cells. When the germ cells divide in the two-step process of meiosis, the chromosomes are separated in such a way that each daughter cell receives a haploid (half the diploid) number of chromosomes. Fusion of the male and female gametes in fertilization restores the diploid number in the fertilized egg, or zygote, which thus contains two sets of homologous chromosomes, one from each parent. The principal constituents of the chromosomes are nucleoproteins containing deoxyribonucleic acid, or DNA (see nucleic acid). Chromosomes appear microscopically as a linear arrangement of genes, the factors that determine the inherited characteristics of all living organisms. The very large chromosomes in the salivary gland cells of Drosophila and other insects have furnished valuable material for the study of genetics.

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chromosome

chromosome A threadlike structure several to many of which are found in the nucleus of plant and animal (eukaryotic) cells. Chromosomes are composed of chromatin and carry the genes in a linear sequence; these determine the individual characteristics of an organism. When the nucleus is not dividing, individual chromosomes cannot be identified with a light microscope. During the first stage of nuclear division, however, the chromosomes contract and, when stained, can be clearly seen under a microscope. Each consists of two chromatids held together at the centromere (see also meiosis; mitosis). The number of chromosomes in each cell is constant for and characteristic of the species concerned. In the normal body cells of diploid organisms the chromosomes occur in pairs (see homologous chromosomes); in the gamete-forming germ cells, however, the diploid number is halved and each cell contains only one member of each chromosome pair. Thus in humans each body cell contains 46 chromosomes (22 matched pairs and one pair of sex chromosomes) and each germ cell 23. Abnormalities in the number or structure of chromosomes may give rise to abnormalities in the individual; Down's syndrome is the result of one such abnormality. See chromosome mutation.

Bacterial and viral cells contain only a single chromosome; it differs from the eukaryotic chromosome in being much simpler, lacking histones and consisting simply of a single or double strand of DNA or (in some viruses) RNA. See also artificial chromosome.

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chromosome

chromosome A DNA-histone protein thread, usually associated with RNA, occurring in the nucleus of a cell. Although chromosomes are found in all animals and plants, bacteria and viruses contain structures that lack protein and contain only DNA or RNA: these are not chromosomes, though they serve a similar function. Chromosomes occur in pairs, which associate in a particular way during meiosis. Each species tends to have a characteristic number of chromosomes (e.g. 20 in maize, 23 in humans), found in most nucleated cells within most organisms. The presence of pairs of homologous chromosomes is referred to as the diploid state and is normal for the sexual phase of an organism. Gametes (reproductive cells), and cells of the gametophyte (gamete-producing phase) of plants (see alternation of generations), however, are haploid with only one member of each pair in their nuclei. Usually chromosomes are visible only during mitosis or meiosis when they contract to form short thick rods coiled into a spiral. Each chromosome possesses chromomeres and a centromere, and some contain a nucleolar organizer. Chromosomes contain a line of different genes, a spindle attachment at some point along their length, and regions of heterochromatin, which stains strongly with basic dyes.

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chromosome

chromosome A DNA-histone protein thread, usually associated with RNA, occurring in the nucleus of a cell. Although chromosomes are found in all animals and plants, bacteria and viruses contain structures that lack protein and contain only DNA or RNA: these are not chromosomes, though they serve a similar function. Chromosomes occur in pairs, which associate in a particular way during meiosis. Each species tends to have a characteristic number of chromosomes (e.g. 20 in maize), found in most nucleated cells within most organisms. The presence of pairs of homologous chromosomes is referred to as the diploid state and is normal for the sexual phase of an organism. Gametes, and cells of the gametophyte of plants, however, are haploid with only one member of each pair in their nuclei. Usually chromosomes are visible only during mitosis or meiosis when they contract to form short thick rods coiled into a spiral. Each chromosome possesses chromomeres and a centromere, and some contain a nucleolar organizer. Chromosomes contain a line of different genes, a spindle-attachment at some point along their length, and regions of heterochromatin, which stains strongly with basic dyes.

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chromosome walking

chromosome walking A technique, used in constructing a physical map, for selecting contiguous overlapping clones from a DNA library and thus reconstructing the order of genes along a segment of chromosome. For example, one can effectively ‘walk’ in either direction from a known marker gene to identify adjacent genes, a technique that is useful in fine-structure mapping of a genome. Essentially, the initial clone containing the marker gene is fragmented and each fragment subcloned for use as a gene probe to identify other clones containing adjacent and overlapping segments. In turn, these adjacent segments are fragmented and subcloned and used to probe for further overlaps, and so on. The cloned segments can then be placed in order corresponding to that on the chromosome. In a refinement of the technique, called chromosome jumping, only the ends of segments are identified, allowing the investigator to ‘jump’ over the middle regions. This speeds up the process, and also is a way of bypassing stretches of repetitive DNA, which are not amenable to chromosome walking.

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chromosome

chromosome A DNA-histone protein thread, usually associated with RNA, occurring in the nucleus of a cell. Although chromosomes are found in all animals and plants, bacteria and viruses contain structures that lack protein and contain only DNA or RNA; these are not chromosomes, though they serve a similar function. The presence of pairs of homologous chromosomes is referred to as the diploid state; in diploid organisms the chromosomes associate in a particular way during meiosis. Gametes are haploid, with only one member of each pair in their nuclei. Each species tends to have a characteristic number of chromosomes (46 in humans), found in most nucleated cells within most organisms. Usually chromosomes are visible only during mitosis or meiosis when they contract to form short thick rods coiled into a spiral. Each chromosome possesses two sets of chromomeres and a centromere, and some contain a nucleolar organizer. Chromosomes contain a line of different genes, a spindle-attachment at some point along their length, and regions of heterochromatin, which stains strongly with basophilic dyes.

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chromosome

chromosome A protein thread composed of DNA and histone, usually associated with RNA, occurring in the nucleus of a cell. Although chromosomes are found in all animals and plants, bacteria and viruses contain structures that lack protein and contain only DNA or RNA: these are not chromosomes, though they serve a similar function. Chromosomes occur in pairs. Each species tends to have a characteristic number of pairs of chromosomes (23 in humans), found in most nucleated cells within most organisms. The presence of pairs of homologous chromosomes is referred to as the diploid state and is normal for the sexual phase of an organism. Gametes (reproductive cells), and cells of the gametophyte (gamete-producing phase) of plants, however, have only one member of each pair in their nuclei (the haploid state).

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Chromosomes

Chromosomes

The genetic material in plants, animals, and fungi is called deoxyribonucleic acid (DNA), a long, linear polymer that is physically organized at the microscopic level into chromosomes. Chromosomes are threadlike cellular structures made up of elaborately packaged DNA complexed with proteins. When a cell reproduces itself to make two identical daughter cells, the chromosomes are replicated and divided so that each daughter cell has the same genetic and DNA content. The chromosome division process is called mitosis. During mitosis the individual chromosomes can be stained and seen under a microscope.

Genes code for the production of structural proteins and enzymes and are located at specific sites along the DNA. These sites are called loci (singular: locus) and represent a sort of chromosomal street address for the basic units of heredity, the genes. Genetic loci number in the tens of thousands for most plant species, and they are physically linked if they reside on the same chromosome.

Plant chromosomes replicate and divide in a typical fashion. They are also subject to a type of molecular infection by small, self-replicating, or mobile, pieces of DNA called transposable DNA elements (or transposons), which can hop from one chromosome to another, as described below.

Historically, some important basic principles of genetics and heredity have come from the scientific study of plants. In his classic work on the transmission of traits (such as wrinkled seed) in peas, Gregor Mendel discovered the basic rules of heredity. Mendel showed that both mother (egg) and father (pollen sperm) contribute genetic factors to the next generation by cell union at fertilization. Similarly, the discovery of the existence of jumping genes (described below) was made by Barbara McClintock in her work on corn (Zea mays ).

Plant chromosome research has come full circle in the new millenium with the ability to relate molecular structure to whole plant function. For instance, the wrinkled seed trait studied by Mendel was recently discovered to have been caused by a transposon that hopped into and broke a gene involved in filling the pea seed with starch. Mendel was able to track the broken gene through multiple generations by observing the inheritance of the wrinkled seed trait. Understanding plant chromosome structure and function helps bridge the gap between molecular biology and whole plant biology.

Physical Description

DNA does not exist in the cell as an isolated chemical, but rather as an elaborately packaged and microscopically visible structure called a chromosome. All chromosomes are comprised of both DNA and proteins, although only the DNA contains the genetic code. Each chromosome carries thousands of genes, and each time a cell divides all of the cell's chromosomes are replicated, divided, and sorted into two pools, one for each new daughter cell. Each chromosome has a centromere (the site on the chromosome where the spindle attaches), which functions as a "luggage handle" for the genetic cargo. This attachment provides the mechanical basis for movement of chromosomes toward one of the two pointed ends (poles) of the football-shaped spindle apparatus.

The entire complement of chromosomes in a given cell or for a given species is referred to as the genome. Plant genomes vary in total DNA content from one species to the next, yet they all have a similar number of functional genes (between fifty and one hundred thousand per individual) required to support the life cycle of a typical plant.

Chromosome Pairing and Segregation

Because most plant species reproduce sexually, they have genomes consisting of two complete sets of genetic instructions, one from each parent, just like humanoids. Most cells of the plant body (stems, roots, leaves) carry this duplicate set, which makes them diploid .

During meiosis, the genome content gets reduced to one complete set of chromosomes per cell, producing gamete cells that are said to be haploid . The male haploid cells in flowering plants give rise to the pollen grains (sperm) whereas the female haploid cells give rise to eggs. As with animals, the diploid state is restored at fertilization by the union of DNA from the sperm and egg cells. Thus the plant life cycle is frequently divided into two major stages: the diploid stage (2N), which occurs after fertilization; and the haploid stage (1N), which occurs after meiosis.

A replicated chromosome consists of two identical sister chromatids that remain connected by a centromere. At mitosis, all the chromosomes attach their replicated and connected centromeres to a bipolar spindle apparatus. For each replicated chromosome, the two centromeres become attached to spindle fibers pointing in opposite directions (the metaphase stage of mitosis). Moving along the spindle fibers (the anaphase stage of mitosis), the sister chromatids of each replicated chromosome separate and move to opposite poles. Thus mitosis ensures that when a single cell divides into two, each new daughter cell is equipped with a complete and equal set of genetic instructions. After fertilization, the zygote grows into an embryo and then an adult by using mitosis until the time for sexual reproduction (flowering).

When producing sperm and egg cells for sexual reproduction, the genetic content must first be reduced from diploid to haploid. This reduction is accomplished by meiosis, a specialized process involving two sequential nuclear DNA divisions without an intervening DNA replication step. The first division requires the matching diploid chromosomes to pair, two-bytwo, then segregate away from each other to reduce the genome from diploid to haploid. This chromosome pairing is necessary for proper chromosome segregation and much of the genetic shuffling that takes place from one generation to the next. The second meiotic division is like mitosis and divides replicated chromosomes into the haploid gamete-producing cells. Plant pollen mother cells that undergo meiosis provide excellent cytogenetic specimens to study because the cells and chromosomes are easy to see under the microscope.

Transposable Elements

Transposable DNA elements are sometimes called "jumping genes" because they can move around within the genome. The earliest evidence for the existence of these transposons came from analysis of certain strains of corn by McClintock. At the time in the 1940s, the idea that some parts of the chromosome could be mobile contradicted the notion that the chromosome was a stable, single structure. McClintock's pioneering work on transposons was formally recognized in 1983 when she was awarded a Nobel Prize. The activity of transposons sometimes causes visible features such as stripes and speckles on seeds (such as maize or beans) or flowers (such as petunias).

Transposons are active in most species of plants and animals, and their hopping around can change or even break individual genes. Thus transposons are thought to provide a source of genetic variation within the gene pool of a breeding population. In recent years, many plant transposons have been isolated molecularly (cloned) and used as tools to study plant genetics and create new genetic variations (mutations) by a technique called transposon mutagenesis.

see also Cell Cycle; Flowers; Genetic Engineering; McClintock, Barbara; Reproduction, Sexual.

Hank W. Bass

Bibliography

John, Bernard. Meiosis. New York: Cambridge University Press, 1990.

Fedoroff, Nina, and David Botstein. The Dynamic Genome: Barbara McClintock's Ideas in the Century of Genetics. Plainview, NY: Cold Spring Harbor Laboratory Press, 1992.

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Chromosome

CHROMOSOME

Chromosomes are structures in the nucleus of the cell that contain the DNA or hereditary material which form genes. Genes are the commonly known units of heredity, and some may contribute to a tendency toward addiction in ways that are not yet understood. Each chromo-some is an elongated structure that is clearly visible during cell division. Humans possess twenty-three pairs including the sex chromosomes. A male has an X and a Y sex chromosome, whereas a female has two X sex chromosomes. One of each pair comes from each parent.

Michael J. Kuhar

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chromosomes

chromosomes The nuclei of the body-cells of humans and other complex animals and plants contain a number (fixed for each species—twenty-three in the human case) of pairs of thread-like structures. These carry the genes, the basic units of material inheritance, arranged in a linear sequence. When cells divide to form egg and sperm, chromosomes pair up and then divide in ways which randomly allocate genetic material to the resulting gametes. This process accounts for the inherited diversity among the individual progeny of sexual reproduction.

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chromosome

chromosome Structure carrying genetic information, found only in the cell nucleus of eukaryotes. Thread-like and composed of dna, chromosomes carry a specific set of genes. Each species usually has a characteristic number of chromosomes; these occur in pairs, members of which carry identical genes, so that most cells are diploid (two sets of chromosomes in each cell). Gametes carry a haploid number of chromosomes (a single set). See also heredity

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chromosome

chromosome (kroh-mŏ-sohm) n. one of the threadlike structures in a cell nucleus that carry the genetic information in the form of genes. It is composed of a long double filament of DNA coiled into a helix together with associated proteins. The nucleus of each human somatic cell contains 46 chromosomes, 23 being of maternal and 23 of paternal origin. See also chromatid, centromere, sex chromosome.
chromosomal adj.

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chromosome

chro·mo·some / ˈkrōməˌsōm/ • n. Biol. a threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes. DERIVATIVES: chro·mo·so·mal / ˌkrōməˈsōməl/ adj.

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chromosome diminution

chromosome diminution A phenomenon seen in embryonic nematode worms in which the germ cells become separated from the somatic cells at an early stage of development. The chromosomes of the somatic cells tend to break down, with only a few remaining intact, whereas the integrity of the germ-cell chromosomes is maintained.

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Chromosome

Chromosome

A chromosome is a threadlike structure found in the nucleus of most cells. It carries genetic material in the form of a linear sequence of deoxyribonucleic acid (DNA ). In prokaryotes, or cells without a nucleus, the chromosome represents circular DNA containing the entire genome. In eukaryotes, or cells with a distinct nucleus, chromosomes are much more complex in structure. The function of chromosomes is to package the extremely long DNA sequence. A single chromosome (uncoiled) could be as long as three inches and therefore visible to the naked eye. If DNA were not coiled within chromosomes, the total DNA in a typical eukaryotic cell would extend thousands of times the length of the cell nucleus.

DNA is the genetic material of all cells and contains information necessary for the synthesis of proteins. DNA is composed of two strands of nucleic acids arranged in a double helix. The nucleic acid strands are composed of a sequence of nucleotides. The nucleotides in DNA have four kinds of nitrogen containing bases: adenine (A), guanine (G), cytosine (C), and thymine (T). Within DNA, each strand of nucleic acid is partnered with the other strand by bonds that form between these nucleotides. Complementary base pairing dictates that adenine pairs only with thymine, and guanine pairs only with cytosine (and vice versa). Thus, by knowing the sequence of bases in one strand of the DNA helix, you can determine the sequence on the other strand. For instance, if the sequence in one strand of DNA were ATTCG, the other strand's sequence would be TAAGC.

DNA functions in the cell by providing a template by which another nucleic acid, called ribonucleic acid (RNA), is formed. Like DNA, RNA is also composed of nucleotides. Unlike DNA, RNA is single stranded and does not form a helix. In addition, the RNA bases are the same as in DNA, except that uracil replaces thymine. RNA is transcribed from DNA in the nucleus of the cell. Genes are expressed when the chromosome uncoils with the help of enzymes called helicases and specific DNA binding proteins. DNA is transcribed into RNA.

Newly transcribed RNA is called messenger RNA (mRNA). Messenger RNA leaves the nucleus through the nuclear pore and enters into the cytoplasm. There, the mRNA molecule binds to a ribosome (also composed of RNA) and initiates protein synthesis. Each block of three nucleotides, called codons, in the mRNA sequence encodes for a specific amino acid, the building blocks of a protein.

Genes are part of the DNA sequence called coding DNA. Noncoding DNA represents sequences that do not have genes and only recently have been found to have many new important functions. Out of the 3 billion base pairs that exist in the human DNA, there are only about 40,000 genes. The noncoding sections of DNA within a gene are called introns, while the coding sections of DNA are called exons. After transcription of DNA to RNA, the RNA is processed. Introns from the mRNA are excised out of the newly formed mRNA molecule before it leaves the nucleus.

The human genome (which represents the total amount of DNA in a typical human cell) has approximately 3 × 109 base pairs. If these nucleotide pairs were letters, the genome book would number over a million pages. There are 23 pairs of chromosomes, for a total number of 46 chromosomes in a diploid cell, or a cell having all the genetic material. In a haploid cell, there is only half the genetic material. For example, sex cells (the sperm or the egg) are haploid, while many other cells in the body are diploid. One of the chromosomes in the set of 23 is an X or Y (sex chromosomes), while the rest are assigned numbers 1 through 22. In a diploid cell, males have both an X and a Y chromosome, while females have two X chromosomes. During fertilization, the sex cell of the father combines with the sex cell of the mother to form a new cell, the zygote, which eventually develops into an embryo. If the one of the sex cells has the full complement of chromosomes (diploidy), then the zygote would have an extra set of chromosomes. This is called triploidy and represents an anomaly that usually results in a miscarriage. Sex cells are formed in a special kind of cell division called meiosis. During meiosis, two rounds of cell division ensure that the sex cells receive the haploid number of chromosomes.

Chromosomes can be visible using a microscope just prior to cell division, when the DNA within the nucleus uncoils as it replicates. By visualizing a cell during metaphase, a stage of cell division or mitosis, researchers can take pictures of the duplicated chromosome and match the pairs of chromosomes using the characteristic patterns of bands that appear on the chromosomes when they are stained with a dye called giemsa. The resulting arrangement is called a karyotype. The ends of the chromosome are referred to as telomeres, which are required to maintain stability and recently have been associated with aging. An enzyme called telomerase maintains the length of the telomere. Older cells tend to have shorter telomeres. The telomere has a repeated sequence (TTAGGG) and intact telomeres are important for proper DNA replication processes.

Karyotypes are useful in diagnosing some genetic conditions, because the karyotype can reveal an aberration in chromosome number or large alterations in structure. For example, Down's syndrome is caused by an extra chromosome 21, called trisomy 21. A karyotype of a child with Down's syndrome would reveal this extra chromosome.

A chromosome usually appears to be a long, slender rod of DNA. Pairs of chromosomes are called homologues. Each separate chromosome within the duplicate is called a sister chromatid. The sister chromatids are attached to each other by a structure called the centromere. Chromosomes appear to be in the shape of an X after the material is duplicated. The bottom, longer portion of the X is called the long arm of the chromosome (q-arm), and the top, shorter portion is called the short arm of the chromosome (p-arm).

DNA in chromosomes is associated with proteins and this complex is called chromatin. Euchromatin refers to parts of the chromosome that have coding regions or genes, while heterchromatin refers to regions that are devoid of genes or regions where gene transcription is turned off. DNA binding proteins can attach to specific regions of chromatin. These proteins mediate DNA replication, gene expression, or represent structural proteins important in packaging the chromosomes. Histones are structural proteins of chromatin and are the most abundant protein in the nucleus. In fact, the mass of histones in a chromosome is almost equal to that of DNA. Chromosomes contain five types of these small proteins: H1, H2A, H2B, H3, and H4. There are two of each of latter four histones that form a structure called the octomeric histone core. The H1 histone is larger than the other histones, and performs a structural role separate from the octomeric histone core in organizing DNA within the chromosome.

The octomeric histone core functions as a spool from which DNA is wound two times. Each histone-DNA spool is called a nucleosome. Nucleosomes occur at intervals of every 200 base pairs of the DNA helix. In photographs taken with the help of powerful microscopes , DNA wrapped around nucleosomes resembles beads (the nucleosome) threaded on a string (the DNA molecule). The DNA that exists between nucleosomes is called linker DNA. Chromosomes can contain some very long stretches of linker DNA. Often, these long linker DNA sequences are the regulatory portions of genes. These regulatory portions switch genes on when certain molecules bind to them.

Nucleosomes are the most fundamental organizing structure in the chromosome. They are packaged into structures that are 30 nanometers in size and called the chromatin fiber (compared to the 2 nm DNA double helix, and 11 nm histone core). The 30 nanometer fibers are sometimes then further folded into a larger chromatin fiber that is approximately 300 nanometers thick and represented on of the arms of the chromsome. The chromatin fibers are formed into loops by another structural protein. Each loop contains 20,00030,000 nucleotide pairs. These loops are then arranged within the chromosomes, held in place by more structural proteins. Metaphase chromosomes are approximately 1400 nm wide.

Chromosomes in eukaryotes perform a useful function during mitosis, the process in which cells replicate their genetic material and then divide into two new cells (also called daughter cells). Because the DNA is packaged within chromosomes, the distribution of the correct amount of genetic material to the daughter cells is maintained during the complex process of cell division.

Before a cell divides, the chromosomes are replicated within the nucleus. In a human cell, the nucleus just prior to cell division contains 46 pairs of chromosomes. When the cell divides, the sister chromatids from each duplicated chromosome separate. Each daughter cell ends up with 23 pairs of chromosomes and after DNA replication, the daughter cells have a diploid number of chromosomes.

In meiosis, the type of cell division that leads to the production of sex cells, the division process is more complicated. Two rounds of cell division occur in meiosis. Before meiosis, the chromosomes replicate, and the nucleus has 46 pairs of chromosomes. In the first round of meiotic cell division, the homologous chromosomes pairs separate as in mitosis (a stage called meiosis I). In the second round of cell division (meisosis II), the sister chromatids of each chromosome separate at the centromere, so that each of the four daughter cells receives the haploid number of chromosomes.

see also DNA; DNA databanks; DNA fingerprint; DNA mixtures, forensic interpretation of mass graves; DNA profiling; Evidence; Gene; STR (short tandem repeat) analysis; War forensics.

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Chromosome

Chromosome

DNA and protein synthesis

Genes

Chromosome numbers

Chromosome shape

The role of proteins in packaging DNA

Chromosomes and mitosis

Protein synthesis and chromosomes

Resources

A chromosome is a threadlike structure found in the nucleus of most cells that carries the genetic material in the form of a linear sequence of deoxyribonucleic acid (DNA). In prokaryotes, or cells without a nucleus, the chromosome represents circular DNA containing the entire genome. In eukaryotes, or cells with a distinct nucleus, chromosomes are much more complex in structure. The function of chromosomes is to package the extremely long DNA sequence. A single uncoiled chromosome could be as long as 3 inches (7.6 centimeters) and therefore visible to the naked eye. If DNA were not coiled within chromosomes, the total DNA in a typical eukaryotic cell would extend thousands of times the length of the cell nucleus.

DNA and protein synthesis

DNA is the genetic material of all cells and contains information necessary for the synthesis of proteins. DNA is composed of two strands of nucleic acids arranged in a double helix. The nucleic acid strands are composed of a sequence of nucleotides. The nucleotides in DNA have four kinds of nitrogen-containing bases: adenine, guanine, cytosine, and thy-mine. Within DNA, each strand of nucleic acid is partnered with the other strand by bonds that form between these nucleotides. Complementary base pairing dictates that adenine pairs only with thymine, and guanine pairs only with cytosine (and vice versa). Thus, by knowing the sequence of bases in one strand of the DNA helix, the sequence on the other strand is determined. For instance, if the sequence in one strand of DNA were ATTCG, the other strands sequence would be TAAGC.

DNA functions in the cell by providing a template by which another nucleic acid, called ribonucleic acid (RNA), is formed. Like DNA, RNA is also composed of nucleotides. Unlike DNA, RNA is single stranded and does not form a helix. In addition, the RNA bases are the same as in DNA, except that uracil replaces thymine. RNA is transcribed from DNA in the nucleus of the cell. Genes are expressed when the chromosome uncoils with the help of enzymes called helicases and specific DNA binding proteins. DNA is transcribed into RNA.

Newly transcribed RNA is called messenger RNA (mRNA). Messenger RNA leaves the nucleus through the nuclear pore and enters the cytoplasm. There, the mRNA molecule binds to a ribosome (also composed of RNA) and initiates protein synthesis. Each block of three nucleotides, called codons, in the mRNA sequence encodes for a specific amino acid, the building blocks of a protein.

Genes

Genes are part of the DNA sequence called coding DNA. Noncoding DNA represents sequences that do not have genes and only recently have been found to have many new important functions. Out of the 3 billion base pairs that exist in human DNA, the sequence obtained from the Human Genome Project has revealed that there are only about 30, 000 genes. The noncoding sections of DNA within a gene are called introns, while the coding sections of DNA are called exons. After transcription of DNA to RNA, the RNA is processed. Introns from the mRNA are excised out of the newly formed mRNA molecule before it leaves the nucleus.

Chromosome numbers

The human genome (which represents the total amount of DNA in a typical human cell) has approximately 3× 109 base pairs. If these nucleotide pairs were letters, the genome book would number over a million pages. There are 23 pairs of chromosomes, for a total number of 46 chromosomes in a diploid cell, or a cell having all the genetic material. In a haploid cell, there is only half the genetic material. For example, sex cells (the sperm or the egg) are haploid, while many other cells in the body are diploid. One of the chromosomes in the set of 23 is X or Y (sex chromosomes), while the rest are assigned numbers 1 through 22. In a diploid cell, males have both an X and a Y chromosome, while females have two X chromosomes. During fertilization, the sex cell of the father combines with the sex cell of the mother to form a new cell, the zygote, which eventually develops into an embryo. If the one of the sex cells has the full complement of chromosomes (diploidy), then the zygote would have an extra set of chromosomes. This is called triploidy and represents an anomaly that usually results in a miscarriage. Sex cells are formed in a special kind of cell division called meiosis. During meiosis, two rounds of cell division ensure that the sex cells receive the haploid number of chromosomes.

Other species have different numbers of chromosomes in their nuclei. As some examples, mosquitoes have 6 chromosomes, lilies have 24 chromosomes, earthworms have 36 chromosomes, chimps have 48 chromosomes, and horses have 64 chromosomes.

Chromosome shape

Chromosomes can be visualized using a microscope just prior to cell division, when the DNA within the nucleus uncoils as it replicates. By visualizing a cell during metaphase, a stage of cell division or mitosis, researchers can take pictures of the duplicated chromosome and match the pairs of chromosomes using the characteristic patterns of bands that appear on the chromosomes when they are specially stained. The resulting arrangement is called a karyotype.

The ends of the chromosome are referred to as telomeres, which are required to maintain stability and recently have been associated with aging. An enzyme called telomerase maintains the length of the telomere. Older cells tend to have shorter telomeres. The telomere has a repeated sequence (TTAGGG), and intact telomeres are important for proper DNA replication processes.

Karyotypes are useful in diagnosing some genetic conditions, because the karyotype can reveal an aberration in chromosome number or large alterations in structure. For example, Down syndrome is caused by an extra chromosome 21, a condition called trisomy 21. A karyotype of a child with Down syndrome would reveal this extra chromosome.

A chromosome usually appears to be a long, slender rod of DNA. Pairs of chromosomes are called homologues. Each separate chromosome within the duplicate is called a sister chromatid. The sister chromatids are attached to each other by a structure called the centromere. Chromosomes appear to be in the shape of an X after the material is duplicated. The bottom, longer portion of the X is called the long arm of the chromosome (q-arm), and the top, shorter portion is called the short arm of the chromosome (p-arm).

The role of proteins in packaging DNA

Several kinds of proteins are important for maintaining chromosomes, in terms of organization and gene expression. Some proteins initiate DNA replication when the cell prepares to divide. Other proteins control gene transcription in the preliminary stages of protein synthesis. Structural proteins help the DNA fold into the intricate configurations within the packaged chromosome. DNA in chromosomes is associated with proteins and this complex of DNA and proteins is called chromatin. Euchromatin refers to parts of the chromosome that have coding regions or genes, while heterchromatin refers to regions that are devoid of genes or regions where gene transcription is turned off. DNA binding proteins can attach to specific regions of chromatin. These proteins mediate DNA replication, gene expression, or represent structural proteins important in packaging the chromosomes. Histones are structural proteins of chromatin

and are the most abundant protein in the nucleus. In fact, the mass of histones in a chromosome is almost equal to that of DNA. Chromosomes contain five types of these small proteins, which participate in organizing DNA within the chromosome.

A histone complex functions as a spool from which DNA is wound two times. Each histone-DNA spool is called a nucleosome. Nucleosomes occur at intervals of every 200 base pairs of the DNA helix. In photographs taken with the help of powerful microscopes, DNA wrapped around nucleosomes resembles beads (the nucleosomeS) threaded on a string (the DNA molecule). The DNA that exists between nucleosomes is called linker DNA. Chromosomes can contain some very long stretches of linker DNA. Often, these long linker DNA sequences are the regulatory portions of genes. These regulatory portions switch genes on when certain molecules bind to them.

Chromosomes and mitosis

Chromosomes in eukaryotes perform a useful function during mitosis, the process in which cells replicate their genetic material and then divide into two new cells (also called daughter cells). Because the DNA is packaged within chromosomes, the distribution of the correct amount of genetic material to the daughter cells is maintained during the complex process of cell division.

Before a cell divides, the chromosomes are replicated within the nucleus. In a human cell, the nucleus just prior to cell division contains 46 pairs of chromosomes. When the cell divides, the sister chromatids from each duplicated chromosome separate. Each daughter cell ends up with 23 pairs of chromosomes, and after DNA replication the daughter cells will have a diploid number of chromosomes.

In meiosis, the type of cell division that leads to the production of sex cells, the division process is more complicated. Two rounds of cell division occur in meiosis. Before meiosis, the chromosomes replicate, and the nucleus has 46 pairs of chromosomes. In the first round of meiotic cell division, the homologous chromosomes pairs separate as in mitosis (a stage called meiosis I). In the second round of cell division (meisosis II), the sister chromatids of each chromosome separate at the centromere, so that each of the four daughter cells receives the haploid number of chromosomes.

Protein synthesis and chromosomes

DNA is bound up within chromatids, which serve as storage units for the DNA. In order for an mRNA molecule to be transcribed from a DNA template, the DNA needs to be freed from its tightly bound and condensed conformation so that the RNA molecule can form on its exposed strands during transcription. Some evidence exists that transcription can take place through histones. However, most often the genes on the DNA are activated after a DNA binding protein unwinds the chromatid structure. Thus loosened, transcriptionally active regions of DNA then resemble microscopic puffs on the chromosomes. When RNA transcription concludes, the puffs recede, and the chromosome is thought to resume its original unwound conformation.

Resources

BOOKS

Bainbridge, David. The X in Sex: How the X Chromosome Controls Our Lives. Boston: Harvard University Press, 2004.

Harper, Peter. The First Years of Human Chromosomes. Bloxham, UK: Scion Publishing, 2006.

Watson, James D. and Andrew Berry. DNA: The Secret of Life. New York: Knopf, 2004.

Walker, Richard. Genes and DNA. London: Kingfisher, 2003.

Kathleen Scogna

KEY TERMS

Chromatin The material that comprises chromosomes; consists of DNA and proteins.

Chromatin fiber The fiber that is formed by the gathering of nucleosomes by H1 histones.

Chromosome puffs The regions of active DNA that are transcribing RNA; appear as puffed regions in a chromosome.

Deoxyribonucleic acid (DNA) The genetic material of cells that are packed into chromosomes.

Eukaryote A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.

Exons The regions of DNA that code for a protein or form tRNA or mRNA.

Genome The complete set of genes an organism carries.

Histone A structural protein that functions in packaging DNA in chromosomes.

Homologue The partner of a chromosome in a chromosome pair.

Introns The sections of DNA that do not code for proteins or RNAs.

Karyotype An arrangement of chromosomes according to number.

Linker DNA The sections of DNA between nucleosomes.

Meiosis The process of sex cell division; results in four haploid daughter cells.

Messenger RNA The RNA that is transcribed from DNA in the nucleus; functions in protein synthesis.

Mitosis The process of body cell division; results in two diploid daughter cells.

Nitrogen-containingt base Part of a nucleotide. In DNA, the bases are adenine, guanine, thymine, and cytosine. In RNA, the bases are adenine, guanine, uracil, and cytosine.

Nucleic acid The chemical component of DNA and RNA.

Nucleosome DNA wrapped around a histone core. NucleotideThe building blocks of nucleic acids.

Octomeric histone core The spool in a nucleosome; consists of four small histones.

Ribonucleic acid RNA; the molecule translated from DNA in the nucleus that directs protein synthesis in the cytoplasm; it is also the genetic material of many viruses.

Ribosomal RNA A type of RNA that functions in protein synthesis.

Sister chromatids Two copies of the same chromosome produced by DNA replication.

Transcription The process of synthesizing RNA from DNA.

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Chromosome

Chromosome

Chromosomes are microscopic units containing organized genetic information, located in the nuclei of diploid and haploid cells (e.g. human somatic and sex cells), and are also present in one-cell non-nucleated organisms (unicellular microorganisms), like bacteria, which do not have an organized nucleus. The sum-total of genetic information contained in different chromosomes of a given individual or species are generically referred to as the genome.

In humans, chromosomes are structurally made of roughly equal amounts of proteins and DNA . Each chromosome contains a double-strand DNA molecule, arranged as a double helix, and tightly coiled and neatly packed by a family of proteins called histones. DNA strands are comprised of linked nucleotides. Each nucleotide has a sugar (deoxyribose), a nitrogenous base, plus one to three phosphate groups. Each nucleotide is linked to adjacent nucleotides in the same DNA strand by phosphodiester bonds. Phosphodiester is another sugar, made of sugar-phosphate. Nucleotides of one DNA strand link to their complementary nucleotide on the opposite DNA strand by hydrogen bonds, thus forming a pair of nucleotides, known as a base pair, or nucleotide base. Genes contain up to thousands of sequences of these base pairs. What distinguishes one gene from another is the sequence of nucleotides that code for the synthesis of a specific protein or portion of a protein. Some proteins are necessary for the structure of cells and tissues. Others, like enzymes, a class of active (catalyst) proteins, promote essential biochemical reactions, such as digestion, energy generation for cellular activity, or metabolism of toxic compounds. Some genes produce several slightly different versions of a given protein through a process of alternate transcription of base pair segments known as codons.

Amounts of autosomal chromosomes differ in cells of different species; but are usually the same in every cell of a given species. Sex determination cells (mature ovum and sperm) are an exception, where the number of chromosomes is halved. Chromosomes also differ in size. For instance, the smallest human chromosome, the sex chromosome Y, contains 50 million base pairs (bp), whereas the largest one, chromosome 1, contains 250 million base pairs. All three billion base pairs in the human genome are stored in 48 chromosomes. Human genetic information is therefore stored in 24 pairs of chromosomes (totaling 48), 24 inherited from the mother, and 24 from the father. Two of these chromosomes are sex chromosomes (chromosomes X and Y). The remaining 46 are autosomes, meaning that they are not sex chromosomes and are present in all somatic cells (i.e., any other body cell that is not a germinal cell for spermatozoa in males or an ovum in females). Sex chromosomes specify the offspring gender: normal females have two X chromosomes and normal males have one X and one Y chromosome.

Each set of 24 chromosomes constitutes one allele, containing gene copies inherited from one of the parents. The other allele is complementary or homologous, meaning that it contains copies of the same genes and on the same positions, but originated from the other parent. As an example, every normal child inherits one set of copies of gene BRCA1, located on chromosome 13, from the mother and another set of BRCA1 from the father, located on the other allelic chromosome 13. Allele is a Greek-derived word that means "one of a pair," or any one of a series of genes having the same locus (position) on homologous chromosomes.

The first chromosome observations were made under light microscopes, revealing rod-shaped structures in varied sizes and conformations; commonly J-, or V-shaped in eukaryotic cells and ring-shaped chromosome in bacteria. Staining reveals a pattern of light and dark bands. Today those bands are known to correspond to regional variations in the amounts of the two nucleotide base pairs: adenine-thymine (A-T or T-A) in contrast with amounts of guanine-cytosine (G-C or C-G).

Genetic abnormalities and diseases occur when one of the following events happens: a) one chromosome copy is missing, b) extra copies of a chromosome are present, c) a chromosome breaks and its fragment is fused into another chromosome (insertion), d) a fragment is deleted, e) a gene is transferred from one chromosome to another (translocation), f) duplication of a chromosomal segment occurs, g) inversion of a chromosomal segment occurs. Down syndrome , for instance, is caused by the presence of a third copy of chromosome 21.

In non-dividing cells, it is not possible to distinguish morphological details of individual chromosomes because they remain elongated and entangled to each other. However, when a cell is dividing, i.e., undergoing mitosis, chromosomes become highly condensed and each individual chromosome occupies a well-defined spatial location.

Mitotic chromosomes present a constricted region, to which the spindle fibers attach during cellular division. Such a constricted region, known as a centromere or primary constriction, may be located in three different positions in chromosomes. Centromeric position allows the classification of chromosomes in three groups: a) acrocentric: centromere lies very near one end; b) metacentric: centromere at the middle, dividing the chromosome in two equal parts or arms; and c) submetacentric: centromere near middle, but dividing chromosome in two unequal arms.

When a chromosome loses its centromere, it is known as acentric. As the centromere is essential for both division and retention of chromosome copies in the new cells, acentric chromosomes will not pass to the daughter cells during the parental cell division. Therefore, daughter cells will miss one chromosome in their karyotype . A karyotype map shows mitotic chromosomes in the mitotic phase, known as metaphase. In metaphase, chromosomes align in pairs. In a normal human karyotype, there are 22 pairs of autosomal chromosomes and two sex chromosomes (X and Y). Each pair of autosomal chromosomes contains two complementary or homologous chromosomes, a maternal and a paternal copy.

Some chromosomes also present a secondary constriction that always appears at the same site. They are also useful, along with centromere position and chromosome size, for identifying and characterizing individual chromosomes, in a karyotype.

Karyotype analysis was the first genetic screening utilized by geneticists to assess inherited abnormalities, like additional copies of a chromosome or a missing copy, as well as DNA content and gender of the individual. With the development of new molecular screening techniques and the growing number of identified individual genes, detection of other more subtle chromosomal mutations is now possible (e.g., determinations of gene mutations, levels of gene expression, etc). Such data allow scientists to better understand disease causation and to develop new therapies and medicines for those diseases.

Sandra Galeotti, MS

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Chromosome

Chromosome

A chromosome is a threadlike structure found in the nucleus of most cells. that carries the genetic material in the form of a linear sequence of deoxyribonucleic acid (DNA) . In prokaryotes, or cells without a nucleus, the chromosome represents circular DNA containing the entire genome . In eukaryotes, or cells with a distinct nucleus, chromosomes are much more complex in structure. The function of chromosomes is to package the extremely long DNA sequence. A single chromosome (uncoiled) could be as long as 3 in (7.6 cm) and therefore visible to the naked eye . If DNA were not coiled within chromosomes, the total DNA in a typical eukaryotic cell would extend thousands of times the length of the cell nucleus.


DNA and protein synthesis

DNA is the genetic material of all cells and contains information necessary for the synthesis of proteins . DNA is composed of two strands of nucleic acids arranged in a double helix . The nucleic acid strands are composed of a sequence of nucleotides. The nucleotides in DNA have four kinds of nitrogen-containing bases: adenine, guanine, cytosine, and thymine. Within DNA, each strand of nucleic acid is partnered with the other strand by bonds that form between these nucleotides. Complementary base pairing dictates that adenine pairs only with thymine, and guanine pairs only with cytosine (and vice versa). Thus, by knowing the sequence of bases in one strand of the DNA helix, you can determine the sequence on the other strand. For instance, if the sequence in one strand of DNA were ATTCG, the other strand's sequence would be TAAGC.

DNA functions in the cell by providing a template by which another nucleic acid, called ribonucleic acid (RNA) , is formed. Like DNA, RNA is also composed of nucleotides. Unlike DNA, RNA is single stranded and does not form a helix. In addition, the RNA bases are the same as in DNA, except that uracil replaces thymine. RNA is transcribed from DNA in the nucleus of the cell. Gene are expressed when the chromosome uncoils with the help of enzymes called helicases and specific DNA binding proteins. DNA is transcribed into RNA.

Newly transcribed RNA is called messenger RNA (mRNA). Messenger RNA leaves the nucleus through the nuclear pore and enters into the cytoplasm. There, the mRNA molecule binds to a ribosome (also composed of RNA) and initiates protein synthesis. Each block of three nucleotides called codons in the mRNA sequence encodes for a specific amino acid , the building blocks of a protein.

Genes

Genes are part of the DNA sequence called coding DNA. Noncoding DNA represents sequences that do not have genes and only recently have found to have many new important functions. Out of the 3 billion base pairs that exist in the human DNA, there are only about 40,000 genes. The noncoding sections of DNA within a gene are called introns, while the coding sections of DNA are called exons. After transcription of DNA to RNA, the RNA is processed. Introns from the mRNA are excised out of the newly formed mRNA molecule before it leaves the nucleus.


Chromosome numbers

The human genome (which represents the total amount of DNA in a typical human cell) has approximately 3 × 109 base pairs. If these nucleotide pairs were letters, the genome book would number over a million pages. There are 23 pairs of chromosomes, for a total number of 46 chromosomes in a dipoid cell, or a cell having all the genetic material. In a haploid cell, there is only half the genetic material. For example, sex cells (the sperm or the egg) are haploid, while many other cells in the body are diploid. One of the chromosomes in the set of 23 are X or Y (sex chromosomes), while the rest are assigned numbers 1 through 22. In a diplod cell, males have both an X and a Y chromosome, while females have two X chromosomes. During fertilization , the sex cell of the father combines with the sex cell of the mother to form a new cell, the zygote, which eventually develops into an embryo. If the one of the sex cells has the full complement of chromosomes (diploidy), then the zygote would have an extra set of chromosomes. This is called triploidy and represents an anomaly that usually results in a miscarriage. Sex cells are formed in a special kind of cell division called meiosis . During meiosis, two rounds of cell division ensure that the sex cells receive the haploid number of chromosomes.

Other species have different numbers of chromosomes in their nuclei. Mosquitos, for instance, have 6 chromosomes. Lilies have 24 chromosomes, earthworms have 36 chromosomes, chimps have 48 chromosomes, and horses have 64 chromosomes. The largest number of chromosomes are found in the Adders tongue fern, which has more than 1,000 chromosomes. Most species have, on average, 10–50 chromosomes.


Chromosome shape

Chromosomes can be visible using a microscope just prior to cell division, when the DNA within the nucleus uncoils as it replicates. By visualizing a cell during metaphase, a stage of cell division or mitosis , researchers can take pictures of the duplicated chromosome and match the pairs of chromosomes using the characteristic patterns of bands that appear on the chromosomes when they are stained with a dye called giemsa. The resulting arrangement is called a karyotype. The ends of the chromosome are referred to as telomeres, which are required to maintain stablility and recently have been associated with aging. An enzyme called telomerase maintains the length of the telomere. Older cells tend to have shorter telomeres. The telomere has a repeated sequence (TTAGGG) and intact telomeres are important for proper DNA replication processes.

Karyotypes are useful in diagnosing some genetic conditions, because the karyotype can reveal an aberration in chromosome number or large alterations in structure. For example, Down syndrome is can be caused by an extra chromosome 21 called trisomy 21. A karyotype of a child with Down syndrome would reveal this extra chromosome.

A chromosome usually appears to be a long, slender rod of DNA. Pairs of chromosomes are called homologues. Each separate chromosome within the duplicate is called a sister chromatid. The sister chromatids are attached to each other by a structure called the centromere. Chromosomes appear to be in the shape of an X after the material is duplicated. The bottom, longer portion of the X is called the long arm of the chromosome (q-arm), and the top, shorter portion is called the short arm of the chromosome (p-arm).


The role of proteins in packaging DNA

Several kinds of proteins are important for maintaining chromosomes in terms of its organization and gene expression. Some proteins initiate DNA replication when the cell prepares to divide. Other proteins control gene transcription in the preliminary stages of protein synthesis. Structural proteins help the DNA fold into the intricate configurations within the packaged chromosome.

DNA in chromosomes is associated with proteins and this complex is called chromatin . Euchromatin refers to parts of the chromosome that have coding regions or genes, while heterchromatin refers to regions that are devoid of genes or regions where gene transcription is turned off. DNA binding proteins can attach to specific regions of chromatin. These proteins mediate DNA replication, gene expression, or represent structural proteins important in packaging the chromosomes. Histones are structural proteins of chromatin and are the most abundant protein in the nucleus. In fact, the mass of histones in a chromosome is almost equal to that of DNA. Chromosomes contain five types of these small

proteins: H1, H2A, H2B, H3, and H4. There are two of each of latter four histones that form a structure called the octomeric histone core. The H1 histone is larger than the other histones, and performs a structural role separate from the octomeric histone core in organizing DNA within the chromosome.

The octomeric histone core functions as a spool from which DNA is wound two times. Each histone-DNA spool is called a nucleosome. Nucleosomes occur at intervals of every 200 bases pairs of the DNA helix. In photographs taken with the help of powerful microscopes, DNA wrapped around nucleosomes resembles beads (the nucleosome) threaded on a string (the DNA molecule). The DNA that exists between nucleosomes is called linker DNA. Chromosomes can contain some very long stretches of linker DNA. Often, these long linker DNA sequences are the regulatory portions of genes. These regulatory portions switch genes on when certain molecules bind to them.

Nucleosomes are only the most fundamental organizing structure in the chromosome. They are packaged into structures that are 30 nanometers in size and called the chromatin fiber (compared to the 2 nm DNA double helix, and 11 nm histone core). The 30 nanometer fibers are then further folded into a larger chromatin fiber sometimes that is approximately 300 nanometers thick and represent on of the arms of the chromsome. The chromatin fibers are formed into loops by another structural protein. Each loop contains 20,000–30,000 nucleotide pairs. These loops are then arranged within the chromosomes, held in place by more structural proteins. Metaphase chromosomes are approximately 1400 nm wide.


Chromosomes and mitosis

Chromosomes in eukaryotes perform a useful function during mitosis, the process in which cells replicate their genetic material and then divide into two new cells (also called daughter cells). Because the DNA is packaged within chromosomes, the distribution of the correct amount of genetic material to the daughter cells is maintained during the complex process of cell division.

Before a cell divides, the chromosomes are replicated within the nucleus. In a human cell, the nucleus just prior to cell division contains 46 pairs of chromosomes. When the cell divides, the sister chromatids from each duplicated chromosome separate. Each daughter cell ends up with 23 pairs of chromosomes and after DNA replication, the daughter cells will have a diploid number of chromosomes.

In meiosis, the type of cell division that leads to the production of sex cells, the division process is more complicated. Two rounds of cell division occur in meiosis. Before meiosis, the chromosomes replicate, and the nucleus has 46 pairs of chromosomes. In the first round of meiotic cell division, the homologous chromosomes pairs separate as in mitosis (a stage called meiosis I). In the second round of cell division (meisosis II), the sister chromatids of each chromosome separate at the centromere, so that each of the four daughter cells receives the haploid number of chromosomes.



Protein synthesis and chromosomes

DNA is bound up within chromatids, which serve as storage unit for the DNA. In order for an mRNA molecule to be transcribed from a DNA template, the DNA needs to be freed from its tightly bound and condensed conformation so that the RNA molecule can form on its exposed strands during transcription. Some evidence exists that transcription can take place through histones. However, most often the genes on the DNA that have been activated after DNA binding protein unwind the chromatid structure. This loosened, transcriptionally active regions of DNA is microscopically resembles puffs on the chromosomes. When RNA transcription concludes, the puffs receed and the chromosome is thought to resume its original conformation.

See also Genetics; Nucleus, cellular; Prokaryote.


Resources

books

Nussbaum, Robert L., Roderick R. McInnes, Huntington F. Willard. Genetics in Medicine. Philadelphia: Saunders, 2001.

Rimoin, David L. Emery and Rimoin's Principles and Practice of Medical Genetics. London; New York: Churchill Livingstone, 2002.

other

United States Department of Energy Office of Science. "Human Genome Project Information." (October 28, 2002). <http://www.ornl.gov/Tech Resources/Human_Genome/home.html>.


Kathleen Scogna

KEY TERMS

. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .
Chromatin

—The material that comprises chromosomes; consists of DNA and proteins.

Chromatin fiber

—The fiber that is formed by the gathering of nucleosomes by H1 histones.

Chromosome puffs

—The regions of active DNA that are transcribing RNA; appear as puffed regions in a chromosome.

Deoxyribonucleic acid (DNA)

—The genetic material of cells that are packed into chromosomes.

Eukaryote

—A cell whose genetic material is carried on chromosomes inside a nucleus encased in a membrane. Eukaryotic cells also have organelles that perform specific metabolic tasks and are supported by a cytoskeleton which runs through the cytoplasm, giving the cell form and shape.

Exons

—The regions of DNA that code for a protein or form tRNA or mRNA.

Genome

—The complete set of genes an organism carries.

Histone

—A structural protein that functions in packaging DNA in chromosomes.

Homologue

—The partner of a chromosome in a chromosome pair.

Introns

—The sections of DNA that do not code for proteins or RNAs.

Karyotype

—An arrangement of chromosomes according to number.

Linker DNA

—The sections of DNA between nucleosomes.

Meiosis

—The process of sex cell division; results in four haploid daughter cells.

Messenger RNA

—The RNA that is transcribed from DNA in the nucleus; functions in protein synthesis.

Mitosis

—The process of body cell division; results in two diploid daughter cells.

Nitrogen-containing base

—Part of a nucleotide; in DNA, the bases are adenine, guanine, thymine, and cytosine; in RNA, the bases are adenine, guanine, uracil, and cytosine.

Nucleic acid

—The chemical component of DNA and RNA.

Nucleosome

—DNA wrapped around a histone core.

Nucleotide

—The building blocks of nucleic acids.

Octomeric histone core

—The "spool" in a nucleosome; consists of four small histones.

Ribonucleic acid

—RNA; the molecule translated from DNA in the nucleus that directs protein synthesis in the cytoplasm; it is also the genetic material of many viruses.

Ribosomal RNA

—A type of RNA that functions in protein synthesis.

Sister chromatids

—Two copies of the same chromosome produced by DNA replication.

Transcription

—The process of synthesizing RNA from DNA.

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Chromosome

Chromosome


A chromosome is a coiled structure in the nucleus of a cell that carries the cell's deoxyribonucleic acid (DNA). DNA is the genetic blueprint that contains the genes that both direct the cell's activities and determine the characteristics of the organism. Chromosomes are found in nearly every cell of the body, and different species have different numbers of chromosomes. They are probably the most important part of a living cell since they contain all the necessary information to make a cell work.

Chromosomes were given their name, which means "color body," because they easily take up the dye stain that biologists use to study cell structures under a microscope. Despite this, chromosomes are only visible in the nucleus (a cell's control center) when the cell is dividing (although they are always present). Just before division occurs in a cell, chromosomes are easily seen because they condense or bunch together forming tightly coiled, rodlike shapes (many look like little "X's"). Until this happens, chromosomes exist in the nucleus as unwound, extremely fine threads or strings of protein and DNA that are called chromatin. These loose, strung-out forms of chromatin contain DNA which, in turn, consists of genes. In humans, there are forty-six of these ropelike fibers in the nucleus, which condense or contract into a chromosome just before cell division. Humans have forty-six chromosomes. Chromosomes are bunched together strings of DNA and proteins, and it is these DNA molecules that contain the cellular instructions or coded information that we call genes. The gene is considered the basic unit of heredity. In summary, chromosomes are found in nearly every cell of our bodies. Chromosomes are made of DNA, and DNA stores genes. It is genes that carry the vital codes and information that not only tell a cell what to do, but which get passed on to the next generation by sexual reproduction.

THOMAS HUNT MORGAN

American geneticist (a person specializing in the study of genes) Thomas Hunt Morgan's work with the fruit fly established the chromosome (a coiled structure in the nucleus of a cell that carries the cell's deoxyribonucleic acid, or DNA) theory of inheritance. He discovered that chromosomes are composed of discrete units called genes which are the actual carriers of specific traits. He also showed that when genes mutate, or change, the traits they control also change.

Thomas Hunt Morgan (1866–1945) was born in Lexington, Kentucky and grew up surrounded by nature and wildlife. As a youngster, he had an intense interest in biology and later majored in zoology in college. After obtaining his Ph.D. from Johns Hopkins University in 1890, he taught for awhile, and in 1904 became professor of experimental zoology at Columbia University where he would remain until 1928. Morgan had long been interested in heredity and believed that it was an important phenomenon barely understood. When Morgan began his research in 1904, the world had just learned of Austrian monk Gregor Mendel's laws of heredity. One of these laws stated that mixing traits did not result in a blend of traits, but instead these traits sorted themselves out according to a fixed ratio. Most scientists realized that the behavior of newly discovered chromosomes during cell division seemed to match Mendel's laws. However, everyone knew that there were only slightly more than two dozen pairs of chromosomes in the human cell, and it did not seem possible that they alone could account for the huge range of inherited characteristics exhibited by people. The explanation might be that each chromosome contained large numbers of different "factors," or "genes."

In 1907, Morgan decided to attack this problem using a new tool to science called the Drosophila melanogaster, or the common fruit fly. Fruit flies are the very tiny flies attracted by the smell of fruit. Leave a bowl of fruit out in the summer, and chances are these flies will somehow get in the house and swarm onto the peaches and plums, attracted by the odor. For Morgan, however, these flies proved perfect for genetic research since they were inexpensive, very easily bred in large numbers, multiplied rapidly, and best of all, their cells possessed only four chromosomes. Morgan would therefore tackle the problem of inheritance by closely following the generations of flies. It was in his famous "Fly room" with his undergraduate students Calvin Blackman Bridges (1889–1938), Alfred Henry Sturtevant (1891–1970), and Hermann Joseph Muller (1890–1967), all of whom went on to do major work in genetics, that Morgan discovered many instances of mutations. He was able to trace these in later generations and to prove that genes were linked in a series on chromosomes (or inherited together) and were responsible for identifiable, hereditary traits. Morgan was also the first to explain sex-linked inheritance when he located the mutant white-eye gene on the male sex chromosome (fruit flies should all have red eyes). He further explained the "mistake" phenomenon called crossing-over in which traits found on the same chromosome are not always inherited together. In this, Morgan showed that one chromosome actually exchanged material with (or crossed over to) another chromosome. This mistake proved to be an important source for genetic diversity since it can possibly add unpredictable variety.

By 1911, Morgan and his "Fly room" team had created the first chromosome maps for fruit flies. In 1915, Morgan and his students published a summary of their work, The Mechanism of Mendelian Heredity, which would lay the groundwork for all future genetically based research. Morgan's rigorous work with the humble fruit fly enabled him to discover how genes are transmitted through the action of chromosomes, thus confirming Mendel's laws of heredity and laying the foundation for modern experimental genetics. In 1933, Morgan was awarded the Nobel Prize in Physiology or Medicine for his work on heredity. His student, Hermann Muller, also experimented with fruit flies and proved that x rays can damage genetic material. For this, Muller received the 1946 Nobel Prize.

Genes have been compared to instructions or to a recipe for making proteins. Proteins can be found in virtually every part of the body, and they help cells do all the complicated things they have to do. There are somewhere between 50,000 and 80,000 genes in the human body, and each contains instructions on making a protein that has a specific purpose. There are genes for proteins that make our eyes, our organs, our hair, and our skin. There are genes that influence how tall we will be or what our skin color is. One of the main functions of chromosomes is to package the DNA that contains these genes that tell our cells what type of proteins to make. This protein-making is a nearly constant activity in the cell and can be considered a kind of biological housekeeping.

MITOSIS

Every day our bodies make billions of new cells that are identical to the ones they will replace. This is because every cell in our bodies has its own life cycle, and some, like skin cells, complete their full cycle in only twenty-four hours. In this steady production of identical or "sister" cells, called mitosis, each chromosome makes a copy of itself, moves to the opposite ends of the cell membrane, and splits into two identical cells after a membrane forms across the cell's middle. This process assures that each new cell gets the correct genetic material.

MEIOSIS

During sexual reproduction, however, an entirely different process called meiosis takes place that involves chromosomes. When a sperm fertilizes an egg, each sex cell (sperm and egg) starts with only twenty-three individual chromosomes, unlike all other cells in the body that have a full set of forty-six (or twenty-three pairs). When the sperm and egg join together, the first new cell created gets twenty-three chromosomes from the mother (egg cell) and twenty-three chromosomes from the father (sperm cell), to form a full complement of forty-six chromosomes. Meiosis also adds a final shuffling of genes that happens before division takes place. During this shuffle, chromosomes cross over each other and actually swap genes, thus further assuring that each sex cell has its own unique combination of genetic instructions. Unlike mitosis, the new cell (and eventually new organism) created is not identical to the cells that formed it but is rather a mixture of the chromosomes of two organisms. This is why in

organisms that reproduce sexually, an offspring does not look exactly like either parent since it inherited genes from both.

At the chromosome level, the difference between a male and a female is only one gene on one chromosome. Chromosomes are always in pairs, and those that determine the sex of an individual make up two of our forty-six chromosomes. These two are known as sex chromosomes. The other forty-four chromosomes are not involved in determining sex and are called autosomes. Females have a pair of sex chromosomes called XX, while males have a pair called XY. Thus the difference is only one (Y) chromosome. If a human embryo is given two X chromosomes, a certain area of cells becomes the egg-making part (the ovaries) and the embryo will develop into a female. If it has an X and a Y chromosome, the Y signals the cells to start producing sperm-making parts (testes), and the offspring develops into a male.

Nature, however, can and does make mistakes (usually during meiosis), and when they occur at the chromosome level, they can be disastrous. The most common mistake in humans is called aneuploidy. This occurs when an offspring has an extra or a missing chromosome. Most cases of aneuploidy result in the mother aborting her fetus spontaneously (called a miscarriage). This can be considered nature's way of putting an end to a mistake. One instance in which fetuses do develop and are born is that of Down's syndrome in which the offspring has an extra chromosome. However, these people suffer from mental retardation and some physical deformities.

[See alsoCell; DNA; Genetic Disorders; Genetic Engineering; Genetics; Mendelian Laws of Inheritance; Mutation; Nucleic Acid; Protein ]

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Chromosome

Chromosome

Chromosomes are microscopic units containing organized genetic information, located in the nuclei of diploid and haploid cells (e.g. human somatic and sex cells), and are also present in one-cell non-nucleated organisms (unicellular

microorganisms), like bacteria, which do not have an organized nucleus. The sum-total of genetic information contained in different chromosomes of a given individual or species are generically referred to as the genome.

In humans, chromosomes are structurally made of roughly equal amounts of proteins and DNA . Each chromosome contains a double-strand DNA molecule, arranged as a double helix, and tightly coiled and neatly packed by a family of proteins called histones. DNA strands are comprised of linked nucleotides. Each nucleotide has a sugar (deoxyribose), a nitrogenous base, plus one to three phosphate groups. Each nucleotide is linked to adjacent nucleotides in the same DNA strand by phosphodiester bonds. Phosphodiester is another sugar, made of sugar-phosphate. Nucleotides of one DNA strand link to their complementary nucleotide on the opposite DNA strand by hydrogen bonds, thus forming a pair of nucleotides, known as a base pair, or nucleotide base. Genes contain up to thousands of sequences of these base pairs. What distinguishes one gene from another is the sequence of nucleotides that code for the synthesis of a specific protein or portion of a protein. Some proteins are necessary for the structure of cells and tissues. Others, like enzymes, a class of active (catalyst) proteins, promote essential biochemical reactions, such as digestion, energy generation for cellular activity, or metabolism of toxic compounds. Some genes produce several slightly different versions of a given protein through a process of alternate transcription of bases pairs segments known as codons.

Amounts of autosomal chromosomes differ in cells of different species; but are usually the same in every cell of a given species. Sex determination cells (mature ovum and sperm) are an exception, where the number of chromosomes is halved. Chromosomes also differ in size. For instance, the smallest human chromosome, the sex chromosome Y, contains 50 million base pairs (bp), whereas the largest one, chromosome 1, contains 250 million base pairs. All three billion base pairs in the human genome are stored in 48 chromosomes. Human genetic information is therefore stored in 24 pairs of chromosomes (totaling 48), 24 inherited from the mother, and 24 from the father. Two of these chromosomes are sex chromosomes (chromosomes X and Y). The remaining 46 are autosomes, meaning that they are not sex chromosomes and are present in all somatic cells (i.e., any other body cell that is not a germinal cell for spermatozoa in males or an ovum in females). Sex chromosomes specify the offspring gender: normal females have two X chromosomes and normal males have one X and one Y chromosome.

Each set of 24 chromosomes constitutes one allele, containing gene copies inherited from one of the progenitors. The other allele is complementary or homologous, meaning that it contains copies of the same genes and on the same positions, but originated from the other parent. As an example, every normal child inherits one set of copies of gene BRCA1, located on chromosome 13, from the mother and another set of BRCA1 from the father, located on the other allelic chromosome 13. Allele is a Greek-derived word that means "one of a pair," or any one of a series of genes having the same locus (position) on homologous chromosomes.

The first chromosome observations were made under light microscopes, revealing rod-shaped structures in varied sizes and conformations, commonly J- or V-shaped in eukaryotic cells and ring-shaped in bacteria. Staining reveals a pattern of light and dark bands. Today those bands are known to correspond to regional variations in the amounts of the two nucleotide base pairs: adenine-thymine (A-T or T-A) in contrast with amounts of guanine-cytosine (G-C or C-G).

Genetic abnormalities and diseases occur when one of the following events happens: a) one chromosome copy is missing, b) extra copies of a chromosome are present, c) a chromosome breaks and its fragment is fused into another chromosome (insertion), d) a fragment is deleted, e) a gene is transferred from one chromosome to another (translocation), f) duplication of a chromosomal segment occurs, g) inversion of a chromosomal segment occurs. Down syndrome , for instance, is caused by the presence of a third copy of chromosome 21.

In non-dividing cells, it is not possible to distinguish morphological details of individual chromosomes, because they remain elongated and entangled to each other. However, when a cell is dividing, i.e., undergoing mitosis, chromosomes become highly condensed and each individual chromosome occupies a well-defined spatial location.

Mitotic chromosomes present a constricted region, to which the spindle fibers attach during cellular division. Such constricted region, known as centromere or primary constriction, may be located in three different positions in chromosomes. Centromeric position allows the classification of chromosomes in three groups: a) acrocentric: centromere lies very near one end; b) metacentric: centromere at the middle, dividing the chromosome in two equal parts or arms; and c) submetacentric: centromere near middle, but dividing chromosome in two unequal arms.

When a chromosome loses its centromere, it is known as acentric. As the centromere is essential for both division and retention of chromosome copies in the new cells, acentric chromosomes will not pass to the daughter cells during the parental cell division. Therefore, daughter cells will miss one chromosome in their karyotype . A karyotype map shows mitotic chromosomes in the mitotic phase, known as metaphase. In metaphase, chromosomes align in pairs. In a normal human karyotype, there are 22 pairs of autosomal chromosomes and two sex chromosomes (X and Y). Each pair of autosomal chromosomes contains two complementary or homologous chromosomes, a maternal and a paternal copy.

Some chromosomes also present a secondary constriction that always appears at the same site. They are also useful, along with centromere position and chromosome size, for identifying and characterizing individual chromosomes, in a karyotype.

Karyotype analysis was the first genetic screening utilized by geneticists to assess inherited abnormalities, like additional copies of a chromosome or a missing copy, as well as DNA content and gender of the individual. With the development of new molecular screening techniques and the growing number of identified individual genes, detection of other more subtle chromosomal mutations is now possible (e.g., determinations of gene mutations , levels of gene expression, etc.). Such data allow scientists to better understand disease causation and to develop new therapies and medicines for those diseases.

Sandra Galeotti, MS

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