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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." The Oxford Pocket Dictionary of Current English. . Encyclopedia.com. 21 Aug. 2017 <http://www.encyclopedia.com>.

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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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