Chromosome

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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.