Chromosome. I INTRODUCTION Chromosome, microscopic structure within cells that carries the molecule deoxyribonucleic acid (DNA)--the hereditary material that influences the development and characteristics of each organism. In bacteria and bacteria-like organisms called archaebacteria, chromosomes consist of simple circles of DNA floating freely in the organism. In all other life forms, collectively called eukaryotes, chromosomes reside within a well-defined nucleus. In eukaryotes, chromosomes are highly complex structures in which the shape of the DNA molecules is linear, rather than circular. II CHROMOSOME STRUCTURE Chromosomes consist chiefly of proteins and DNA. Tiny chemical subunits called nucleotide bases form the structure of DNA. A sequence of bases along a DNA strand that codes for the production of a protein is known as a gene (see Genetics). Genes occupy precise locations on the chromosome. Each cell contains enough DNA to form a thread extending about 2 m (about 7 ft). Proteins called histones play a key role in packaging DNA within chromosomes. Sections of the DNA molecule wind around clusters of histones to form units called nucleosomes, which resemble spools encircled with thread. Another type of protein, called nonhistone chromosomal protein, further compresses nucleosomes into a compact, narrow coil. Chromosomes become most condensed when a cell is preparing to divide. The chromosome structure ensures that even when the DNA is highly confined, it is free to carry out transcription, or the production of messenger ribonucleic acid (mRNA). Messenger ribonucleic acid is the molecule that carries the DNA instructions that determine the types of proteins a cell will reproduce to the sites where proteins are constructed. In addition, chromosomes permit DNA to replicate, or reproduce itself, so that as a cell divides to produce two cells, each of these new cells will contain all of the necessary genetic information. Scientists are learning how DNA loosens its connection with histones in order to replicate itself and participate in the synthesis of mRNA. Evidence suggests that enzymes interact with the tails of histones, which protrude from the nucleosomes. These interactions may temporarily disrupt the nucleosome structure so that the DNA is free to interact with the enzymes that help to generate either mRNA or new copies of DNA. III CENTROMERES AND TELOMERES The chromosomes of nearly all eukaryotic life forms contain two important structures: centromeres and telomeres. During cell division, the centromere--visible through a microscope as a knotlike structure--connects to an apparatus called the spindle. The spindle contains fibers that move the centromeres around, causing the rest of each chromosome to follow. This process ensures that each chromosome moves to its proper place during mitosis, when a cell divides to give rise to two cells, and during meiosis, the process of cell division that gives rise to eggs or sperm. Telomeres are specialized sequences of DNA that are found at the tips of chromosomes. Telomeres serve as a kind of cap that prevents the ends of chromosomes from attaching to the ends of other chromosomes. Scientists suspect that telomeres may influence the activity of nearby genes and may play a role in determining the life span of a cell. IV CHROMOSOME NUMBER In the cells of most organisms that reproduce sexually, chromosomes occur in pairs: One chromosome is inherited from the female parent, and one is inherited from the male parent. The two chromosomes of each pair contain genes that correspond to the same inherited characteristics. Each pair of chromosomes is different from every other pair of chromosomes in the same cell. The number of chromosome pairs in an organism varies depending on the species. The number of chromosomes characteristic of a particular organism is known as the diploid number. Dogs, for example, have 39 pairs of chromosomes and a diploid number of 78, while tomato plants have 12 pairs of chromosomes and a diploid number of 24. Sex cells (eggs or sperm) contain only half the number of chromosomes found in the other cells of an organism. This reduced number of chromosomes in the sex cells is known as the haploid number. During fertilization, an egg and sperm unite to form a cell known as a zygote, the first cell of the offspring. The zygote contains the diploid number of chromosomes characteristic of the species. Most organisms have complete sets of matching chromosomal pairs, known as autosomes. In mammals, birds, and some other organisms, one pair of chromosomes is not identical. Known as the sex chromosomes, this pair plays a dominant role in determining the sex of an organism. Females have two copies of the X chromosome, while males have one Y chromosome and one X chromosome. Both males and females inherit one sex chromosome from the mother (always an X chromosome) and one sex chromosome from the father (an X in female offspring and a Y in male offspring). The presence of the Y chromosome determines that a zygote will develop into a male. The Y chromosome is about one-third the size of the X chromosome and contains only a fraction of the number of genes. At one point in evolutionary history, the X and Y chromosomes were equal in size and gene number, but the two chromosomes gradually diverged over the course of 300 million years. These unmatched sex chromosomes produce a pattern of gene inheritance known as sex-linked inheritance, which differs from genes found on autosomes. In males, which carry an X and a Y chromosome, some genes found on the X chromosome may be missing on the Y chromosome. As a result, the organism will usually develop the trait associated with the gene on the X chromosome. In fruit flies, for instance, the gene for eye color is located on the X chromosome. A male fruit fly will inherit the eye color found on the X chromosome, since no gene for eye color is found on the Y chromosome. V HUMAN CHROMOSOMES Humans have 23 pairs of chromosomes, with a diploid number of 46. Scientists number these chromosome pairs according to their size--the largest is chromosome 1 and the smallest is chromosome 23. In human chromosomes, errors may occur that give rise to embryos with more or less genetic material, sometimes resulting in developmental disabilities or health problems (see Genetic Disorders). In a process called nondisjunction, paired members of chromosomes fail to separate from one another during meiosis. Nondisjunction can lead to a condition known as Down syndrome, in which a person inherits three copies of chromosome 21. Another condition that may result from nondisjunction is Turner syndrome, a disorder in which a female inherits only a single X chromosome. Genetic errors occur if part of a chromosome is either missing or duplicated. Chromosomes sometimes undergo changes called translocations, in which part of one chromosome breaks off and attaches to another chromosome. A translocation involving chromosomes 9 and 22 is linked to a type of leukemia called chronic myelogenic leukemia. On the sex chromosomes, problems arise in men when an abnormal gene is present on the X chromosome. With no healthy gene found on the Y chromosome to override the abnormal gene, disease may result. For example, men who inherit a mutated gene that causes hemophilia from their mother on the X chromosome will develop this bleeding disorder since they are missing a normal version of the gene on their Y chromosome. Scientists called cytogeneticists look at a person's chromosomes in the laboratory to determine whether the individual has the usual number of chromosomes and whether these chromosomes have missing or extra segments. To examine chromosomes, cytogeneticists grow samples of a person's blood cells in the laboratory and expose the cells to a chemical called colchicine, which disrupts the spindle apparatus that is normally present in dividing cells. This disruption immobilizes the chromosomes during cell division, when they are most condensed and visible. Chromosomes are then stained with various dyes, which produce a pattern of vertical bands. Cytogeneticists take photographs of the banded chromosomes through a microscope to create images called karyotypes, in which the members of each chromosome pair are arranged next to each other for easy comparison. The analysis of karyotypes reveals whether a person has extra or missing chromosomes, as well as whether large segments of chromosomes are absent, rearranged, or duplicated. Experiments involving artificial chromosomes--chromosomes that are synthesized in the laboratory--are providing new insights into the structure and function of chromosomes. The first artificial chromosomes, produced in the 1980s, were chromosomes of yeast cells. The first artificial human chromosomes were created in 1997. Researchers have successfully identified all the genes located on chromosomes 5, 16, 19, 21, and 22. This research has revealed a number of disease-causing genes associated with these chromosomes. For instance, genes found on chromosome 5 have been linked to colorectal cancer, basal cell carcinoma (a form of skin cancer), and a type of dwarfism. Chromosome 16 contains genes implicated in adult polycystic renal disease, which affects 5 million people worldwide. Identifying diseasecausing genes and their chromosome locations will help researchers devise new diagnostic tools to determine a person's risk for disease as well as new therapies to replace or repair faulty genes. Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.
