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.

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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.

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