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.