DNA DNA, deoxyribonucleic acid, is the genetic substance of all living cells and many viruses. Hereditary information transmitted from each generation to the next is encoded in the structure of DNA molecules. These make long chains that are composed of four types of subunits. The sequence of these subunits makes up the genetic code. DNA was discovered by the Swiss biochemist Friedrich Miescher, in Its structure was established in 1953, largely through the combined efforts of the American biochemist James D. Watson, the British molecular biologist Francis Crick, the British biophysicist Maurice Wilkins, and the British physical chemist Rosalind Franklin.

STRUCTURE Each DNA subunit, or nucleotide, is composed of a sugar (deoxyribose), a phosphate group, and a base, with the phosphate linking each deoxyribose component to the deoxyribose of the next subunit. The four DNA bases are adenine, guanine, cytosine, and thymine. A DNA molecule usually exists as two polynucleotide chains. The chains intertwine to form a double helix. The nucleotides in the two chains are matched in such a way that adenine in one chain is always paired with thymine in the other; and guanine in one chain is always paired with cytosine in the other. Thus the two chains have a complementary relationship. As a result, the nucleotide sequence of one chain specifies the sequence of the other.

STRUCTURE A virus particle contains only one DNA molecule. Depending on the virus, the DNA molecule ranges in length from 5,000 to over 200,000 subunits. In contrast, a human cell contains more than 3 billion pairs of linked nucleotide bases. Over 99 percent of human DNA is found in chromosomes (structures composed of DNA and protein) in the cell nucleus. However, a small but distinct fraction of DNA is found in mitochondria. Mitochondria are membrane- bound organelles in a cell's cytoplasm that provide energy to the cell. In plant cells, a small amount of DNA is found in chloroplasts (chlorophyll-containing organelles).

FUNCTION The hereditary instructions contained in DNA determine the sequence of amino acids making up proteins. The location of each amino acid is dictated by a specific sequence of three nucleotides. Protein molecules, in turn, comprise the structural elements and enzymes of the cell. All forms of life employ the same three-nucleotide code for a particular amino acid. A gene consists of the entire sequence of nucleotides making up a protein.

FUNCTION Elaborate machinery exists within the cell to synthesize protein according to DNA instructions and to exert a variety of specific controls on the process. This machinery involves RNA (ribonucleic acid), which is synthesized on the DNA template.

FUNCTION Another portion of the total DNA blueprint is directly related to the process of control and specification of the beginning and ending points of message sequences for particular proteins. Although all cells of a higher organism contain the complete DNA complement, in any specialized cell only a small percentage of the messages are operating at any time. Proteins called histones also appear to play an important role in regulating genetic activity. A histone provides a structural core for the long DNA strands to wind around, making compact coils of the lengthy DNA molecules. Evidently histones serve as "on-off" switches for genes on the DNA molecule as well, thereby determining which message sequences are functional. In addition to gene regulation, histones are involved in DNA repair and in the process of mitosis.