(Deoxyribonucleic Acid). 1. The molecular chain found in genes within the nucleus
of each cell, which carries the genetic information that enables cells to reproduce. 2. DNA
is the principal constituent of chromosomes, the structures that transmit hereditary characteristics. The amount of DNA
is constant for all typical cells of any given species of plant or animal (including humans), regardless of the size or function of that cell. Each DNA molecule
is a long, two-stranded chain made up of subunits, called nucleotides, containing a sugar (deoxyribose), a phosphate group and one of four nitrogenous bases: adenine (A), guanine (G), thymine (T) and cytosine (C). In 1953 J.D. Watson and F.H. Crick proposed that the strands, connected by hydrogen bonds between the bases, were coiled in a double helix. Adenine bonds only with thymine (A-T or T-A) and guanine only with cytosine (G-C or C-G). The complementarity of this bonding ensures that DNA
can be replicated (i.e., that identical copies can be made in order to transmit genetic information to the next generation).