Adenine and guanine are double-ring structures termed purines,
and thymine and cytosine are single-ring structures termed
pyrimidines (Fig. 2). Within the overall composition of DNA,
the concentration of thymine is always equal to the concentration
of adenine, and the concentration of cytosine is always
equal to guanine (22,23). Thus, the total concentration of pyrimidines
always equals the total concentration of purines. These
monomeric units are linked together into the polymeric structure
by 3′, 5′-phosphodiester bonds (Fig. 3). Natural DNAs display
widely varying sizes depending on the source. Relative molecular
weights range from 1.6 × 106 Daltons for bacteriophage
DNA to 1 × 1011 Daltons for a human chromosome.