ABSTRACT
Deoxyribonucleic acid (DNA) consists of four kinds of deoxyribonucleotides linked together in a specific sequence. DNA is usually double-stranded. Of the four kinds of ribonucleoside bases, there are two subsets of hydrogen-bonded pairings, adenine with thymine and guanine with cytosine. RNA (ribonucleic acid) is also composed of four types of bases, but with uracil substituted for thymine. Each nucleoside base is joined to the carbon-1 of a pentose sugar (deoxyribose in DNA, ribose in RNA). A phosphate molecule is joined through an ester linkage to the carbon-5 and the resulting molecule is known as a nucleotide. The nucleotides are joined by linking a hydroxyl group on the carbon-3 of a pentose to the phosphate group on the carbon-5 of another pentose to form a phosphodiester bridge. The pentose carbons are primed (" ' ") in order to distinguish them from carbons in the bases; thus the two ends of single strand DNA are known as the 5'-phosphate end and the 3'-hydroxyl end. DNA from different species have characteristic relative amounts of the four nucleotides and this property has served to help identify unknown species of bacteria and establish evolutionary relationships among the species (Figure 9, Figure 10).
