STRUCTURAL CONSIDERATIONS.
June 1st, 2008 | by admin |The basic repeating units of the DNA polymer are nucleotides (Figure ). Nucleotides consist of an invariant portion, a five-carbon deoxyribose sugar with a phosphate group, and a variable portion, the “base.” Of
the four bases that appear in the nucleotides of DNA, two are purines, adenine (A) and guanine (G), and two are pyrimidines, cytosine (C) and thymine (T).
Nucleotides are connected to each other in the polymer through their phosphate groups, leaving the bases free to interactwith each other through hydrogen bonding. This “base pairing” is specific, so that A interacts with T, and C interacts with G. DNA is ordinarily double-stranded, that is, two linear polymers of DNA are aligned so that the bases of the two strands face each other. Base pairing makes this alignment specific so that one DNA strand is a perfectly complementary copy of the other.
In every strand of a DNA polymer, the phosphate substitutions alternate between the 5 ′ and 3′ carbons of the deoxyribose molecules. Thus, there is a directionality to DNA: the genetic code reads in the 5 ′ to 3´ direction. In double-stranded DNA, the strand that carries the translatable code in the 5 ′ to 3′ direction is called the “sense” strand, while its complementary partner is the “antisense” strand.
