| Some DNA details. Apart from
reproductive gametes, each cell of the human body
contains 23 pairs of chromosomes, each a packet of
compressed and entwined DNA. Every strand of the DNA is a
huge natural polymer of repeating nucleotide units, each
of which comprises a phosphate group, a sugar
(deoxyribose), and a base (either adenine, thymine,
cytosine, or guanine). Every strand thus embodies a code
of four characters (A's, T's, C's, and G's), the recipe
for the machinery of human life. In its normal state, DNA
takes the form of a highly regular double-stranded helix,
the strands of which are linked by hydrogen bonds between
adenine and thymine (AÐT) and between cytosine and
guanine (CÐG). Each such linkage is said to constitute a
base pair; some three billion base pairs constitute the
human genome. It is the specificity of these base-pair
linkages that underlies the mechanism of DNA replication
illustrated here. Each strand of the double helix serves
as a template for the synthesis of a new strand, the
nucleotide sequence of which is strictly determined.
Replication thus produces twin daughter helices, each an
exact replica of its sole parent. |
