Self and the Beginning of Life 21“9x6” b2726 Self and the Phenomenon of Life: A Biologist Examines Life from Molecules to HumanityFig. 3.2. Structure of DNA. (A) The nucleobases in DNA consist of adenine (A),
guanine (G), cytosine (C), and thymine (T). The base pairing between two strands of
DNA is A-T and G-C. (The bases in RNA differ by the fact that thymine is replaced by
uracil, which forms A-U pairing.) (B) A nucleotide is composed of a nucleobase (rectan-
gle) combined with a 5-carbon sugar (pentagon) which in turn is joined to a phosphate
group (circle). In DNA, the sugar is deoxyribose, whereas in RNA it is ribose. A nucle-
otide minus the phosphate group is called nucleoside. (C) Two complementary chains
of DNA joined by the hydrogen bonds of A-T and G-C pairing. Each individual chain
is formed by alternating the sugar (S) and phosphate (P) moieties. The right hand side
shows a 3-dimensional structure of DNA, with the two strands coiled up to form a dou-
ble helix, in which the complementary nucleobases form the rungs of a twisted ladder,
and the phosphate-sugar backbones form the two side-rails. (In contrast, RNA chains
are flexible and do not form a double helix.) (D) Drawing showing how DNA replicates
by unwinding the double helix, followed by the building of two new complementary
strands. [US Natl. Lib. Med.]