which are responsible for the peculiar kind of pairing which takes place
between strands. Each base in one strand faces a complementary base in the
other strand, and binds to it. The complements are as in Typogenetics: A
pairs up with T, and C with G. Always one purine pairs up with a
pyrimidine.
Compared to the strong covalent bonds along the backbone, the in-
terstrand bonds are quite weak. They are not covalent bonds, but hydrogen
bonds. A hydrogen bond arises when two molecular complexes are aligned
in such a way that a hydrogen atom which originally belonged to one of
them becomes "confused" about which one it belongs to, and it hovers
between the two complexes, vacillating as to which one to join. Because the
two halves of double-stranded DNA are held together only by hydrogen
bonds, they may come apart or be put together relatively easily; and this
fact is of great import for the workings of the cell.
When DNA forms double strands, the two strands curl around each
other like twisting vines (Fig. 93). There are exactly ten nucleotide pairs
per revolution; in other words, at each nucleotide, the "twist" is 36 degrees.
Single-stranded DNA does not exhibit this kind of coiling, for it is a
consequence of the base-pairing.
516FIGURE 93. Molecular model of the
DNA double helix. [From Vernon M. Ingram,
Biosynthesis (Menlo Park, Calif.: W. A. Ben-
jamin, 1972), p. 13.]Self-Ref and Self-Rep