Page 30 - Academic Press Encyclopedia of Physical Science and Technology 3rd Polymer

P1: FPP 2nd Revised Pages
Encyclopedia of Physical Science and Technology EN002C-64 May 19, 2001 20:39

234 Biopolymers

FIGURE 18 (a) Formation of a dinucleotide. (b) The chain structure of a polynucleotide or nucleic acid. X = H in DNA
and OH in RNA; Y is often a triphosphate group.

the helix are held together by hydrophobic associations so the base pair must always consist of one double-ring
between the stacked bases and also by specific hydrogen base, a purine (A or G), hydrogen bonded to a single-ring
bonding between pairs of bases, where one base of a pair base, a pyrimidine (T or C). This base-pairing is specific;
is contributed by each chain. In an unstrained structure adenine on one chain always bonds to thymine on another,
there are 10 base pairs per complete turn of the helix. while guanine on one chain bonds to cytosine on the other.
The two chains of the helix are always approximately the The result is that the sequence of bases on one chain deter-
˚
same distance apart (the helix has a diameter of 20 A) and mines the sequence on the second chain, if base-pairing is