Conformational analysis of DNA shows that the origin of the B-form double helix can be substantially attributed to the atomic charge pattern in the base pairs. The pattern favors specific helical stacking of the various base pairs. The base pairs alone, without the sugar-phosphate backbone have a strong tendency to be helical. The backbone appears to play a relatively passive role in determining the helix form of DNA. Both electrostatic and van der WaaLs interactions play a role, but electrostatic interactions are particularly important in A-form DNA. Sequence studies of bases flanking homooligomer tracts indicate that there are strong biases: usually preferred are bases complementary or identical to the run. A or T runs are preferentially flanked by A's or T's whereas C or C runs are preferentially flanked by G's or C's.
