This long molecule resembles a twisting rope ladder, the legs of which are identical groups of sugars and phosphates. The rungs, however, are composed of four kinds of bases. In genes, regions that code for proteins, the bases exist in any of 64 different groupings of three. Over the years scientists have learned that a source of nuance in the meaning of this genetic language lies in structural variations that arise from differences in the sequence not of codons but of the bases themselves. Differences in base sequence encourage certain stretches of the double helix to fold back upon itself as if making a U-turn, while other stretches are straight as rods.
Some stretches twist tightly, like an overwound rubber band, whereas others curve gently.
These twists and bends often influence whether a gene sits dormant or turns on to crank out thousands of copies of a protein. Today genetic engineers can design DNA with any base sequence, but they still cannot predict how it will twist or bend. Last updated May. 18, 1999. All rights reserved.