H I

ch2oh

h oh

The P = O groups, on the other hand, are inclined an angle of —55° to the axis. X-ray diagrams indicated that the planes of the bases were separated by a distance of 3.4 A. Impressive advances in knowledge have come from the recognition by Watson and Crick (14,15) of the specific base pairing in DNA. Helical DNA has a pitch of 34 A with 10 repeat units per turn, and its fundamental structure is not one helix but two right-handed helices of radius about 8.5 A, spaced about one-half the pitch apart. The- pairs of purines and pyrimidines that occur in equal amounts in DNA, namely, thymine-adenine and cytosine-guanine, have complementary structures that easily form hydrogen bonds. Watson and Crick found that the bases would be in a proper position for strong hydrogen bonds if the two helical chains had sequences of atoms in opposite directions. The helical chains are the phosphate-sugar backbones. The bases extend toward the center of the helix and hydrogen bond across the axis of the double helix. Thus the DNA spirals are held together by interchain bonding, whereas protein helices keep their configuration by means of intrachain hydrogen bonds.

The double spiral for the structure of DNA has very practical properties concerned with information storage and the duplication of genetic material.

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