Hydrogen Bonds and Stacking Forces Stabilize the Double Helix

Hydrogen Bonds and Stacking Forces Stabilize the Double Helix 

The most common form of the double-helix DNA. The base-paired structure shown in figure 25.5 forms the helix structure shown in (a) and (b) by a right-handed twist. The two strands are antiparallel as indicated by the curved arrows in (a). (Reprinted with permission from Nature (171 737, 1953) Copyright 1953 Macmillan Magazines Limited.) In (b) a space-filling model depicts the sugar-phosphate backbones as 

Different conformations of base-paired DNA (a) the untwisted straight ladder, (b) the normal spiral ladder. The stepladder structure is unstable it can be converted into a spiral ladder by a right-handed twist, a change that permits the planes of the base pairs to come into close contact. 

8 A between adjacent nucleotides in the direction of the long axis. This 6.8-A distance between adjacent base pairs produces a gap that would presumably be filled by water. Such a conformation is unstable because the planes of the bases prefer close contacts with one another as does water. The stepladder structure is related to the helix structure by a simple right-handed (clockwise) twist (see fig. 25.1a). Following this operation the distance between base pairs decreases until they are in close contact, with a spacing of 3.4 A. 

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