DNA’s B Form, A Form and Z Form

In a DNA molecule, the two strands are not parallel, but intertwined with eachother. Each strand looks like a helix. The two strands form a "doublehelix" structure, whichwas first discovered by James D. Watson and Francis Crick in 1953.In this structure, alsoknown as the B form, the helix makes a turn every 3.4 nm, and the distance between twoneighboring base pairs is 0.34 nm. Hence, there are about 10 pairs per turn.The intertwined strands make two grooves of different widths, referred to as the majorgroove and the minor groove, which may facilitate binding withspecific proteins.

Figure 3-B-3. The normal right-handed "doublehelix" structure of DNA, also known as the B form.

In a solution with higher salt concentrations or with alcohol added, the DNA structuremay change to an A form, which is still right-handed, but every 2.3 nm makesa turn and there are 11 base pairs per turn.

Another DNA structure is called the Zform, because its bases seem to zigzag. Z DNA is left-handed. One turn spans 4.6 nm, comprising 12base pairs. The DNA molecule with alternating G-C sequences in alcohol orhigh salt solution tends to have such structure.

Figure 3-B-4. Comparison between B formand Z form.

Follow this link:
DNA's B Form, A Form and Z Form