Question

List and describe the three forms of DNA.

Please can you type the answer. I don't understand the writing hand. Sometime.

Thanks

Answer 1

Mainly there are three forms of DNA:- A-DNA, B-DNA & Z-DNA.

DNA exist in four major interconvertible forms, A, B, C and D. These are distinguished on the basis of sugar puckering and the number of base pairs turn. A- DNA has 3' endopuckering, B & D types have 3'- exopuckering and C-DNA has 2' endopuckering.

B-DNA

Typical or classical type. Proposed by Watson & Crick. It consist of antiparallel, right handed double helix of sugar phosphate backbone with purine-pyrimidine basepairs held at right angles to the axis of double helix. In case of the tilt of a base pair lambda to the normal of the helix is about 6.3°. One turn of the helix consist of 10.5 basepairs(10.5 fold helix). The rise of the helix per base pair (h) is 3.37A°, and the pitch of the helix ( rotation per base pair) is 34A°.

A,C & D-DNAs

A-DNA has right handed double helix. But it is wider and less flexible than the B-DNA . Also it incorporates more base pairs per helical turn and has 11-fold helix. In it lambda(tilt of base pair)= 7.8°, h= 2.56A°, and the pitch of helix is 28.15A°.

C-DNA has 9 1/3fold helix. In it lambda=7.8°, h=3.32A°, and the pitch of helix is 31A°. D-DNA has8 fold helix. In it, lambda =16.7° and h=3.0A°.

Z-DNA

Characterised by a left handed, zig zag double helix and only minor grooves. The name denotes that the sugar phosohate backbone presents a zig zag conformation. It was discovered by Alexander Rich and his team in 1979. It may look like B-DNA. But it rotates at 180°, producing a zig zag left handed structure. The non rotated configuration of the base is called anti- configuration, and the rotated configuration is called syn configuration. Specific segments of a DNA molecule can undergo conformational changes from B-form to Z- form or vice versa with the help of specific regulator proteins.

Z-DNA is not a simply mirror image of the right handed B-DNA. At the same time, it appears to be almost inverse to the A-DNA in respect to the grooves. In it the minor groove is enormously large, and the major groove may be flat, convex, or altogether absent. Another unique feature is it composed of repeating C-G sequences on the companion strand.

differs from B-DNA

1. It is left handed, zig zag double helix rather than right handed.

2. In Z- DNA, there are 12 basepairs in each turn if the helix, instead of the 10.5 pairs in B-DNA.

3. The pitch of the helix is 45A° in Z- DNA in pkace of the 34A° in B-DNA.

4. The axial rise of base pairs is 3.7A° in Z-DNA, and 3.4A° in B-DNA

5. There is only single groove in the helix, in place of the major and minor grooves of B-DNA.