Question

One turn of the helix in a B-form DNA is
approximately
(a) 3.4 nm (b) 2 nm
(c) 20 nm (d) 0.34 nm

Answer 1

0.34 nm.... is the correct answer

Answer 2

(d) 0.34 nm

Explanation:

DNA-duplex model given by Watson and Crick is right handed spiral and is called B-DNA (Balanced DNA ) .There are diferent forms of DNA-duplex model was found later such as A-DNA , C-DNA ,D-DNA and Z-DNA etc . In DNA-duplex model all forms are right handed except Z-DNA . Every form of DNA-duplex model has diferent Pitch of helix per turn , Diameter of helix , Base pairs per turn of helix and distance between two base pairs .While  B-DNA duplex model is right handed  ,stable and repeating unit is mononucleotide . One turn of helix (Pitch of helix per turn) of B-DNA duplex model  is 0.34nm  and each turn of B-DNA duplex model contain 10 base pairs.