One turn of the helix in a B-form DNA is approximately 
(a) 0.34 nm
(b) 3.4 nm
(c) 2 nm
(d) 20 nm
(b) B-DNA is helical structure with 20Å and the distance between the two base pairs is 3.4Å and there are 10 base pairs in each turn or pitch (one round). Hence, one turn of the helix is approximately 34Å or 3.4 nm (10Å = 1.0 nm). Z-DNA (in comparison to B-DNA) is left handed double helical structure in which double helix winds to left in zig-zag pattern (instead of right, like B-DNA).