One turn of the helix in a B-form DNA is approximately:
One turn of the helix in a B-form DNA is approximately:
$2 \mathrm{~nm}$
$20 \mathrm{~nm}$
$0.34 \mathrm{~nm}$
$3.4 \mathrm{~nm}$
Solution
B-DNA is a helical structure with a diameter of $20 Å$ and the distance between the two base pairs is $3.4 Å$. There are 10 base pairs in each turn; hence, one turn of the helix (pitch of each helix) is approximately $34 Å$ or $3.4 \mathrm{~nm}(10=1.0 \mathrm{~nm})$.
Related Theory
There are three different DNA types:
(i) A-DNA: It is a right-handed double helix similar to the B-DNA form. Dehydrated DNA takes an A form that protects the DNA during extreme condition such as desiccation.
(ii) B-DNA: The most common DNA conformation which is a right-handed helix, normal physiological conditions.
(iii) Z-DNA: Z-DNA is a left-handed DNA where the double helix winds to the left in a zig-zag pattern and play some role in gene regulation.