Question

Length of one loop of B-DNA is

• A. 3.4 nm
• B. 0.34 nm
• C. 20 nm
• D. 10 nm

Answer 1

Answer: (A)

$B-DNA$ is an antiparallel double helix. The double strand or duplex is coiled plectonemically in right handed fashion around a common axis like a rope stair case twisted in a spiral. The coiling produces alternate major and minor grooves. One turn of $360^{\circ }$ of the spiral has about 10 nucleotides on each strand of $DNA$. It occupies a distance of about $3.4nm$ $(34\mathring{A})$ i.e., pitch of $DNA$ is $34\mathring{A}$ so that adjacent nucleotides or their bases are separated by a space $0.34nm(3.4\mathring{A})$.