Question

Diameter of DNA double helix is

A) 34 $\overset{o}{A}$

B) 20 $\overset{o}{A}$

C) 3.4 $\overset{o}{A}$

D) 340 $\overset{o}{A}$

Answer 1

Hint:  DNA consists of two right-handed helical chains or threads coiled through an axis to create a double helix. Bonds between bases bind the two strands together, adenine forming a base pair with thymine, and cytosine forming a base pair with guanine.

Complete step by step solution: To answer this question, at first we should know about DNA double helix. DNA has a double helix structure, with sugar and phosphate on the exterior of the helix, forming the DNA backbone of sugar-phosphates. The nitrogen bases are bound by hydrogen bonding between them.

Now lets us find the solution from the given options-

- The polymer’s general presentation indicates a periodicity of 3.4 $\overset{o}{A}$, corresponding to the distance between the two bases, and another of 34 $\overset{o}{A}$, equivalent to a helix turn. So, 34 $\overset{o}{A}$ is the incorrect answer. Thus, option A is an incorrect option.

- The B-DNA is formed in a double helix on the right; the helix is 20 angstroms broader and the number of base pairs per helical spin is 10.5. So, 20 $\overset{o}{A}$ is the correct answer. 

- 3.4 $\overset{o}{A}$ is the distance between base pairs. So, 3.4 $\overset{o}{A}$ cannot be the correct answer. Thus, option C is an incorrect option. 

- B-DNA has a diameter of 20 $\overset{o}{A}$. So 340 $\overset{o}{A}$ cannot be the correct answer. Thus, option D is an incorrect option. 

Thus, option B is the correct option.

Note: With respect to the helix axis, the base pair plane in B-DNA is bent around 6 degrees. One complete double helix turn is 3.4 nm long and each has 10.5 base pairs, the difference between two consecutive base pairs is 0.34 nm.