Question

What is the length of one spiral DNA?
A. 10 Aͦ
B. 20 Aͦ
C. 32 Aͦ
D. 34 Aͦ

Answer 1

Hint: DNA is the genetic hereditary material that carries genetic information down the race of a species. It is double-stranded and spirals in nature. DNA is of various types. Typically, 4 types of DNA are known- A type, B type, C type, Z type. The most common type which Watson and Crick studied is the B type. Thus, this question refers to the length of a typical spiral turn of B DNA. The B DNA is the type of DNA that is found in humans.

Complete answer:
 A typical comparison of the types of DNA is discussed below:

DNA	Base pairs per turn (n)	Direction of rotation of the helix	Vertical rise per base pair	Helical Diameter	Vertical distance between two strands
A	11	Right-handed	2.56 Aͦ	23 Aͦ	28.16 Aͦ
B	10	Right-handed	3.4 Aͦ	20 Aͦ	34 Aͦ
C	9.33	Right-handed	3.3 Aͦ	19 Aͦ	30.78 Aͦ
Z	12	Left-handed	3.8 Aͦ	18.4 Aͦ	45.6 Aͦ

Thus, we can see that a typical DNA (B-DNA) having a typical spiral length of 34 Aͦ.

Note:
One point to note down here is that in this question, the one spiral DNA categorically does not mean the length of a whole DNA molecule that is spiral in nature. Rather it means the length of each DNA segment that lies between the two turns.
The vertical distance between two turns can be easily be calculated by a simple formula:
Vertical distance = Vertical rise per base pair ✕ Base pairs per turn.