Question

Length of DNA with 23 base pairs is
a) 78.4 A°
b) 78.2 A°
c) 78 A °
d) 4.8 A °

Answer 1

Hint: DNA is genetic material. In humans, it is dsDNA (double-stranded). It is of many types like A, B, C, D, and Z The commonest type is B DNA. It consists of 10 bp (base pairs) per turn and distance between two base pairs is 3.4 A °.

Complete answer:
DNA is the genetic material. The nucleotides are the major structural units of the DNA. Nucleotides are
Phosphate + Pentose Sugar + Nitrogen Base
The DNA in humans is double-stranded. Phosphate forms the backbone of the strands and nitrogen bases are in the centre. The Nitrogen bases of two strands are connected by hydrogen bonds. The Phosphodiester bond connects the two nucleotides on the same strand.
The nitrogen bases can be Adenine, Guanine, Thymine and Cytosine. Adenine and Guanine are Purines (as they have a double ring) and Thymine and Cytosine are Pyrimidines (as they have a single ring). Purines always pair with pyrimidines.

As mentioned, DNA is double-stranded and hence the two strands are coiled and thus the strands are in a helical form showing many turns. The different DNA types have a different number of base pairs per turn. A DNA has 11 bp per turn. B DNA has 10 bp per turn, C DNA has 9 bp, D DNA has 8 bp, Z DNA has 11 bp.

The Question has given the length of DNA. As no type is mentioned, it is considered as B DNA.
Thus B DNA has 10 bp per turn.
Distance between two base pairs = 3.4 A°
Total number of base pairs (in question) = 23
Hence to calculate distance we will consider 22 bp
So the total distance = 22 x 3.4 = 74.8 A°

Thus the answer is option ‘D’.

Note: The main point to note in such questions is the type of DNA and number of base pairs. Suppose if the number of base pairs is 'n' , then to calculate the distance, we will consider n-1 x 3.4
A, B, C,and D DNA's are right-handed helix while Z DNA is left-handed helix.