Question

Variable part of a DNA molecule is

• A. Nitrogen base
• B. Phosphate
• C. Both $\left(b\right)$ and $\left(c\right)$
• D. Sugar

Answer 1

The correct option is A Nitrogen base

Structure of DNA
DNA is a double stranded helical molecule in which each strand is composed of repeating units of deoxyribonucleotides.
Each deoxyribonucleotide is composed of a deoxyribose sugar with a phosphate attached to its $5^{\prime }$ carbon and a nitrogenous base attached to its $1^{\prime }$ carbon.
The nitrogenous base in a nucleotide of DNA can be either adenine, guanine, cytosine or thymine.
Thus the only variable component in the structure of a DNA is the nitrogenous base.

The adjacent nucleotides are connected by phosphodiester bonds between the $3^{\prime }$ hydroxyl group of one nucleotide and the $5^{\prime }$ phosphate group of the next one.