Question

The diameter of the DNA molecule is
A. $10 Å$
B. $100 Å$
C. $200 Å$
D. $20 Å$

Answer 1

Hint: DNA is a hereditary molecule that is made up of a sugar backbone, a nitrogenous base, and phosphate groups. This genetic material is present in the nucleus of every cell. It carries information from parent to offspring from generation to generation.

Complete answer:
DNA is made up of two antiparallel sugar-phosphate chains. The sugar is deoxyribose. One chain of the DNA starts from 3’ and ends in 5’. Another chain starts with 5’ and ends with 3’ – thus, it is called antiparallel. The DNA has two types of nitrogen bases – purine and pyrimidine. The purine base is made up of Adenine (A) and Guanine (G). The pyrimidine base is made up of Thymine (T) and Cytosine (C).
One purine attaches with one pyrimidine by a hydrogen bond. For example - A binds with T by two hydrogen bonds and C binds with G by three hydrogen bonds. A phosphate group is attached to sugars by a phosphodiester bond. The nucleotide unit is made up of deoxyribose sugar, a phosphate group, and a nitrogenous base. The diameter of the DNA is 20 Å and the length per turn is about 34 Å. Each turn contains 10 base pairs. Thus, the length of 10 base pairs is 34 Å.
So, the correct answer is option D, i.e., 20 Å.

Note: One interesting thing that should be noticed in the structure of DNA is that the chain of sugar backbone is named as 3’ to 5’. The 3 prime and 5 prime namings is done on the basis of carbon atoms present on the sugar molecule and the nitrogenous bases. The carbon atoms of the nitrogenous base are named as 1, 2, 3, and so on; and the naming of carbon atoms of the sugar molecule is done as 1’, 2’, 3’, and so on.