Which of the following statements is not correct about the `DNA` molecules?

Options

A. The base composition of `DNA` varies from one species to another
B. In all `DNA` molecules, regardless of the species, the number of adenine residues is equal to the number of cytosine residues
C. In all `DNA` molecules, regardless of the species, the number of guanine residues is equal to the number of thymine residues
D. Both (2) and (3)

Answer

D

Explanation

Molecules of `DNA` isolated from different tissues of the same species have the same proportine, but molecules from different species can have quite different proportions.
`DNA` is made up of two polynucleotide chains wound about each other to form a double helix `20 Å` in diameter. Each helix is right-handed and has ten nucleotide units for each complete turn, which occurs every `34 Å` along the exist. The two chains heat in opposite directions, that is, the deoxyribose units are oriented in opposite ways, so that the sequence is `C - 3'`, `C - 5'` in one chain and `C - 5'`, `C -3'` in the other.
The chains are held together at intervals by hydrogen bonds These are linear hydrogen bons between adenine and thymine and between guanine and cytosine. Quite simply, `A = T` and `G = C` because A is always bonded to `T` and `G` is always boned to `C`. Hydrogen bonding between o her pairs of bases would not allow them to fit into the double helical structure. The two strands are thus not identical but complementary opposite every `A` of one chain is `T` in the other, and opposite every `G` is a `C`