Question

Which of the following agents causes mutation through deamination?
A) $2 - a\min opurine$
B) $5 - bromouracil$ and X rays
C) $HN{O_2}{\text{ and UV}}$
D) Alkyl sulfonates and ${N_2}$ mustards
E) $HN{O_2}$

Answer 1

A mutation is defined as a change in a DNA sequence. It can be due to DNA copying mistakes which are made during the process of cell division, exposure to ionizing radiation, exposure to chemicals which are called mutagens, or also by infection of viruses. Deamination as the name suggests is the elimination of an amino group from a molecule.

Complete answer:
Enzymes that catalyze deamination reactions are called deaminases. Mutagens are chemical compounds or forms of radiation that can cause irreversible and heritable changes in the cellular genetic material, which is DNA.

$2 - a\min opurine$ is a base analog that causes both transition and frameshift mutations in Escherichia coli. It can cause mutations by two mechanisms, either by directly mispairing with cytosine, or indirectly, by saturation of mismatch repair. Hence, it does not cause mutation through deamination.

$5 - bromouracil$ is a base analogue of thymine which can be incorporated into DNA and is a well-known mutagen, causing transition mutations by mispairing with guanine (G) rather than pairing with adenine (A) during replication. X-ray causes chromosomal aberrations by introducing chromosomal breaks which cause alteration of nitrogenous bases, or by initiating a chain of chemical reactions. Therefore, $5 - bromouracil$and X rays also do not cause mutation through deamination.

Nitrous acid, which is a potent chemical mutagen, exerts its effect on DNA by the deamination of the amino group of adenine, cytosine and guanine residues and also cross- links of undefined structure. Whereas UV absorption induces dimerization of thymine in DNA which disrupts the hydrogen bonding between the bases by forming bumps in the DNA and also on the complementary strands. Therefore, though nitrous acid causes mutation by deamination, UV cannot cause mutation through deamination.

Alkylating agents undergo metabolic activation, by cytochromes $P450$. Nitrogen mustards replace chloride by amine nitrogen and form aziridinium rings which in turn alkylate DNA. Therefore, they do not cause mutation by deamination.

As discussed earlier in option C, nitrous acid, which is a potent chemical mutagen, exerts its effect on DNA by the deamination of the amino group of adenine, cytosine and guanine residues and also cross- links of undefined structure.

Therefore, the correct answer is option ‘E’.

Note:
- There are a wide range of mutations. Two major categories of mutations are;
Germline mutations occur in gametes, they are significant because they can be transferred to the offspring and every cell will have the mutation.
Somatic mutations occur in other cells of the body and have little effect on the organism because they are confined to just one cell and its daughter cells.