Mutations and Their Effects Questions and Answers

Answer: D

The mutagen is EMS (ethyl methanesulfonate).

EMS is an alkylating agent. It adds an ethyl group to the O⁶ position of guanine, and O⁶-ethylguanine pairs with thymine instead of cytosine. After replication this gives a G:C to A:T transition, which is why EMS is widely used to induce point mutations.

5-Bromouracil is a base analogue of thymine, UV rays form pyrimidine dimers, and acridine orange intercalates between bases and causes frameshifts. None of them alkylates guanine.

Answer: D

X-rays induce mutations chiefly by chromosomal breakage.

X-rays are ionising radiation: they knock electrons from atoms, producing free radicals that break the sugar-phosphate backbone. Double-strand breaks lead to deletions, inversions, translocations and other chromosomal aberrations, the hallmark of radiation damage that H.J. Muller first exploited in 1927.

Transitions and transversions are base substitutions caused mainly by base analogues, deaminating and alkylating agents, and frameshifts by intercalating agents such as acridines. X-rays can cause point mutations too, but breakage is their characteristic effect.

Answer: C

The agent that is not a DNA depurinating agent is Dimethyl nitrosamine.

Alkylating agents such as ethyl ethanesulphonate, methyl ethanesulphonate (written 'ethane sulphonate' in the paper) and nitrosoguanidine react directly with DNA. They add alkyl groups to the N7 position of guanine and the N3 position of adenine; these alkylated purines weaken the glycosidic bond, and the base falls out, leaving an apurinic site.

Dimethylnitrosamine is different: it does not react with DNA as it is. It is a pro-mutagen that has to be activated by liver cytochrome P450 enzymes before it can methylate anything, which is why the key singles it out from the three direct-acting depurinating agents.

Answer: B

Deamination of adenine gives Hypoxanthine.

Removing the amino group at C6 of adenine leaves a keto group there, producing hypoxanthine (whose nucleoside is inosine). Hypoxanthine pairs with cytosine rather than thymine, so if it is not removed, an A-T pair becomes a G-C pair after replication: deamination is a source of transition mutations.

The other products come from different bases. Deamination of cytosine gives uracil, and deamination of guanine gives xanthine. Cytosine is not formed by deaminating any base; it is itself a base that can be deaminated.

Answer: C

A single amino acid change is most likely from ethyl methane sulphonate.

EMS is an alkylating agent: it ethylates guanine to O⁶-ethylguanine, which pairs with thymine, producing G:C to A:T transitions. A base substitution in a coding sequence typically changes one codon and hence one amino acid (a missense mutation).

Ethidium bromide, acriflavine and acridine orange are intercalating agents. They slip between base pairs and cause insertions or deletions of a base, which shift the reading frame and alter every amino acid downstream rather than just one.

Answer: B

The most likely consequence is disruption of protein folding.

A globular protein folds so that its hydrophobic side chains are buried in the core, away from water. This hydrophobic effect is one of the main forces holding the folded shape together. A hydrophilic or charged residue placed in the core cannot be shielded from water and cannot pack against its neighbours, so the core is destabilised and the protein tends to misfold or unfold.

'No change' ignores the hydrophobic effect. The protein becomes less stable, not more. Stronger ionic bonds are unlikely, because a lone polar group buried in a nonpolar core has no partner to bond with.