Question

A certain type of mutation converts the base cytosine into uracil. If this mutation is not repaired when the DNA replicates, as shown in the diagram, which of the following changes is most likely to result?


The uracil will be replaced with cytosine in the replicated DNA because uracil is in RNA and not DNA.

Option A,The uracil will be replaced with cytosine in the replicated DNA because uracil is in RNA and not DNA.

A

Uracil will base pair with adenine, resulting in a thymine-to-adenine base pair in the replicated DNA.

Option B,Uracil will base pair with adenine, resulting in a thymine-to-adenine base pair in the replicated DNA.

B

The DNA will become single stranded because uracil cannot base pair with adenine or guanine.

Option C,The DNA will become single stranded because uracil cannot base pair with adenine or guanine.

C

The DNA will no longer encode a protein because uracil is not recognized by ribosomes during protein synthesis.

Answer 1

Answer: B.

Explanation: Deamination of cytosine yields uracil, which (if unrepaired) pairs with adenine during replication. That converts an original C–G pair into a U–A pair in the first round and ultimately into a T–A pair in daughter DNA — a C:G → T:A transition.

Notes: A is wrong because if unrepaired the U is not automatically replaced with C (only repair enzymes would do that). C is wrong because uracil does base-pair with adenine. D is wrong because ribosomes read RNA (where U is normal); a U in DNA causes a base-change mutation, not a direct failure of translation.