Molecular biology and genetics: CpG mutation hotspots: practice question

A Biochemistry question in the style of the NEET PG paper. Answer it first, then read why the right option is right and why each of the others is wrong.

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Biochemistry · Molecular biology and genetics: CpG mutation hotspots

Sample item

Single-base C to T transitions occur far more often at CpG dinucleotides than elsewhere in human genes. Why?

Original practice item, not a recalled exam question. Nothing here is recorded.

The answer and the reasoning

A. Deamination of 5-methylcytosine gives thymine, which repair does not flag as damage

Cytosines in CpG are often methylated. Spontaneous deamination of 5-methylcytosine produces thymine, a normal DNA base that forms a G:T mismatch and is repaired inefficiently, so the change becomes permanent. Deamination of ordinary cytosine gives uracil, which uracil DNA glycosylase removes.

Option by option

  • A. Deamination of 5-methylcytosine gives thymine, which repair does not flag as damage (correct)

    Correct. Thymine from methylcytosine deamination is not recognised as foreign, so the mutation persists.

  • B. Deamination of cytosine gives uracil, which glycosylase cannot excise in CpG sites

    Uracil from ordinary cytosine is removed by uracil DNA glycosylase, so this is not the explanation.

  • C. DNA polymerase has lower fidelity when copying guanine located next to cytosine

    Polymerase fidelity is not specifically reduced at CpG sequences.

  • D. Guanine methylation in CpG pairs adenine with cytosine in the next round of replication

    Methylation occurs on cytosine in CpG, not on guanine.

Source note: Original NextPrep sample item — pending expert review. This item is original practice material awaiting independent expert review, so confirm important facts in a standard textbook.

More Biochemistry questions

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, Deamination of 5-methylcytosine gives thymine, which repair does not flag as damage. Cytosines in CpG are often methylated.

Why is "Deamination of cytosine gives uracil, which glycosylase cannot excise…" not the answer?

Uracil from ordinary cytosine is removed by uracil DNA glycosylase, so this is not the explanation.

Why is "DNA polymerase has lower fidelity when copying guanine located next t…" not the answer?

Polymerase fidelity is not specifically reduced at CpG sequences.

Why is "Guanine methylation in CpG pairs adenine with cytosine in the next ro…" not the answer?

Methylation occurs on cytosine in CpG, not on guanine.

What concept does this question test?

It tests Molecular biology and genetics: CpG mutation hotspots in Biochemistry. Spontaneous deamination of 5-methylcytosine produces thymine, a normal DNA base that forms a G:T mismatch and is repaired inefficiently, so the change becomes permanent. Deamination of ordinary cytosine gives uracil, which uracil DNA glycosylase removes.

How hard is this question?

We tag it as a multi-step question: it asks you to combine more than one idea. The tag is our own editorial judgement, not an official difficulty rating, so read the reason for every option, not only the answer.

How should I study Molecular biology and genetics: CpG mutation hotspots?

Build one table of enzyme deficiency, disease, inheritance and key finding, then revise it by spaced recall. Then use the flashcards and the other Biochemistry questions on this site to test what stuck.

Is this a real NEET PG question?

No. It is an original practice item (editorial status: sample, version 1). NextPrep does not publish recalled or leaked questions, and independent faculty review of this item has not been completed yet, so confirm important facts in a standard textbook.