DNA repair: 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 · DNA repair

Sample item

Spontaneous deamination of cytosine produces uracil in DNA. Which enzyme initiates the repair of this lesion?

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

The answer and the reasoning

C. Uracil DNA glycosylase

Uracil DNA glycosylase starts base excision repair by removing the uracil base, leaving an abasic site. An endonuclease then cuts the backbone, DNA polymerase beta fills the gap and DNA ligase seals the nick.

Option by option

  • A. Helicase

    Helicase unwinds DNA and is part of other pathways but does not remove uracil.

  • B. Photolyase

    Photolyase directly reverses pyrimidine dimers using light and is absent in humans.

  • C. Uracil DNA glycosylase (correct)

    Correct. Uracil DNA glycosylase recognises and excises uracil, starting base excision repair.

  • D. DNA ligase

    DNA ligase only seals the final nick and does not recognise the damaged base.

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 C, Uracil DNA glycosylase. Uracil DNA glycosylase starts base excision repair by removing the uracil base, leaving an abasic site.

Why is "Helicase" not the answer?

Helicase unwinds DNA and is part of other pathways but does not remove uracil.

Why is "Photolyase" not the answer?

Photolyase directly reverses pyrimidine dimers using light and is absent in humans.

Why is "DNA ligase" not the answer?

DNA ligase only seals the final nick and does not recognise the damaged base.

What concept does this question test?

It tests DNA repair in Biochemistry. An endonuclease then cuts the backbone, DNA polymerase beta fills the gap and DNA ligase seals the nick.

How hard is this question?

We tag it as an application question: it asks you to use a fact on a patient or a finding. 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 DNA repair?

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.