Chromosome ends: telomerase: 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 · Chromosome ends: telomerase

Sample item

A 12-year-old boy has nail dystrophy, oral leukoplakia, reticulate skin pigmentation and progressive bone marrow failure. Mutations affect a ribonucleoprotein that maintains chromosome ends. How does the normal enzyme work?

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

The answer and the reasoning

A. It uses an internal RNA template to add TTAGGG repeats to the 3' end of the chromosome

Telomerase is a reverse transcriptase that carries its own RNA component (TERC) and extends the 3' overhang of telomeric DNA with TTAGGG repeats. This solves the end-replication problem, in which the lagging strand cannot be fully copied. Defective telomerase causes dyskeratosis congenita, with stem cell failure from critically short telomeres.

Option by option

  • A. It uses an internal RNA template to add TTAGGG repeats to the 3' end of the chromosome (correct)

    Telomerase extends the 3' end using its RNA template, preserving telomere length.

  • B. It uses a DNA template to add repeats to the 5' end of the lagging strand

    Telomerase uses an RNA, not a DNA, template and extends the 3' end.

  • C. It removes RNA primers from the 5' end of the lagging strand

    Primer removal is the job of DNA polymerase I in prokaryotes and of FEN1 and RNase H in eukaryotes.

  • D. It repairs thymine dimers at chromosome ends by excision

    Thymine dimer repair is nucleotide excision repair, a different process.

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.

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Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is A, It uses an internal RNA template to add TTAGGG repeats to the 3' end of the chromosome. Telomerase is a reverse transcriptase that carries its own RNA component (TERC) and extends the 3' overhang of telomeric DNA with TTAGGG repeats.

Why is "It uses a DNA template to add repeats to the 5' end of the lagging st…" not the answer?

Telomerase uses an RNA, not a DNA, template and extends the 3' end.

Why is "It removes RNA primers from the 5' end of the lagging strand" not the answer?

Primer removal is the job of DNA polymerase I in prokaryotes and of FEN1 and RNase H in eukaryotes.

Why is "It repairs thymine dimers at chromosome ends by excision" not the answer?

Thymine dimer repair is nucleotide excision repair, a different process.

What concept does this question test?

It tests Chromosome ends: telomerase in Biochemistry. This solves the end-replication problem, in which the lagging strand cannot be fully copied. Defective telomerase causes dyskeratosis congenita, with stem cell failure from critically short telomeres.

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 Chromosome ends: telomerase?

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.