Molecular techniques: Sanger sequencing: 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 techniques: Sanger sequencing

Sample item

In a Sanger sequencing reaction, the newly synthesised strand reads 5'-ATGCCG-3' when fragments are ordered from shortest to longest. What is the sequence of the template strand?

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

The answer and the reasoning

B. 3'-TACGGC-5'

Dideoxynucleotides lack a 3' hydroxyl group, so chain synthesis stops where they are incorporated; fragments ordered by size reveal the new strand 5' to 3'. The template is complementary and antiparallel: A pairs with T, T with A, G with C, C with G, so the template reads 3'-TACGGC-5'.

Option by option

  • A. 5'-ATGCCG-3'

    This repeats the newly synthesised strand and is not complementary to it.

  • B. 3'-TACGGC-5' (correct)

    Base-pairing with the new strand and antiparallel orientation give 3'-TACGGC-5'.

  • C. 3'-ATGCCG-5'

    These are the same letters as the new strand with the polarity reversed and are not complementary.

  • D. 5'-TACGGC-3'

    This has the correct complementary letters but the wrong polarity, since the strands are antiparallel.

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 B, 3'-TACGGC-5'. Dideoxynucleotides lack a 3' hydroxyl group, so chain synthesis stops where they are incorporated; fragments ordered by size reveal the new strand 5' to 3'.

Why is "5'-ATGCCG-3'" not the answer?

This repeats the newly synthesised strand and is not complementary to it.

Why is "3'-ATGCCG-5'" not the answer?

These are the same letters as the new strand with the polarity reversed and are not complementary.

Why is "5'-TACGGC-3'" not the answer?

This has the correct complementary letters but the wrong polarity, since the strands are antiparallel.

What concept does this question test?

It tests Molecular techniques: Sanger sequencing in Biochemistry. The template is complementary and antiparallel: A pairs with T, T with A, G with C, C with G, so the template reads 3'-TACGGC-5'.

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 Molecular techniques: Sanger sequencing?

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.