Nucleotide metabolism: nucleoside linkage: 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 · Nucleotide metabolism: nucleoside linkage

Sample item

In a purine nucleoside such as adenosine, C1 of ribose is joined by a beta-N-glycosidic bond to which ring atom of the base?

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

The answer and the reasoning

B. N9

In purine nucleosides the sugar attaches to N9 of the base, whereas in pyrimidine nucleosides it attaches to N1. In the nucleotide, phosphate is esterified to the 5' hydroxyl of the sugar.

Option by option

  • A. N1

    N1 is the attachment point in pyrimidines, not purines.

  • B. N9 (correct)

    Correct. The glycosidic bond links C1' of ribose to N9 of purines.

  • C. N7

    N7 receives its nitrogen from glycine in synthesis but is not the attachment point.

  • D. N3

    N3 is not a glycosidic attachment site in standard nucleosides.

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 B, N9. In purine nucleosides the sugar attaches to N9 of the base, whereas in pyrimidine nucleosides it attaches to N1.

Why is "N1" not the answer?

N1 is the attachment point in pyrimidines, not purines.

Why is "N7" not the answer?

N7 receives its nitrogen from glycine in synthesis but is not the attachment point.

Why is "N3" not the answer?

N3 is not a glycosidic attachment site in standard nucleosides.

What concept does this question test?

It tests Nucleotide metabolism: nucleoside linkage in Biochemistry. In the nucleotide, phosphate is esterified to the 5' hydroxyl of the sugar.

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 Nucleotide metabolism: nucleoside linkage?

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.