Molecular biology and genetics: tetracycline mechanism: 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: tetracycline mechanism

Sample item

A teenager with acne is treated with doxycycline. Which step of bacterial protein synthesis does the drug block?

ATranslocation of the ribosome along the mRNABEntry of aminoacyl-tRNA into the ribosomal A siteCAssembly of the 70S initiation complexDPeptide bond formation by peptidyl transferase

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

The answer and the reasoning

B. Entry of aminoacyl-tRNA into the ribosomal A site

Tetracyclines bind the 30S subunit and block attachment of aminoacyl-tRNA to the A site. Chloramphenicol blocks peptidyl transferase, macrolides block translocation, and aminoglycosides disturb initiation and cause misreading. The 30S site of binding explains the bacterial selectivity.

Option by option

A. Translocation of the ribosome along the mRNA
Translocation is blocked by macrolides such as erythromycin.
B. Entry of aminoacyl-tRNA into the ribosomal A site (correct)
Correct. Tetracyclines block aminoacyl-tRNA access to the A site on the 30S subunit.
C. Assembly of the 70S initiation complex
Aminoglycosides, not tetracyclines, interfere mainly with initiation.
D. Peptide bond formation by peptidyl transferase
Peptidyl transferase is inhibited by chloramphenicol.

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.

Questions about this Biochemistry question

What is the answer to this Biochemistry question?

The best answer is B, Entry of aminoacyl-tRNA into the ribosomal A site. Tetracyclines bind the 30S subunit and block attachment of aminoacyl-tRNA to the A site.

Why is "Translocation of the ribosome along the mRNA" not the answer?

Translocation is blocked by macrolides such as erythromycin.

Why is "Assembly of the 70S initiation complex" not the answer?

Aminoglycosides, not tetracyclines, interfere mainly with initiation.

Why is "Peptide bond formation by peptidyl transferase" not the answer?

Peptidyl transferase is inhibited by chloramphenicol.

What concept does this question test?

It tests Molecular biology and genetics: tetracycline mechanism in Biochemistry. Chloramphenicol blocks peptidyl transferase, macrolides block translocation, and aminoglycosides disturb initiation and cause misreading. The 30S site of binding explains the bacterial selectivity.

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 biology and genetics: tetracycline mechanism?

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.