Molecular biology: translation initiation: 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: translation initiation

Sample item

A bacterial polycistronic mRNA encodes three proteins, and ribosomes initiate translation at each start codon independently. Which feature makes this possible?

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

The answer and the reasoning

B. A Shine-Dalgarno sequence upstream of each AUG that pairs with the 3' end of 16S rRNA

In prokaryotes the Shine-Dalgarno sequence lies a few bases upstream of each initiation codon and pairs with the 3' end of 16S rRNA, positioning the 30S subunit. Each cistron has its own site, so several proteins can be made from one transcript. Eukaryotic mRNAs are usually monocistronic and use cap-dependent scanning.

Option by option

  • A. A Kozak sequence that base-pairs with 18S rRNA

    The Kozak sequence is a eukaryotic context around the start codon, and it does not pair with 18S rRNA in this way.

  • B. A Shine-Dalgarno sequence upstream of each AUG that pairs with the 3' end of 16S rRNA (correct)

    Correct. Each cistron carries its own ribosome-binding site pairing with 16S rRNA.

  • C. A poly-A tail bound by poly-A binding protein

    Bacterial mRNAs lack a functional poly-A tail for initiation.

  • D. A 5' cap bound by eIF4E before scanning

    A cap and eIF4E belong to eukaryotic translation, and bacterial mRNA is uncapped.

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, A Shine-Dalgarno sequence upstream of each AUG that pairs with the 3' end of 16S rRNA. In prokaryotes the Shine-Dalgarno sequence lies a few bases upstream of each initiation codon and pairs with the 3' end of 16S rRNA, positioning the 30S subunit.

Why is "A Kozak sequence that base-pairs with 18S rRNA" not the answer?

The Kozak sequence is a eukaryotic context around the start codon, and it does not pair with 18S rRNA in this way.

Why is "A poly-A tail bound by poly-A binding protein" not the answer?

Bacterial mRNAs lack a functional poly-A tail for initiation.

Why is "A 5' cap bound by eIF4E before scanning" not the answer?

A cap and eIF4E belong to eukaryotic translation, and bacterial mRNA is uncapped.

What concept does this question test?

It tests Molecular biology: translation initiation in Biochemistry. Each cistron has its own site, so several proteins can be made from one transcript. Eukaryotic mRNAs are usually monocistronic and use cap-dependent scanning.

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: translation initiation?

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.