Question

A eukaryotic cell is exposed to a chemical that inhibits the 5' capping of pre-mRNA. What is the most likely effect on translation?

• A. Translation will initiate but elongation will be impaired.
• B. Translation will fail to initiate due to improper ribosome binding.
• C. The polyadenylation will not occur.
• D. Splicing of introns will be unaffected.

Hint:

Think of the eukaryotic mRNA modifications as "passports" and "instructions." The 5' cap is the "address label" that tells the ribosome where to bind and start reading. Without this label, the ribosome is lost and cannot begin its job of translation.

Answer 1

The Correct Option is B

Step 1: Understanding the Concept:
The question asks about the consequence of inhibiting the 5' cap addition to eukaryotic pre-mRNA, specifically its effect on translation. The 5' cap is a modified guanine nucleotide added to the 5' end of the mRNA transcript.
Step 2: Detailed Explanation:
The 5' cap has several crucial functions in eukaryotes:

Protection: It protects the mRNA from degradation by 5' exonucleases.
Nuclear Export: It is essential for the transport of the mRNA from the nucleus to the cytoplasm.
Translation Initiation: It serves as the primary recognition site for the ribosome to bind to the mRNA. The cap is recognized by the cap-binding complex (eIF4F), which then recruits the small ribosomal subunit (40S) to the mRNA. This is a critical step for initiating translation.
Let's analyze the options based on these functions:

(A) Translation will initiate but elongation will be impaired: This is incorrect. The cap is essential for initiation, not elongation. Without the cap, initiation itself is blocked.
(B) Translation will fail to initiate due to improper ribosome binding: This is correct. Without the 5' cap, the ribosome cannot be recruited to the mRNA efficiently, and thus translation cannot begin.
(C) The polyadenylation will not occur: Polyadenylation (addition of a poly-A tail) occurs at the 3' end and is a separate process from 5' capping, although they are often coupled. Inhibiting capping doesn't directly prevent polyadenylation.
(D) Splicing of introns will be unaffected: This is not the most likely or direct effect on translation. While capping can influence the efficiency of splicing for some introns, its most direct and critical role is in translation initiation. The question asks for the effect on {translation}.
Step 3: Final Answer:
Inhibiting 5' capping will prevent the proper binding of the ribosome to the mRNA, thereby causing a failure in the initiation of translation.