Eukaryotic mRNA processing: Immediately after transcription begins, which modification is added to the 5' end of the nascent transcript?

Introduction / Context:
In eukaryotes, the primary RNA transcript undergoes co-transcriptional and post-transcriptional processing, including 5' capping, intron splicing, and 3' polyadenylation, to form a stable, translatable mRNA.

Given Data / Assumptions:

• Question asks about the earliest modification following transcription initiation.
• The 5' cap protects RNA and facilitates ribosome recruitment.

Concept / Approach:
A 7-methylguanosine cap is added via a 5'–5' triphosphate linkage shortly after the 5' end emerges from RNA polymerase II. Poly(A) addition occurs downstream after cleavage directed by the AAUAAA signal.

Step-by-Step Solution:
Identify timing: capping occurs early and at the 5' end. Reject options placing the cap at the 3' end or describing poly(A) signal as an “added” moiety (it is encoded, while the tail is enzymatically added). Select the accurate description of 5' capping.

Verification / Alternative check:
Capping enzymes associate with the polymerase CTD; capped RNAs show increased stability and translation efficiency.

Why Other Options Are Wrong:
3' capping does not occur; polyadenylation is distinct and later; riboswitches are bacterial regulatory RNAs, not added domains in eukaryotic mRNA.

Common Pitfalls:
Confusing the poly(A) signal (a sequence) with the poly(A) tail (an added stretch of adenosines); mixing up 5' and 3' ends.

Final Answer:
A methylated guanine cap is added to the 5' end.