mRNA capping — Which modified base is found at the 5′ end (cap) of a typical eukaryotic mRNA?

Introduction:
Eukaryotic mRNAs are modified co-transcriptionally at the 5′ end with a specialized cap that protects the transcript and promotes translation. This question asks you to identify the specific modified base present in that cap structure.

Given Data / Assumptions:

• The 5′ cap is linked via a unique 5′–5′ triphosphate bridge.
• Cap structure enhances stability, nuclear export, and ribosome recruitment.
• Cap-binding proteins recognize the methylated base.

Concept / Approach:
The canonical eukaryotic cap (cap 0) contains 7-methyl-guanosine (m7G) attached to the first transcribed nucleotide via a 5′–5′ triphosphate linkage. Additional 2′-O-methylations on the first one or two riboses (cap 1 and cap 2) may occur, but the defining modified base at the cap itself is m7G.

Step-by-Step Solution:

Verification / Alternative check:
Cap-binding complex eIF4E specifically recognizes the m7G cap, facilitating translation initiation and interactions with eIF4G and the poly(A)-binding protein loop.

Why Other Options Are Wrong:

• 1-methyl-adenosine / 1-methyl-guanosine: not the defining cap base.
• 2′-O-methyl-guanosine: refers to ribose methylation, not the signature N7 base methylation.
• pAp: a metabolite, not the cap base.

Common Pitfalls:
Confusing sugar methylations near the cap with the essential N7 methylation of guanosine that defines the mRNA cap.

Final Answer:
7-methyl-guanosine.