Initiation in eukaryotes — how does mRNA bind the ribosome? Eukaryotic mRNA recruitment to the small ribosomal subunit during translation initiation is primarily facilitated by which feature?

Introduction:
Eukaryotic translation initiation relies on recognition of the 5′ cap structure by cap-binding factors, which recruit the small ribosomal subunit to scan for the start codon. This differs from bacteria, which use base-pairing between the Shine–Dalgarno sequence and 16S rRNA.

Given Data / Assumptions:

• Eukaryotic mRNAs are capped at the 5′ end with 7-methylguanosine.
• Initiation factors (eIF4E, eIF4G, eIF4A, and others) form a complex that binds the cap.
• Poly(A) tails and PABP synergize with cap-binding to enhance initiation but are not the primary docking feature.

Concept / Approach:

The cap-dependent scanning model posits that the small subunit (40S) is recruited to the mRNA via eIF4E recognition of the m7G cap. The preinitiation complex then scans 5′→3′ to locate an AUG in a favorable Kozak context. Some specialized mRNAs use IRES elements, but these are exceptions.

Step-by-Step Solution:

Verification / Alternative check:

Biochemical reconstitution shows cap dependence for most cellular mRNAs; cap analogs competitively inhibit initiation, confirming the central role of the m7G cap.

Why Other Options Are Wrong:

Shine–Dalgarno is bacterial, not eukaryotic. tRNA alone cannot mediate mRNA binding. The poly(A) tail aids initiation but is not sufficient by itself. IRES elements are not universal.

Common Pitfalls:

Assuming that all eukaryotic mRNAs use IRES or that the poly(A) tail replaces the cap in typical initiation.

Final Answer:

The 7-methylguanosine (m7G) cap at the 5′ end of the mRNA