In eukaryotic mRNA processing, which modification protects the mRNA from 5' exonuclease degradation and is required for ribosomal cap-dependent translation initiation?

• A Poly-A tail addition to the 3' end by poly-A polymerase
• B Removal of introns by the spliceosome
• C Pseudouridylation of specific uridines in the mRNA backbone
• D 7-methylguanosine (m7G) cap added to the 5' end by guanylyl transferase and methyltransferase ✓

Explanation

Co-transcriptional 5' capping adds a 7-methylguanosine in an unusual 5'–5' triphosphate linkage, creating a structure unrecognised by 5'–3' exonucleases and providing protection against degradation. The m7G cap is also recognised by eIF4E (eukaryotic initiation factor 4E), which bridges the cap to the 43S pre-initiation complex for cap-dependent translation. Loss of eIF4E activity (or competition by 4E-BPs) inhibits translation; mTORC1 promotes translation by phosphorylating and inactivating 4E-BPs.

Reference: Harper's Illustrated Biochemistry, 32nd ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

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