During eukaryotic mRNA processing, the 5' cap is added to the nascent mRNA. The 5' cap structure is:

• A A polyadenosine tail added by poly-A polymerase
• B An N6-methyladenosine modification of internal adenosines
• C A branch-point adenosine in the lariat structure formed during splicing
• D A 7-methylguanosine residue connected via a 5'-5' triphosphate linkage to the first transcribed nucleotide ✓

Explanation

The 5' cap is m7GpppN — a 7-methylguanosine added in a 5'-to-5' triphosphate linkage (unusual, as all other phosphodiester bonds in RNA are 3'-5'). This structure is added co-transcriptionally by capping enzyme (RNA 5'-triphosphatase, guanylyltransferase, and guanine-N7-methyltransferase) when the nascent transcript is ~25 nucleotides. The cap protects mRNA from 5'-exonuclease degradation, facilitates nuclear export, and is recognised by eIF4E for cap-dependent translation initiation.

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

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.