5' cap

Eukaryotic pre-mRNA goes through a process of modification before it is able to bind to the ribosomal RNA. This modification also adds stability and plays an important role in the removal of introns in the pre-mRNA. The process of cap addition takes place rapidly after transcription is initiated, the addition of a guanine base to the 5’ end of the mRNA, followed by the addition of methyl groups (CH₃) to the guanine and the bases of one or two bases at the 5’ end of the mRNA. Unlike the rest of the bases in RNA the guanine base constituting the 5’ cap is connected to the RNA by a 5’-5’ bond. The methyl groups are then added to position 7 on the guanine nucleotide. Cap-binding proteins then bind this structure allowing recognition by the ribosomal RNA. The rRNA will then move downstream until it reaches the start codon, initiating translation. The addition of this cap is catalysed by the enzyme that associates with RNA polymerase II. RNA polymerase I and III do not have this property and so RNAs modified by these enzymes don’t have the 5’ cap.