Capping and polyadenylation
There are three main types of post-transcriptional modification in eukaryotes: capping, polyadenylation and splicing.
All of these modifications occur in the nucleus.
- Capping is the addition of a 7-methylguanosine cap to the 5' end of the mRNA strand. This is a methylated guanine molecule.
- Polyadenylation adds a chain of around 250 adenine nucleotides to added to the 3' end of the mRNA. This is known as the poly(A) tail. It is not encoded in the genome. Instead, the nucleotides are added by an enzyme.
Polyadenylation increases the stability of mRNA. The longer the chain, the more stable the mRNA
Capping is required for the mRNA to be recognised by proteins. If mRNA is not capped, it cannot leave the nucleus.