Post-transcriptional modification is the physical alteration of the newly formed mRNA strand before it starts producing proteins. This process occurs after transcription (DNA to RNA).
Newly transcribed mRNA is often called the primary transcript or pre-mRNA.
Post-transcriptional modification gives an opportunity to alter gene expression. This modification can have significant effects on the amount mRNA that is translated.
Post-transcriptional modification is most common in eukaryotes.
Eukaryotic transcription occurs in the nucleus while translation occurs in the cytoplasm.
This means that there are several opportunities for the mRNA to undergo modification, as transcription and translation are spatially separated.After modification, mRNA leaves the nucleus through the nuclear pores.
There is very little post-transcriptional modification in prokaryotes. As a result, post-transcriptional modification has only a minor influence on gene expression in prokaryotes.
Translation usually occurs immediately after an mRNA strand has been transcribed, so there is no opportunity for post-transcriptional modification. Both translation and transcription occur in the cytosol.
Post-transcriptional modification does, however, frequently occur in the production of prokaryotic rRNA and tRNA.
In this case, transcription is not followed by translation, so there is time to perform modifications.
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.
Splicing is the removal of sections from mRNA. This is a form of post-transcriptional modification.
Eukaryotic splicing is typically catalysed by a spliceosome, a complex composed of RNA and protein sections. A section that is removed is called an intron, while a section that is left is an exon (Expressed regiON).
Alternative splicing is the removal of different introns from mRNA to create slightly different versions of the same protein, called isoforms. Each isoform has a slightly different function.
The majority of human genes can be alternatively spliced. This greatly increases protein diversity.
Parkinson's and Alzheimer's diseases are both thought to be caused by failures in splicing.