Summary
Highlights
Post-transcriptional regulation occurs after DNA has been transcribed into mRNA. This process is exclusive to eukaryotes and helps stabilize mRNA, protecting it from premature degradation before protein translation.
DNA is transcribed into RNA, which contains both coding segments (exons) and non-coding segments (introns). Introns are removed, or 'spliced out,' by a spliceosome, while exons are retained and exit the nucleus to be translated. A mnemonic to remember is 'exons exit the nucleus' and 'introns stay in the nucleus'.
After splicing, the mRNA receives a 5' prime cap. This modification occurs at the phosphate end of the mRNA, protecting it from degradation by exonucleases. The cap also promotes ribosomal binding for translation and regulates the mRNA's export from the nucleus.
The mRNA also gets a 3' prime poly-A tail, which consists of multiple adenosine monophosphates. Similar to the 5' cap, this tail acts as a buffer against exonucleases, increasing the mRNA's half-life and protecting it from degradation. It also helps with translation promotion and nuclear export regulation, and aids in transcription termination for RNA polymerase. The poly-A tail is built to about 250 nucleotides long by polyadenylate polymerase.
Both the 5' prime cap and the poly-A tail are crucial for stabilizing the mRNA, allowing it to be effectively translated into a protein. Once these modifications are complete and introns are removed, the mRNA is ready to leave the nucleus.
RNA editing is another form of RNA regulation that results in sequence variations within the RNA molecule, catalyzed by various enzymes. This rare process can involve insertions, deletions, and base substitutions. Examples include ADAR enzymes converting adenosine to inosine, and CDAR enzymes deaminating cytosine to uridine. RNA editing is being actively studied for its role in infectious diseases, as it can alter viral enzymes and their function.