Post-Transcription: Capping the 5' End of mRNA

So this is a continuation of the transcription saga and consists of the plot after transcription occurs.

Eukaryotic mRNA Processing

There are four important steps regarding post-transcriptional processing of mRNA:

1) Transcription and 5' Capping of mRNA
2) Cleavage at the Poly(A) site
3) Polyadenylation
4) RNA splicing

Capping the 5' End of the mRNA

The mRNA is the primary RNA transcript; the sequence coded from the template DNA strand. To prepare the mRNA to become a fully functioning polypeptide, the introns have to be removed. However, before this happens, the first step that occurs is the capping of the 5' end of the mRNA. Why? Well, capping the mRNA protects it from degradation from nucleases. Exonucleuses recognize the free end of the mRNA of the OH group, so to make this OH group invisible, it must be capped. 7-methylguanosine is added to the 5' end when the transcript is 25-30 nucleotides long even while its still associated with RNA Pol II. This is a dynamic process, so transcription can still be in session while the mRNA is processed. The dimeric capping enzyme associates with the unstructured CTD tail of RNA Pol II, becomes phosphorylated and drags along with it the enzymes required to process the mRNA. One subunit of the capping enzyme removes the gamma-phosphate from the 5' end of the RNA and the other converts GMP from GTP to the 5' diphosphate of the transcript.