There are three major processing events that all eukaryotic pre-mRNA goes through. Following these processing events it is a mature mRNA that can be transported to the cytoplasm and translated.

An overview shows the three processing events. Once transcription initiates, a 5' CAP is added. During transcription, all introns are removed by splicing: there is one splicing reaction per intron. Finally, following an endonucleolytic cleavage a poly(A) tail is added at the 3' end.

The addition of the 5' CAP is shown in more detail here. The CAP is a 7-methyl guanine that is attached to the 5' end by a 5'-5' bond. A standard GTP is added first and this is modified to 7-MG once it is attached. Finally, what had been the 5' nucleotide on the transcript is modified. This CAP structure plays an important role in the initiation of translation.

Splicing occurs while transcription proceeds. The reaction is illustrated below.

The addition of the poly(A) tail is essentially the transcription termination process for PolII. It involves the recognition of the polyadenylation signal on the RNA transcript followed by cleavage and the action of poly(A) polymerase.

A video about mRNA splicing:

Eukaryotic Transcription

Table of Contents