14.6: pre-mRNA processing
In eukaryotic cells, transcripts made by RNA polymerase are modified and processed before exiting the nucleus. Unprocessed RNA is called precursor mRNA or pre-mRNA, to distinguish it from mature mRNA.
Once about 20-40 ribonucleotides have been joined together by RNA polymerase, a group of enzymes adds a “cap” to the 5’ end of the growing transcript. In this process, a 5’ phosphate is replaced by modified guanosine that has a methyl group attached to it. This 5’ cap helps the cell distinguish mRNA from other types of RNA in the cell and plays a role in subsequent translation.
During or shortly after transcription, a large complex called the spliceosome cuts out various parts of the pre-mRNA transcript, rejoining the remaining sequences. RNA sequences that remain in the transcript are called “exons” (expressed sequences) while portions removed are called “introns”. Interestingly, a single RNA segment can be an exon in one cell type and an intron in another. Similarly, a single cell can contain multiple variants of a gene transcript that has been alternatively spliced, enabling the production of multiple proteins from a single gene.
When transcription is completed, an enzyme adds approximately 30-200 adenine nucleotides to the 3’ end of the pre-mRNA molecule. This poly-A tail protects the mRNA from degradation in the cytoplasm. The mature mRNA then exits the nucleus for translation.