mRNA processing
Summary
mRNA processing is the set of changes that convert eukaryotic pre-mRNA into mature mRNA in the nucleus: a 5′ cap, splicing out introns, and a 3′ poly-A tail. Most testable: introns are removed and exons joined, and alternative splicing lets one gene encode several proteins.
Detail
Meaning: In eukaryotes, the primary transcript (pre-mRNA) is modified in the nucleus before translation. A 7-methylguanosine cap is added to the 5′ end. The spliceosome removes introns and joins exons. A poly-A tail is added to the 3′ end after cleavage near the AAUAAA signal. The cap and tail protect against exonucleases and aid export; the cap also helps ribosome binding. Alternative splicing lets one gene encode several proteins.
On the MCAT: Passages describe a mutation in a splice site or poly-A signal and ask about the product: a splice-site mutation can cause exon skipping or intron retention, giving an altered protein. A gel showing a shorter or longer mRNA suggests a splicing change. Questions also ask where processing occurs (nucleus) and contrast prokaryotes, which typically lack this processing and couple transcription to translation.
Common confusion: The poly-A tail is not encoded in the DNA; poly-A polymerase adds it without a template. Also, exons are kept in mature mRNA and introns are removed ("exons exit"), but exons include untranslated regions, so not every exon base is translated. Processing alters the RNA, not the gene.
Sources
- Lehninger Principles of Biochemistry
- Molecular Biology of the Cell
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.