mRNA Processing: Enzyme Regulated Modifications of mRNA transcript:
by Thomas Smith | Feb 18, 2024
- In Eukaryotes, mRNA transcript must be processed before can exit the nucleus and be used for translation → Pre-mRNA longer than post mRNA
- 5’ Cap: is added to the 5’ end of mRNA
- Guanine nucleotide with 2 additional phosphate groups (forms GTP)
- Capping provides stability, protection from nucleases, and a point of attachment for small ribosomal subunit
- Poly-A-tail: attached to 3’ end of mRNA
- Consists of many adenine nucleotides
- Provides stability and controls movement of mRNA across the nuclear envelope
- At 3’ end of the gene, there is a poly-A signal sequence at the last exon of the gene
- It is transcribed into an RNA sequence that signals where the transcript is cleaved and the poly-A-tail is added
- RNA splicing: removes nucleotide segments from mRNA
- mRNA (and DNA) contain two kinds of sequences
- Exons: express code for polypeptide
- Introns: intervening sequences that are noncoding
- Allow for exon shuffling/alternative splicing
- Uses spliceosomes: a large RNA-protein complex made on snRNA and proteins (snRNPs) that catalyzes the removal of introns & join exons
- Alternative Splicing: Selective excision of introns and retention of exons → allows diff mRNAs to be generated from same RNA transcript → each code for diff protein product of diff size
- So single gene can code for proteins that are specific to cell type & developmental stage
- Often diff exon combinations code for diff subunits (4th) of same protein
