mRNA Splicing Flashcards
(17 cards)
What are the key steps in eukaryotic mRNA processing?
1 - 5’ capping - Addition of 7-methyl guanin to stablise mRNA
2 - 3’ polyadenylation - 150-400 A’s addded at mRNA end
3 - RNA splicing - Introns removed, exons connected to form continuous mRNA
What are introns?
Non-coding sequences removed before translation
What are exons?
Protein-coding regions joined together in mature mRNA
What is oc-transcriptional splicing?
Splicing occurs simultaneously with transcription in eukaryotic cells
Is exon order changed during splicing?
No
Exon order emains the same in DNA and mature mRNA
Why must introns be removed?
Un-spliced mRNA contains termination codons, preventing translation
Un-spliced mRNA cannot exit the nucleus
Mature mRNA requires 5’ cap and 3’ poly-A tail for stability
What happes if a splice site mutation prevents correct intron removal?
Intron inclusion disrupts the reading frame, causing premature stop codons
Leads to non-functional proteins or complete loss of translation
What are the conserved splice site sequences?
5’ splice-site (GU) - Splice donor
3’ splice site (AG) - Splice acceptor
Brance point (YURAC sequence) - Essential for lariat formation
What are the two main stages of pre-mRNA splicing?
Stage 1 - 5’ cleavege and lariat formation
Stage 2 - 3’ cleavage and exon ligation (intron is removed)
What enzyme complex carriers out splicing?
The spliceosome - a ribonucleoprotein complex made of snRNPs (snurps)
What is the role of U1 snRNP?
First step of splicing, binds the 5’ splice site
Contains U1 snRNA, which helps recognise splice sites
How deos the splicesome assemble?
1 - U1 binds 5’ splice site
2 - U2AF binds pyrimidine-rich sequence near 3’ splice site
3 - U2 binds brance point and replaces BBP
4 - U5 and U4/U6 snRNPs from the B complex
5 - Catalytic center froms when U4 is released –> splicing occurs
What are self-splicing introns?
Introns that remove themselves without proteins ot a splicesome –> RNA acts as a ribozyme
What is the diffference between Group 1 and Group 2 introns?
Group I - Require a guanine nucleotide, no splicesome
Group II - Lariat formation like splicesome splicing, but no proteins needed
Why is alternative splicing important?
Increases transcript diversity
Can include/exclude coding seuquences to produce different proteins
Can alter mRNA stability, making it more or less stable
More than 90% of mamalian genes undergo alternative splicing
How does alternative splicing influence Drosophila sex determination?
Sxl gene (Sex lethal) is expressed in females, preventing intron inclusion in tra gene
Tra protein is only functional in females
Tra/Tra2 regulate splicing of Dsx (Double-sex) gene, producing female specific or male specific isoforms
Dsx isoforms inhibit either male or female genes, determining sex
