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BIOB11H3 Study Guide - Midterm Guide: Spliceosome, Exonuclease, Normal Function

Biological Sciences
Course Code
Dan Riggs
Study Guide

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BIOB11-L5: RNA Splicing, Catalytic RNAs, RNA interference
Consensus Sequences
- Consensus Sequences exist at splice junction
- Accurate splicing removes the intron and involves the precise joining of
adjacent exons
*Polypyrimidine tract and Branch point sequence near the 3’ splice site
‘Mis-splicing’ Consequences
1. Accumulation of unspliced RNAs
- Not exported to cytoplasm, likely degraded
- No protein produced
2. Frameshift: Resulting in a novel aberrant protein (perhaps containing part of an intron)
- Triplet set of nucleotides encode each AA
- Missense protein produced
- Normal function of the protein may be altered or lost
- Frameshift: Addition / deletion of nucleotides to alter the ‘reading frame’ of the triplet
genetic code
‘Self-Splicing’ RNA found in Tetrahymena
Ribozyme: an RNA that has enzymatic activity
Group I introns: Found in nuclear RNA of lower eukaryotes, some organelles Self-splicing
Forms elaborate 3D structure due to intra-strand base-pairing
Group II introns: Found in organelles, ALSO self-splicing
But mechanism generates a Lariat intermediate
Intrastrand base pairing generates specific 3D structure
Exons become aligned
Branch point A ‘activated’ and located to initiate cleavage reaction
Group II introns
Higher Eukaryotes: Spliceosome Mediated RNA processing
snRNPs: ‘snurps’, small nuclear ribonuclear protein particles
snRNAs: Small nuclear RNAs, RNA components (called U1, U2, U3… ,U6)
Base pairing between snRNAs, between snRNAs & hnRNA intron sequences
strategically position molecules for cleavage and relegation events
1. U1 binds to splice donor
2. U2 binds to branch point A (U1 & U2 both base pair with conserved sequence)
[The A is torqued out of the pairing, not involved]
*The torqued A seems to be interacting with the splice donor sequence
3. U4,5,6 come as a complex, where U6 displace U1, acts as ribozyme
4. Extensive pairing between snRNAs, and between snRNAs / pre-mRNA occur
5. Secondary reaction where exons get ligated and intron lariat is formed
6. Lariat introns is then degraded and recycled
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