Gene organisation and transcription II Flashcards
Transcription in bacteria
Only 1 type of RNA polymerase and no nucleus
Therefore, newly transcribed RNA is ready for use in translation
What is mRNA processing?
Splicing out introns before it exits the nucleus and enters the cytoplasm
pre-mRNA
primary transcript
heterogenous nuclear RNA
Needs to be processed before it can be used as mRNA for protein translation (occurs in nucleus)
Introns
Non coding sequences
Exons
Coding sequences
How is sequence information contained in final mRNA encoded (before producing final mRNA)?
Discontinuously in DNA of a gene
i.e. in exons separated by introns
Splice donor site
Site at 5’ end of intron
GU
Splice acceptor site
Site at 3’ end of intron
15 pyrimidine bases, any base and then CAG
(Pyr15NCAG)
What bases do introns begin with and end with?
GU —-AG
snRNPs
Small ribonuclear proteins
numbered
Describe how small ribonuclear proteins (snRNPs) bind to the mRNA.
U1 binds to the splice donor site
U5 binds to the splice acceptor site
U2, U4 and U6 bind in the middle of the intron
What is the spliceosome made up of?
snRNPs bound to pre-mRNA
Describe how the intron is removed from the mRNA strand.
Formation of spliceosome results in cleavage of the splice donor site (phosphodiester bond broken)
1st G loops round and forms a phosphodiester bond with an A residue in the middle of the intron (branchpoint).
The mRNA then cleaves at the splice acceptor site and the intron is removed as a lariat structure.
RNA ligase joins exons
What 2 structures are added during post-transcriptional modification?
7-methylguanylate cap
Poly-A-tail
Why is the 7-methylguanylate cap added during post-transcriptional modification?
Protects the mRNA from degradation
Enhances translation
Why is the Poly-A-tail added during post-transcriptional modification?
Provides stability
Describe the location of the poly-A-tail
Occurs 11-30 bases downstream of an AAUAAA sequence in the mRNA
Added 1 base at a time
Give an example of a disease that affects 1 of the structures added during post-transcriptional modification.
Polio-myelitis can interfere with 7-methylguanylate cap recognition during translation
Give 2 examples where mutations in splice site features in human disease.
Beta Thalassemia
Duchenne Muscular Dystrophy
Beta Thalassaemia
Splice site mutation in the beta globin gene
Causes imbalances in relative amounts of alpha and beta chains
Leads to severe anaemia and endocrinopathies
Duchenne Muscular Dystrophy
Deletion of >,1 exon(s) in dystrophin gene
Causes muscle wastage
Alternative splicing
Can pharmacologically re-programme spliceosome to favour alternatively spliced dystrophin mRNAs
Forms shorter, functional forms of dystrophin
So exon with mutation is not in final mRNA
