lecture 4: gene organisation and transcription ii - mRNA processing Flashcards
what is the primary transcript/ pre mRNA?
this is the initial RNA produced from a gene
what happens to the pre mRNA? (RNA processing)
- the pre mRNA needs to be processed before it can be used as mRNA in translation - RNA processing occurs in the nucleus before it can be used at mRNA in translation - the fully processed mRNA is moved to the cytoplasm to be translated - RNA processing is basically removing the introns and leaving the exons
* what is the anatomy of the gene?
insert a pic - the gene promotor lies at the 5’ end of the gene - the segments of the DNA which contains sequences that form part of the final mRNA are called exons - introns are transcribed but they are edited out of the final mRNA
what is the splice donor site?
this is the junction between the exon and the intron
what is the splice acceptor site?
they is the junction between the intron and the exon
what do exons normally end with?
they end with AG
what do introns start and end with?
they start with GU and they end with AG
what does RNA processing use?
uses small ribonucleic proteins (snRNPs)
* what is the first stage of RNA processing?
- the snRNP U1 binds to the splice donor sequence insert the pic
* what is there second stage of RNA processing?
after U1, SnRNP U2, U4, U5, U6 also bind to the splice donor sequence - this completes the formation of the spliceosome (because all of the SnRNPs make the spliceosome) - and it results in the cleavage of the splice donor site insert the pic
* What is the third stage of RNA processing?
the end G of the intron bends round, and a A residue in the intron acts as a branch point in the intermediate step in splicing. insert the pic
* what is the fourth stage in the RNA processing?
- phosphodiester bonds form between the G and the A residues - the bond is between the 5’ phosphate on the G and the 2’OH of the branch point of the A insert the pic
* what is the fifth stage of RNA processing?
- the phosphodiester bond between the end G of the intron and the exon breaks so the intron is removed as a lariat structure.
what is the sixth stage of RNA processing?
- the adjacent axons are ligated together
what is the post transcriptional modification?
- a cap is added to the 5’ end of the mRNA
what is the structure of the cap?
- a cap is formed by hydrolysis of the terminal triphosphate of mRNA to a diphosphate - this then reacts with the alpha phosphate of GTP to form 5’ - 5’ phosphate linkage - the cap is then further modified by methylation at the N7 position in the purine ring to form 7- methylguanylate cap
what is the function of the cap?
- the cap acts to protect mRNA at the 5’ end - it greatly enhances the translation of mRNA
what does polio do to the cap?
polio can interfere with the recognition of the cap during translation
what are the symptoms of polio myelitis? who does it affect ? how does the disease enter?
- initial symptoms are fever, fatigue, headache, vomiting, stiff neck - highly infectious disease cause complete paralysis - this mainly affects children under 3 - the virus enters the body through the mouth and multiples in the intestine
what is polyadenylation?
- this is the addition of the poly A tail to the pre mRNA - the poly A tail is added one base at a time - poly A tail is added 11-30 bases downstream of the sequence AAuAAA - AAuAAA is found in all mRNAs
whats an example of a mutation in the splice site featuring in human disease?
- for example thalassemia
what is thalassemia?
- this is an inherited disorder in which there is an imbalance in the relative amounts of alpha and beta chains making haemoglobin
what are the features of thalassemia?
- theres an iron overload due to the chronic anaemia - the iron overload results in hepatic fibrosis and cirrhosis, the darkening of the skin and cardiomyophaty
what mutation happens in beta thalassemia?
- there is a splice site mutation in the beta global gene
