4 Gene Organisation And Transcription II Flashcards
Q: What is RNA processing? Where does it take place? After?
A: Initial RNA produced from a gene is the primary transcript or Pre-mRNA or heterogenous nuclear RNA (hn RNA) and this primary transcript must be processed first before it can be used as mRNA in translation.
RNA Processing occurs in the nucleus and fully processed mRNA is exported into the cell cytoplasm to be translated
Q: Where does the gene promoter lie in the gene?
A: Gene promoter lies at the 5’ end of the gene
Q: What are exons? Usually end with?
A: Segments of DNA which contain sequences that form part of the final mRNA
Exons usually end with AG
Q: What are introns? Start with? End with?
A: transcribed but edited out of final mRNA
INTRONS START WITH GU AND END WITH AG
Q: Which proteins does RNA processing use?
A: small Ribonuclear Proteins (snRNPs)
Q: What is the splice acceptor site?
A: Junction between intron and exon, 3’ end of the intron (end of the intron)- cleaved after
Q: What is the splice donor site?
A: Junction between exon and intron, 5’ end of the intron (start of the intron) - cleaved first
Q: Describe with the aid of diagrams the sequence of events in mRNA splicing. 6 steps.
A: 1. snRNP U1 binds to splice donor sequence
- snRNP U2, U4, U5 and U6 bind and complete the formation of the splicing complex (splicesome) where U5 binds to the splice acceptor site- >
The formation and binding of the spliceosome results in the cleavage of the splice donor sequence. - The end G of the intron bends round and an ‘A residue’ in the intron acts as a branchpoint in an intermediate step in splicing
- Phosphodiester bonds forms between G and A residue: between 5’ phosphate on the G at the start of of the cleaved intron and the 2’ OH of the branchpoint A
- The phosphodiester bond beween end G of intron and the exon breaks so intron is removed as LARIAT structure
- Adjacent exons are ligated together
Q: What are the 2 types of post transcriptional modification of RNA?
A: cap structure, polyA tail
Q: Explain the addition of a “cap” to pre-messenger (hn-) RNA. Where? Cap structure? Further modification?
A: cap is added to the 5’ end of the mRNA
Cap Structure: formed by hydrolysis of terminal triphosphate of mRNA to a diphosphate -> then reacts with a phosphate of GTP to form 5’-5’phosphate linkage.
Cap is further modified by Methylation at the N7 position in the purine ring to form 7-Methylguanylate cap
Q: What does hnRNA stand for?
A: heterogenous nuclear DNA
Q: What is the purpose of the addition of a “cap” to pre-messenger (hn-) RNA? (2)
A: ACTS TO PROTECT mRNA AT 5’ END AND GREATLY ENHANCES THE TRANSLATION OF mRNA.
Q: What can interfere with the recognition of the cap during translation?
A: viruses such as polio
Q: Explain the addition of a “poly A tail” to pre-messenger (hn-) RNA.
A: process of polyadenylation
Poly A tail is added one base at a time- is added 11-30 bases downstream of the sequence AAUAAA which is found in all mRNAs.
Q: What percentage of human genetic disease feature mutations in splice donor/splice acceptor sequences?
A: 33
