gene organisation, transcription and regulation Flashcards
mRNA processing: explain with the aid of diagrams the events that take place in pre-mRNA processing; define what is meant by the terms "splice donor site" and "splice acceptor site", the function of the spliceosome, and explain the addition of a "cap" and "poly A tail" to pre-mRNA
define pre-mRNA
within nucleus before use as mRNA
pre-mRNA processing: diagram
diagram from NAGE 4
exons
ligated together to form mRNA; order of exons equal Hox gene body plan; end with AG
introns
transcribed but edited out; start with GU; end with Pyr15NAG (15 C/U bases; any nitrogenous base; AG)
location of gene promoter
5’ end
define splice donor site
junction between exon and 5’ end of intron
define splice acceptor site
junction between 3’ end of intron and exon
removing intron: formation of spliceosome
snRNP U1 binds to splice donor site; U5 then binds to splice acceptor site and U2, U4 and U5 bind in between
removing intron: formation of spliceosome diagram
diagram from NAGE 4
spliceosome definition
splicing complex which cleaves the splice donor sequence (intron)
removing intron: formation of lariat
A residue in intron used as branchpoint; phosphodiester bond forms between 5’ phosphate group on G of end of intron with 2’ OH on A; intron removed as lariat and broken down
removing intron: formation of lariat diagram
diagram from NAGE 4
post-transcriptional modification of mRNA: CAP
added to 5’ end of mRNA by hydrolysis, forming 5’-5’ phosphate linkage requiring GTP; modified by methylation at N7 → 7-methylguanylate CAP; protects and enhances mRNA transcription; polio interferes with CAP recognition
post-transcriptional modification of mRNA: poly A tail
added 1 at a time downstream of AAUAAA sequence in all mRNA molecules (recognition); stabilises mRNA
genetic diseases caused by mutations in splice donor/acceptor sequence
thalassaemia, DMD
