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MM6 - Transcription (ii) transcription in eukaryotes is complex for 4…
MM6 - Transcription (ii) transcription in eukaryotes is complex for 4 reasons...
1) Spatial + temporal stages
regulatory proteins must separate DNA from histones so transciption can occur
time for mRNA processing is essential before going to ribosomes
2) numerous RNAPs
1 in nucleolus, makes rRNA
2 in nucleoplasm, makes mRNA precursors (primary transcripts)
3 in nucleoplasm, makes tRNA + 5S rRNA
mitochondrial RNAP in mitochondria, makes mtRNA
3) Complex control
multiple genes to be transcribed
needed to be switched on/off
differentiated/ selective expression
promotor not sufficient alone to drive RNAP
activators: spreed up transcription by binding to sites called enhancers (type of upstream promoter element)
coactivators: integrate activator signals
basal transcription factors: position RNAP correctly @ start site
repressors: slow transcription by binding to sides called silencers (type of upstream promotor element)
4) Post-transcriptional mRNA processing
heterogeneous nuclear (hn) RNA collects primary transcripts
primary transcripts must be extensively modified...
5' capping
7-methyl-guanosine inserted upside-down @ 5'end
capping enzyme = guanyltransferase
protects 5' end from nuclease + guides mRNA through nuclear pore
permits translation initiation
Cleaving
polyadenylation sequence recognised, endonuclease recruited to cleave
tailing
poly(A) polymerase adds 40-250 A resides @ cleaved 3' end
tail length determines lifespan of strand in cytoplasm
long - can be translated multiple times
short - translated once
multiple copies of poly(A) binding protein bind to tail
stabilises mRNA
facilitates mRNA exiting from nucleus
retards action of 3'-exonucleases
Splicing
necessary to cut out non-coding regions (introns, 50-10000 nucleotides) + join coding regions (exons)
introns removed after capping, before strand enters cytoplasm
facilitated by small nuclear (sn) RNA binding with proteins to form sn ribonucleoproteins (the spliceosome)
spliceosome recognises sequences @ end of intron
adenine attacks 5' end of intron, forms RNA loop
3' end of adjacent exon joins covalently with 5' end of other econ, excising intron loop