eukaryotic post transcriptional gene regulation Flashcards
Why is RNA less stable than DNA
prone to hydrolysis
OH on 2’ carbon makes it less stable
mRNA processing- 5’ capping
modified guanine nucleotide
5’-5’ triphosphate bridge
Protects mRNA from degradation by 5’ exonucleases
helps ribosomes recognise start of translation
Stability
polyadenylation
once transcription complete, adenines added to 3’ end by polyA polymerase
PolyA tail
stabilises mRNA
protects against 3’ exonucleases
longer tail=longer lifespan in cytoplasm
helps mrna interact with proteins involved in ribosome recruitment to the mrna at 5’ end-loop forms
mRNA degradation
3’ exonucleases remove PolyA tail
then 5’ exonucleases remove 5’ cap and breakdown mRNA 5’ to 3’ direction
purpose of mRNA loop forming
5’ close to 3’
efficient translation: ribosome synthesis around this loop structure and when it gets to the 3’ end, its essentially been brought back to the beginning so can keep translating protein
Enhances stability
as poly A tail degraded, fewer poly A proteins can bind, destabilises interaction between tail and 5’ cap
Without poly A binding proteins, 5’ cap exposed to capping enzymes and the mRNA is susceptible to degradation by nucleases
transcriptome
array of mRNA transcripts produced in a cell
RNA sequencing
poly T beads bind to poly A tails
poly T primers bind to tail-recognised by reverse transcriptase
cDNA
spliceosome
5 small ribonuclear proteins
snips out introns and pulls exons together
mRNA splicing
Spliceosome recognises splice sites (5’ splice site marked as GU and 3’ marked as AG)
Branch point within intron contains an adenine
sequence from this adenine to the 3’ splice site is rich in CU
2 transesterification reactions
mRNA splicing
transesterification 1
attack of the 2’ OH from the branch point adenine onto the phosphate of the guanine at the 5’ splice site
reaction requires ATP
results in the cleavage of the 5’ end of the intron
lariat forms where intron loops back on itself and is held by covalent bond to branch point
mRNA splicing
transesterification 2
the newly exposed 3’ OH at the end of the exon attacks the 3’ splice site phosphate on the guanine, using ATP
leads to cleavage of 3’ end of the intron and ligation of the two exons, resulting in seamless joining of the exons together and the release of the intron in its lariat form
after splicing is complete, the lariat intron is degraded by exonucleases in the nucleus
alternative RNA splicing
expands the proteome: A single gene can
produce multiple proteins variants by using different combinations of exons
RNA editing
post-transcriptional process that modifies RNA molecules, leading to changes in their
sequence
2 types of RNA editing
A to inosine (G equivalent in DNA)
C to uracil (T equivalent in DNA)
adenine and cytosine are deaminated in this process
miRNA
small non coding RNAs
inhibits translation
if an miRNA perfectly binds to mRNA, the mRNA is degraded
if its a partial fit, it is simply blocked
