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CMB2001 TEST QUESTIONS WITH CORRECT ANSWERS
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CMB2001 TEST QUESTIONS WITHCORRECT ANSWERS
Why does RNA degradation take place? - ANSWER Damaged mRNA
Incorrectly transcribed/processed mRNA
Control gene expression
What 2 things control mRNA levels? - ANSWER Transcription and turnover
Why is turnover important? - ANSWER If mRNA has a ridiculously long half-life
and doesn't go away, then if you stop transcription you don't stop protein
expression because the mRNA is still there
Describe what casein mRNA is and explain the importance of turnover - ANSWER
Expressed in mammary glands
mRNA increases ~70fold on stimulation by prolactin
But transcription only increases ~2 fold
HOW?
- Half-life increases dramatically (~40fold) in response to prolactin
- Poly(A) tail length is increased which is an indication of the mRNA being
stabilised
- 3'UTR of RNA binds proteins which aid in this stabilisation
What do mRNAs look like during translation? - ANSWER Circular
Why is the mRNA circular during translation? - ANSWER Monitors mRNA
integrity - it will not be circular if it has lost the cap or poly(A) tail
Bring ribosomes ending translation close to the AUG (it is suggested that
ribosomes recycle onto the 5' end)
Helps translation because it helps reinitiation of translation
,Stabilises the RNA because most processes that target RNA for degradation start
through either the cap or the poly(A) tail and to get to these you have to break open
the circular structure
Describe Phase 1 of mRNA degradation - ANSWER The breakdown of the RNA
Start by either removing the Poly(A) tail and/or the cap to make it accessible for
exonucleases
Decapping enzymes - DCP1, DCP2
Endonucleases - Argonaute, Swt1, Smg6
Deadenylases (remove the poly(A) tail) - Ccr/Not complex
Describe Phase 2 of mRNA degradation - ANSWER Degrade with exonucleases
5' t 3' exonuclease
3' to 5' exonuclease
What is the exosome? - ANSWER The exosome is the main 3' to 5' exonuclease in
the cell
It is involved in RNA turnover and processing
It is a multisubunit complex
It has multiple nuclease activities: RRP6 and RRP44
The rest of the subunits function in RNA binding and unwinding
It degrades the RNA once the Poly A tail has been removed
What is XRN1? - ANSWER 5' to 3' exonuclease
Involved in RNA turnover and processing
Also involved in transcription termination
Functions after decapping of the mRNA
What is Deadenylation-dependent decay? - ANSWER Degradation where the Poly
A tail has to be shortened or removed before degradation occurs
, So generally the length of the PolyA tail is an indication of how long the mRNA
will survive
When is translation initiation stopped? - ANSWER As soon as the cap is removed
What causes Deadenylation-dependent decay? - ANSWER There is a number of
elements within the RNA that target and promote deadenylation:
- AU-rich element (ARE) - sequences that RNAs have that cause them to be
unstable
- Nonsense codon - mutation that introduces a STOP codon in the middle of the
coding sequence
- c-fos major coding determinant
- miRNA recognition site
- miRNA - miRNAs bind 3'UTR of mRNAs and promote deadenylation
Describe Deadenylation-independent decay - ANSWER It is autoregulated
(negative feedback) - this is by Rps28B binding to its own message which recruits
Edc3
Edc3 then promotes the decapping complex and starts degradation
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