Lecture 16 Flashcards
1
Q
Gene expression in prokaryotes compared to eukaryotes:
A
- Ribosomes in eukaryotes are bigger
- Transcription and translation are not coupled
- Internal translation initiation within an mRNA does not usually occur in eukaryotes
- Monocistronic mRNA
- Ribosomes are synthesised in the nucleolus and transported to the cytoplasm in eukaryotes
2
Q
Translation in prokaryotes:
A
Mechanics between prokaryotes and eukaryotes are very similar
3
Q
Cap recognition:
A
- Important in translation
- Indicates where the 5’ end of the transcript is
- Important in determining where initiation is going to happen
4
Q
Pre-initiation compex:
A
- Scans along the RNA until a ATG is reached
- Base pairing between the ATG and tRNA occurs
- 60S subunit binds to elongation to occur
5
Q
Translation initiation codon:
A
- Most translational initiation occurs at the first AUG, closest to the 5’ cap
- Efficiency is influenced by fit to a consensus sequence
- Usually (gcc)gccRccAUGG sequence
6
Q
Dicistronic mRNAs:
A
- Internal translation initiation via IRES (internal ribosome entry sites)
- Recognition of 4F
7
Q
mRNA translatability:
A
- 5’ cap and polyA tail
- PolyA tail determines if the mRNA will be translatable
- De-capping or shortening polyA tail precipitates degradation of mRNA
- A series of polyA binding proteins regulate translation initiation, recruit enzymes to add A’s,
8
Q
What does polyA tail length determine?
A
- It will determine how well it is translated
- The longer the tail, the more translation occurs
- A mechanism of regulating when you will get the product
9
Q
Nonsense mediated mRNA decay (NMD):
A
- An RNA surveillance mechanism
- Directs degradation of mRNAs containing premature stop codons
- Prevents production of truncated gene products and defected proteins
10
Q
NMD mechanism:
A
- Ribosomes displace exon junction complexes on mRNAs
- When premature termination due to nonsense mutation occurs this is due to ribosomes neglecting to displace complexes.
- Upf decaps transcripts and nucleases can degrade the entire mRNA
11
Q
Premature termination due to abnormal splicing produces:
A
- UpF triggers mRNA degradation
- The mRNA is degraded
12
Q
Non-stop mediated decay means that splicing is fine, but there is an error changing the reading frame so the correct stop codon is missed:
A
- Ribosome translates polyA tail (lys) and stalls at the end
- The poly-lycine tail produces a localised depletion of lycine, so the ribosome stalls to wait for more activated tRNAs.
- The stalling causes the nonstop mediate decay complex to interact, kick off ribosome and an exosome will degrade from the 3’ end.
13
Q
Programmed frame-shifting:
A
- Gag protein is generated when no frameshift mutation is present
- When a frameshift mutation is present everything in the protein is different from the frameshift onward
- A number of genes are overlapping in the viral genome, so frameshifting is not a problem! The frameshift produces a different protein
14
Q
Polypeptide maturation is a post translation method of modifying:
A
- Folding, co-factor binding, interaction with other polypeptides
- Chaperones assist correct folding, using energy from ATP hydrolysis
- Incorrectly folded and aberrant proteins are degraded
15
Q
Ubiquitin:
A
- Degraded by covalent attachment of Ub to lysine
- Extent and position of ubiquitiation affects the fate of the protein
- Attachment of Ubiquitin doesn’t always mean the protein will be degraded
