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Flashcards in Lecture 16 Deck (19):

Gene expression in prokaryotes compared to eukaryotes:

- 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


Translation in prokaryotes:

Mechanics between prokaryotes and eukaryotes are very similar


Cap recognition:

- Important in translation
- Indicates where the 5' end of the transcript is
- Important in determining where initiation is going to happen


Pre-initiation compex:

- Scans along the RNA until a ATG is reached
- Base pairing between the ATG and tRNA occurs
- 60S subunit binds to elongation to occur


Translation initiation codon:

- 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


Dicistronic mRNAs:

- Internal translation initiation via IRES (internal ribosome entry sites)
- Recognition of 4F


mRNA translatability:

- 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,


What does polyA tail length determine?

- 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


Nonsense mediated mRNA decay (NMD):

- An RNA surveillance mechanism
- Directs degradation of mRNAs containing premature stop codons
- Prevents production of truncated gene products and defected proteins


NMD mechanism:

- 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


Premature termination due to abnormal splicing produces:

- UpF triggers mRNA degradation
- The mRNA is degraded


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:

- 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.


Programmed frame-shifting:

- 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


Polypeptide maturation is a post translation method of modifying:

- Folding, co-factor binding, interaction with other polypeptides
- Chaperones assist correct folding, using energy from ATP hydrolysis
- Incorrectly folded and aberrant proteins are degraded



- 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


Mono ubiquitylation:

- Histone regulation






- Ub joined end to end on the protein
- Proteasomal degradation
- DNA repair



- Intervening protein segments
- Removed from proteins by self-splicing (autocatalytic)
- Rejoin flanking protein (exteins)
- Similar to transposons but are protein based, not DNA based.