15 - Translation

Question 1

Function of ribosomes

Answer 1

Site of protein manufacture via translation
- provides a framework where tRNAs can be matched to codons

Question 2

What does the S mean in 70S ribosomes

Answer 2

Svedberg unit
- sedimentation coefficient
- offers a measure of a particles size based on its sedimentation rate
- I.e. how fast a particle of given size and shape ‘settles’ to the bottom of a solution

Question 3

Function of ribosomal proteins

Answer 3

Stabilise the ribosome

Question 4

How does a ribosome find mRNA to read and translate

Answer 4

• a few bases upstream of the start codon is a short stretch dominated by purines
• called Shine-Dalgarno sequence
• this sequence on mRNA base pairs with section of 16S rRNA and recruits riboosme

Question 5

Initiation of translation in bacteria - mechanism

Answer 5

• initiation factor 1 (IF-1)blocks the A site
• IF-3 prevents the premature assembly of the ribosomal S subunit and the ribosomal P subunit
• until both mRNA and an initiating tRNA are available
• S subunit binds the Shine-Dalgarno sequence (S-D sequence)
• IF-2 guides the initiating aminoacyl tRNA to tge P site
• L subunit is now recruited to 30S initiation complex
• initiation factors IF-1 and IF-2 are released
• the assembled ribosome is now ready to enter the elongation phase

Question 6

Key principle in initiation of translation in bacteria

Answer 6

• Initiation factors coordinate ribosomal assembly
• the result is a ribosome positioned around a specific initiator tRNA base-paired to a start codon in a specific site (the P site)

Question 7

Bacterial translational elongation mechanism

Question 8

Bacterial translation termination- mechansim

Question 9

How do peptide bonds form between amino acids in the ribosome

Answer 9

• formation of peptide bonds requires a peptidyl transferase activity
• ribosome acts as its own catalyst - a ribozyme
• reaction is catalysed by the 23S rRNA of the L subunit
• so the ribosome is a giant catalytic ribozyme
  (Findings have led to speculations of an RNA as a precursor to current life forms)

Question 10

Bacterial vs eukaryotic ribosomes

Answer 10

Bacterial:
- 70S ribosomes

Eukaryotic:
- 80S ribosomes
- bigger, more complex
- no clearly defined E site

Question 11

Eukaryotic vs bacterial translation - initiation differences

Answer 11

• eukaryotic mRNAs are capped (and tailed) by factors recruited by the C-terminal tail of RNA pol
• expect differences in initial mRNA recognition step
• eukaryotic mRNAs often (pseudo)circularised
• the 5’ and 3’ ends are bound by specific proteins that interact
• facilitates the re-binding of ribosomes

Question 12

Nomenclature of eukaryotic initiation factors (ribosomes)

Question 13

General functions of eukaryotic initial factors (eIFs)

Answer 13

• prepare the S small subunits
• to allow binding of charged initiator tRNA and the message
• to promote scanning to find the initiator codon by assembled ribosome
• facilitate scanning by unwinding secondary structures in mRNA
• to circularise mRNAs
• to facilitate dissociation of inactive 80S into 40S and 60S subunits

Question 14

Eukaryotic translational elongation

Question 15

Puromycin antibiotic function

Answer 15

• inhibitor of protein synthesis in bacteria
• Causes premature chain termination by acting as an analog of aminoacyl-tRNA (prokaryotes and eukaryotes)

Question 16

How does ricin work?

Answer 16

• RTB binds to cell surface receptors at plasma membrane via galactose residues
• holotoxin traffics to the ER (endo reticulum)
• PDI breaks the disulphide bond between the RTA and RTB
• RTA retrotranslocates (dislocates) from the ER to the cytosol
• …