Lecture 7- Translation and Protein Synthesis Flashcards

1
Q

In which direction is mRNA translated?

A

5’ –> 3’ or N terminus –> C terminus

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2
Q

Define the term codon

A

A sequence of three nucleotides found on mRNA which code for a specific amino acid

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3
Q

What is the UTR and suggest it’s function

A

UTR= untranslated region

1) enhance translation
2) mRNA stability

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4
Q

How many different codons and amino acids are there?

A

Codons- 64 (4^3)
Amino acids -20
–> universal code is degenerate= amino acids are coded by more than one triplet codon

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5
Q

What is the start codon and amino acid?

A

Methionine = AUG

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6
Q

What are the three different stop codons?

A

UAA, UAG, UGA

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7
Q

5’ CGC UGA AUG CAU GGG CGC CUG AAG GUA AAG ACG UCG GCU GAA GAG CAG GCA GAG GCC AAA AGG CAG UGA GCC 3’

In this sequence where would you start and stop translation and why?

A
  • ribosomes start scanning at 5’ end
  • start at first AUG start codon
  • continues in frame reading triplet codons
  • stops scanning at first in frame stop codon (UGA)
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8
Q

What are the sizes of prokaryotic and eukaryotic ribosomes? Also state the sizes of their subunits

A
Prokaryotic= 70S (50S + 30S)
Eukaryotic= 80S (60S + 40S)
S= Svedberg, the unit of sedimentation in a centrifuge
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9
Q

Explain how the fidelity of the genetic code is maintained

A

Aminoacyl tRNAs
Specific tRNA for the amino acid that it carries on the 3’ receptor
Anticodon on stem hydrogen bonds with the codon on mRNA

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10
Q

Explain how amino acid tRNA synthetase is charged….

A

Specific tRNA synthetase for specific amino acid

  • specific amino acid binds to the enzyme
  • ATP binds to the enzyme
  • covalent bond forms between AMP and amino acid
  • specific tRNA binds to enzyme, anticodon and 3’ receptor are specific to enzyme
  • AMP dissociates from enzyme and phosphodiester bond forms between amino acid and tRNA = charged
  • amino acyl tRNA dissociates from enzyme
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11
Q

What happens during the initiation steps of translation?

A

1) The ribosomal subunits dissociate (40S + 60S)
2) Assembly of pre-initiation complex
- EIF4E and EIF4G bind with the 5’ cap which is recognised by the 40S/ Met-tRNA/eIF2
- 40S= mRNA and tRNA recognition
3) Binding of mRNA to pre-initiation complex. Initiator Met binding sets frame
4) Binding of 60S subunit. GTP–> GDP + Pi . Met tRNA binds to ‘peptidyl’ P site on ribosome
5) eIF2 and GDP dissociate

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12
Q

Describe the steps involved in the ‘elongation’ phase

A

1) Binding of tRNA to amino acyl (A) site of ribosome
2) Catalysis of peptide bond by peptidyl transferase on 60S
3) Translocation of peptidyl tRNA from A to P with help of elongation factors to help move ribosome along mRNA
4) repeat

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13
Q

Describe the steps involved in the ‘termination’ phase

A

1) Recognition of the step codon.
- vacant A site recognised
- Release factors not tRNA bind to stop codon
2) Release of peptide chain.
- Peptidyl transferase catalyses the transfer of the completed protein chain to water
- protein is released from ribosome

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14
Q

What are polyribosomes?

A

One ribosome does not translate one mRNA at a time but multiple ribosomes at a time to increase the rate of protein synthesis

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15
Q

Why do many antibiotics target protein synthesis?

A

Because we can exploit the differences between prokaryotic and eukaryotic ribosome and translation factors

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16
Q

Name some antibiotics and the stages of translation they interfere with

A

1) Streptomycin- inhibits initiation
2) Tetracycline- Inhibits aa-tRNA binding
3) Erythromycin- inhibits translocation
4) Chloramphenicol- inhibits peptide transferase
5) Puromycin- terminates elongation prematurely

17
Q

Where does protein translation take place?

A

In the cytoplasm (+some in mitochondria)

18
Q

What do transmembrane/ secretory proteins have?

A

Signal sequences

  • first 20-24 aa
  • N-terminus of pp
  • mainly hydrophobic aa’s which like to sit within the lipid bilayer
19
Q

Describe the process of synthesising proteins on the RER

A

1) SS is recognised by a protein-RNA complex (signal recognition particle)
- SRP binds to ss
- translation is halted
2) SRP binds to SRP receptor on RER surface
- Translation resumes
3) Translocation of protein chain into RER lumen
- as protein is being translated SRP-receptor binding triggers assembly of protein channel within RER membrane
- PP chain enters lumen across the channel
4) SS is cleaved by signal peptidase once protein has entered. Protein is folded. SS degrades in lumen.

20
Q

What is the difference between the synthesis of transmembrane and secretory proteins?

A

Transmembrane have an extra hydrophobic sequence at the C-terminus holding them in the membrane

21
Q

List some of the post-transitional modifications that could be made.

A
  • disulphide bond formation
  • proteolytic cleavage
  • addition of carbohydrate
  • addition of phosphate
  • addition of lipid groups
  • hydroxylation
22
Q

Describe the post translational modifications that occur to produce insulin

A
  • synthesised on RER ribosomes PREPROINSULIN
    1) SS is cleaved and degraded
    2) Disulphide bonds between two cysteine residues. 3 disulphide bonds form + folding –> PROINSULIN
    3) Proteolytically cleaved in 2 positions which releases C chain.
    4) Packaged into secretory vesicles. Has A and B chains held together by 3 x S-S bonds