Lecture 7- Translation and Protein Synthesis Flashcards
In which direction is mRNA translated?
5’ –> 3’ or N terminus –> C terminus
Define the term codon
A sequence of three nucleotides found on mRNA which code for a specific amino acid
What is the UTR and suggest it’s function
UTR= untranslated region
1) enhance translation
2) mRNA stability
How many different codons and amino acids are there?
Codons- 64 (4^3)
Amino acids -20
–> universal code is degenerate= amino acids are coded by more than one triplet codon
What is the start codon and amino acid?
Methionine = AUG
What are the three different stop codons?
UAA, UAG, UGA
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?
- 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)
What are the sizes of prokaryotic and eukaryotic ribosomes? Also state the sizes of their subunits
Prokaryotic= 70S (50S + 30S) Eukaryotic= 80S (60S + 40S) S= Svedberg, the unit of sedimentation in a centrifuge
Explain how the fidelity of the genetic code is maintained
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
Explain how amino acid tRNA synthetase is charged….
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
What happens during the initiation steps of translation?
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
Describe the steps involved in the ‘elongation’ phase
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
Describe the steps involved in the ‘termination’ phase
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
What are polyribosomes?
One ribosome does not translate one mRNA at a time but multiple ribosomes at a time to increase the rate of protein synthesis
Why do many antibiotics target protein synthesis?
Because we can exploit the differences between prokaryotic and eukaryotic ribosome and translation factors