Lecture 20: Translation Flashcards
For translation to work what needs to happen?
- Attach amino acids to tRNA
- Assemble ribosomes from rRNA and ribosomal proteins
- Then, the tRNA, ribosomes and mRNA can synthesise the new protein
What is tRNA?
Transfer RNA
It is responsible for conveying the correct amino acid to the correct codon during proteins synthesis
General features:
-there is at least one tRNA for each amino acid formed
-all tRNA have an amino acid binding site at the 3’ end where the base sequence is -CCA
-all tRNA’s bind to the target codon via the anticodon loop
-tRNA’s have a generalised clover leaf structure
-when folded the tRNAs have an inverted L shape
How are amino acids bound to tRNAs?
-The synthesis of proteins requires the amino acids are bound together in order determined by the base sequence of the mRNA
-the binding of 2 amino acids together via a peptide bond
-the binding requires an energy input
-therefore amino acids need to undergo an activation step prior to incorporation
-this activation step occurs during their binding to a tRNA molecule
-amino acids are bound to tRNAs by an amino-acyl tRNA synthetase
-these enzymes consume ATP in the process:
Slide 4
If there are 20 amino acids, then how does the right amino acid get to the correct tRNA
This is achieved by the aminoacyl-tRNA synthetase being very specific
- there is at least one enzyme for each amino acid
- these enzymes contain:
- an amino acid recognition site
- a tRNA recognition site
- thus, the specificity of the enzyme ensures that each amino acid is only bound to its corresponding tRNA
What are the various modified bases on a tRNA molecule.
U—> pseudoU, T and dihydroU
G—> methylG and dimethylG
Aminoacyl-tRNA
- binding of the amino to the tRNA activates the amino acid.
- also, by being bound to a tRNA molecule with an anticodon, the amino acid can now be carried to the ribosome when the mRNA has the corresponding codon.
- to achieve specificity the amino acid must be bound to the correct tRNA
What are ribosomes?
-ribosomes are the actual site if protein synthesis
-consists of two molecules:
-rRNA
-protein
-the proportion of RNA and proteins differ between prokaryotes and eukaryotes
Bacterial ribosomes: 50% protein, 50% rRNA
Rat liver ribosomes: 60% protein, 40% rRNA
Ribosomal structure
Consists of 2 subunits- a small and a large subunit
-the overall structure of eukaryotic and prokaryotic ribosomes are very similar
Prokaryotic and eukaryotic ribosomal structure differ considerably in:
- Size of subunits
2. Number of proteins associated
Ribosome role:
Small subunit
Large subunit
Small subunit:
-binds mRNA and
-this complex then binds the first aminoacyl-tRNA
-note that bacteria use a modified methionine (N-formal methionine) to initiate translation
Large subunit:
-binds to the complex above
-contains the enzyme peptidyl transferase
-is responsible for the formation of peptide bonds
Each complete ribosome contains two different binding sites:
- a site (aminoacyl site)- involved in positioning the incoming aminoacyl-tRNA
- P site (peptidyl site)- binds temporarily the growing peptide chain
- E site (also known as the exit site)
What are the 4 steps involved in protein synthesis
- Activation of amino acids.
- Initiation of translation on the ribosomes
- Elongation of the protein chain
- Termination of translation and release of the completed protein
Initiation of translation:
- small subunits binds to the mRNA, seeking out the initiating AUG
- met-tRNA anticodon base pairs with the AUG
- large subunits binds such as the initiating AUG is positioned over the P site
Termination of translation
UAA, UAG, UGA are recognised as STOP codons
- there are no corresponding aminoacyl-tRNAs
- stop codon is recognised by a protein release factor
- this triggers the peptidyl transferase to become a polypeptide releasing enzyme
- severs the linkage between the polypeptide (protein) and the last tRNA
Transfer of the mRNA takes place in what direction
Synthesis of the protein takes place in a
5’–>3’ direction
NH2 terminal –> COOH terminal direction
