Translation Flashcards
What is translation and where is it carried out?
The process of converting the information present in mRNA to proteins.
Translation is carried out by the ribosomes in the cytoplasm.
How many possible codes are there of a codon?
64 (4^3)
How many sense codons and stop codons are there?
61 sense and 3 stop/nonsense.
What is the main exception to the universal genetic code?
Mitochondria
What is the open reading frame?
The protein coding sequences in a gene.
Explain why the genetic code is not ambiguous?
Each codon codes for only one amino acid.
What amino acid does the start codon code for?
Methionine (AUG)
What does the stop codon code for?
No amino acid
What 3 RNAs are involved with translation?
mRNA - gene code
rRNA - components of ribosomes
tRNA - anticodon + amino acid
What is the adaptor between the codon and amino acids?
tRNA.
What is the role of the anticodon?
Recognises one or more codons in mRNA by base pairing, triggering tRNA to bring the correct amino acid.
Three functions of tRNA?
-Binds to an amino acids and is then charged
-It associates with mRNA molecule through anticodon loop
-Interacts with ribosomes.
What is the amino acid attachment site and how does it attach?
3’ OH group of tRNA molecule.
Covalent linkage of tRNA 3’ end to cognate amino acid.
Catalysed by aminoacyl-tRNA synthetase.
Linkage of amino acid is via an ester bond to OH group.
3 features of tRNA structure?
TC loop
D loop
Anticodon
How is accuracy of tRNA charging maintained?
Both amino acid selection and tRNA selection are subject to proofreading.
Proofreading either disfavours the forward reaction (kinetic proofreading) or reverses the catalytic reaction (chemical proofreading) if the wrong component has been selected.
Avoids the need for absolute accuracy and slowness.
Why does proofreading in tRNA charging matter?
Mutation affects the accuracy of the enzyme.
How do the 30 tRNA molecules recognise the 61 sense codons?
Wobble and modified bases allow one tRNA to decode more than one codon.
Wobble hypothesis - base pair rules between anticodon position 1 and codon position 3 are relaxed due to flexibility and modifications of the anticodon loop.
Single tRNA can recognise multiple codons due to inosine wobble as can base pair with uracil cytosine and adenine.
Wobble codon base and possible anticodon bases?
U - A,G OR I
C - G OR I
A - U OR I
G - C OR U
The ribosome - 50S crystal structure?
RNA forms (grey) the bulk of structure.
Protein (gold) fill in gaps between rRNA.
Critical role of rRNA in ribosome?
rRNA is highly structured.
Not just a scaffolding for proteins - most of proteins are non-essential.
Peptide bond formation is catalysed by 23S rRNA.
Subunit association and tRNA binding directly involve rRNA - specific changes in rRNA conformation are associated with these events.
Three sites for tRNA binding?
A (aminoacyl tRNA or entry) site - bind with the anticodon of charged tRNA.
P (peptidyl tRNA or donor) site - where tRNA adds its amino acid to the growing peptide chain.
E (exit) site - where tRNA sits before being released from the ribosome.
Translation steps?
Initiation
Elongation
Termination
Initiation steps?
Nucleotide sequences in mRNA signal where to start protein synthesis.
Start codon AUG methionine.
Recognition of correct start codon requires 3’ end of 16S of the small subunit.
Initiator tRNA in the P-site in bacteria and A - site in all other aminoacyl tRNAs.
Initiation factors facilitate assembly of the initiation complex.
3 steps in elongation?
Codon recognition
Peptide bond formation.
Translocation.
Events of codon recognition elongation?
Aminoacyl-tRNA entry into A-site.
Ribosomes have fidelity function small subunits rRNA validates H-bonds and if they have not formed between all three base pairs, the tRNA must be incorrect match, and its is rejected.
Events of peptide bond formation elongation?
Met is linked to pro in A site.
After peptidyl transfer there is a new peptidyl-tRNA in the A site and a deacylated tRNA in the P site.
Carried out by 23S rRNA.
Occurs when A and P sites are appropriately occupied.
Generates the new peptide bond.
Polypeptide is now in the A site.
Events of translocation in elongation?
Simultaneously, the free tRNA is moved to the E-site, the peptidyl-tRNA is moved to the P-site.
The deacylated tRNA is ejected via E-site the peptidyl-tRNA is moved to the P-site together with its codon and the next codon is exposed in the A-site.
The steps in termination?
Stop codons mark the end of translation.
Stop codons are not recognised by any aa~tRNA but rather by protein release factors.
Released factors promote hydrolysis of completed peptide from the tRNA and expulsion of the tRNA.
RRF promotes dissociation of the ribosomal subunits, tRNA and mRNA.
How are proteins made on polyribosomes (polysomes)
Several ribosomes can work together to translate the same mRNA producing multiple copies of the polypeptide.
A strand of mRNA with associated ribosomes is called polysome.
Cooperation between transcription and translation in bacteria?
During transcription of protein-coding genes, bacterial RNA polymerase is closely followed by a ribosome that is engaged in translation of the newly synthesized transcript.
As a result of translation-transcription coupling the overall elongation rate of transcription is tightly controlled by translation.
How to do some proteins fold?
While still being synthesized.
Growing peptide chain folded N-terminal domain.
Folding C-terminal domain.
Folding protein completed after release from ribosome.
