Gene Expression - Translation Flashcards
How many bases in polynucleotides?
4
What specifies mRNA nucleotide sequence?
DNA
What does the genetic code consist of?
non-overlapping triplets which are read from a fixed starting point
Triplet codons:
64 codons specify 20 amino acids - 61 ‘sense’ and 3 ‘stop’ codons or nonsense codons
Is genetic code identical in all organisms?
yes
Is the genetic code universal?
yes
What is the ‘open reading frame’?
the protein coding sequences in a gene
For most amino acids…
- there is more than one codon
- the genetic code is ‘degenerate’
The genetic code is not ambiguous meaning?
each codon specifies only on amino acid
Types of RNA involved with translation:
- mRNA
- tRNA
- rRNA
Why is an adaptor or interpreter required?
as we have no direct pairing between the codons and amino acids
How do tRNAs act as adapter molecules:
- bridges the codon to a specific amino acid
- each tRNA becomes covalently linked to one specific amino acid
- each tRNA has a triplet anticodon that recognises one or more codons in the mRNA by base pairing
In response to a specific codon, tRNAs…
bring the correct amino acid to the ribosome
What type of structure do tRNAs have?
cloverleaf structure
3 functions of tRNA:
- it binds to an amino acid, and is then “changed”; requires specific features in tRNA
- it associates with mRNA molecules through the anticodon loop
- it interacts with ribosomes
tRNA charging:
specific enzymes couple each amino acid to its appropriate tRNA molecule
What is tRNA catalysed by?
aminoacyl-tRNA synthetase
The accuracy of tRNA charging is controlled by what?
aminoacyl -tRNA synthetase
Amino acid selection and tRNA selection are subject to what?
proofreading
What does proofreading do?
either disfavours the forward reaction or reverses the catalytic reaction if the wrong component has been selected
What does proofreading avoid?
the need for absolute accuracy(which is slow)
tRNA molecules match what to what in mRNA?
amino acids to codons
Where does codon-anticodon recognition occur?
on the ribosome by the base pairing between the codon and anticodon
What does ‘wobble’ and modified bases allow?
one tRNA to decode more than one codon
What is ‘wobble’?
flexibility in pairing
Wobble hypothesis:
Watson-crick base pair rules between anticodon position 1 and codon position 3 are released due to flexibility and modifications of the anticodon loop
Can single tRNAs recognise multiple codons?
yes
Ribosomes:
holds mRNA and charged tRNAs in the correct positions to allow assembly of the polypeptide chain
What type of proteins can ribosomes make?
any meaning they’re not specific
In eukaryotes, how many molecules of rRNA and proteins in a large subunit?
3 rRNA and 49 proteins
In eukaryotes, how many molecules of rRNA and proteins in a small subunit?
1 rRNA and 33 proteins
rRNA structure:
rRNA is highly structured and the major structural and functional component
What is peptide bond formation catalysed by?
23s rRNA
How many sites for tRNA binding on the largest ribosomal subunit?
3
Codon-anticodon interactions between tRNA and mRNA occur at what sites?
P and A sites
A site:
binds with the anticodon of charged tRNA
P site:
where tRNA adds its amino acid to the growing peptide chain
E site:
where tRNA sites before being released from the ribosome
3 phases to polypeptide synthesis:
- initiation
- elongation
- termination
Initiation:
mRNA binding, start codon selection, binding the first aminoacyl-tRNA
Elongation:
sequential addition of amino acids as specified by the codons in the mRNA
Termination:
release of the completed polypeptide in response to a stop codon
What is a fidelity function?
small subunit rRNA validates H-bonds and if they’ve not formed between all three base pairs, the tRNA must be an incorrect match, and it is rejected
Do ribosomes have a fidelity function?
yes
3 steps of elongation:
- codon recognition - aminoacyl-tRNA entry into the A-site)
- peptide bond formation(met is linked to Pro in A-site)
- translocation(simultaneously, the free tRNA is moved to the E-site, the peptidyl-tRNA is moved to the P-site)
Is elongation a repeated process?
yes
What marks the end of translation?
stop codons
What are stop codons recognised by?
protein release factors
What do RRF(ribosome releasing factors) promote?
dissociation of the ribosomal subunits, tRNA and mRNA
What are proteins made on?
polyribosomes
What is a strand of mRNA with associated ribosomes called?
a polysome
