The genetic code/translation Flashcards
What functions do RNAs perform in the ribosome?
rRNAs provide most critical functions of the ribosome
- catalytic activity
- mRNA and tRNA docking
- subunit assembly
- proof reading
- stimulating GTP hydrolysis
Where does energy for translation come from?
GTP hydrolysis
How many ORFs are there in a sequence?
3
What are the major properties of the genetic code?
Specificity - Each codon is translated into only one amino acid
Degenerate - >1 codon can encode most amino acids
Non-overlapping – mRNA is read 3 bp at a time
Almost universal – most organisms use the same code
What changes in base pairing can occur with wobble?
G can bind C or U
U can bind A or G
I can bind U or A or G
How are amino acids linked to tRNAs?
Enzyme is tRNA synthetase
1) Activation of the amino acid
ATP is cleaved and AMP is covalently linked to the amino acid
2) tRNA linkage
covalent linkage of the aminoacyl group to the tRNA3’ aminoacyl esters
Where does the energy for amino acid activation come from?
Cleavage of PPi
How does the cell make sure the right AA is attached to each tRNA?
Synthetases can proof read
- can detect an incorrect aminoacyl-AMP
- will hydrolyze
- can detect an incorrect aminoacyl-tRNA
- will hydrolyze
What are the functions of the 2 ribosomal subunits?
Ribosome has 2 subunits – large and small subunit - for initiation small ribosome subunit binds mRNA
- anti-codon of initiator tRNA binds to initiation codon
- large subunit recruited into complex
What are the major sites in the ribosome/
Ribosome has 2 important sites
- the P site = peptidyl site
- the A site = aminoacyl site
- prokaryotic ribosomes also have an E site, where empty tRNA is bound
How is mRNA read by the ribosome? In what direction is protein made?
mRNA is read 5’ to 3’; protein is made from N-terminus to C-terminus
What are the steps in translational elongation?
3 steps to elongation –
1) codon/anti-codon base pairing in the A site
2) peptide bond formation
3) transfer of the peptidyl-tRNA to the P site
What does peptidyl transferase do?
Peptidyl transferase activity catalyzes peptide bond formation
“transpeptidation rxn – α-amino nitrogen from the A site – nucleophillic
attack on the carbonyl of the P site amino acid
- growing peptide chain is now attached at the A site
How is translation stopped?
Stop codon cannot bind tRNA
Instead of a tRNA, a protein releasing factor binds to the A site
There is a conformation change and the peptidyl transferase activity is
altered – now functions as an esterase
Esterase cleaves the ester between the peptide and the tRNA in the P site
Ribosome releases mRNA and dissociates into its subunit
What is one major difference between prokaryotic and eukaryotic translation?
Eukaryotic translation has lots of translation factors associating with ribosome
