Translation 1- Genetic Code/Translation Machinery Flashcards
Why is the genetic code triplet?
There are 20 types of amino acid and 4 bases, meaning the code must be at least triplet.
Why is the genetic code degenerate?
There is a redundancy and some amino acids are encoded by more than one codon.
Is the redundancy in the genetic code random?
No- it is always in the 3rd base of a codon.
Why is the redundancy in the genetic code always in the 3rd base of a codon?
Due to wobble base pairing- between the 3rd base of the codon and the 1st base of the anticodon of tRNA. This is because of modified bases in the tRNA.
How did Crick and Brenner show that the genetic code was triplet?
By mutational analysis.
What was Nirenberg’s experiment?
Adding synthetic polynucleotides to bacterial extracts and showed they could make polypeptides.
How was the genetic code deciphered?
- Synthesis of all triplet combinations, bound with ribosomes on an assay. 2. Identification of the amino acid produced by each combination.
How are amino acids joined to tRNA molecules?
At CCA found at 3’ end of tRNA.
Describe the structure of tRNA.
L-shaped with the anticodon at one end and the amino acid at the other. Folded on itself due to intramolecular base pairing. Base stacking due to stem loop stacking.
Where are modified bases found in tRNA?
The D arm and the TWC arm.
Where are the variable regions in tRNA?
Bases in the anticodon and the extra arm.
What is tRNA charging?
The covalent linkage of amino acids to tRNAs.
How do tRNAs bind to the correct amino acid?
Aminoacyl tRNA syntetases can recognise subtle differences in amino acid structure.
What happens if a tRNA is mischarged?
tRNA synthetases has proof reading activity and will hydrolyse the incorrect amino acyl- tRNA bond.
What is the size of a ribosome?
~18nm in diameter.
What subunits are found in bacterial ribosomes?
50S and 30S.
What subunits are found in eukaryotic ribosomes?
60S and 40S.
How can RNA form the structures needed for catalysis and what does this suggest?
By folding on itself, can be considered a ribozyme, suggesting that life began in an RNA world.
What is the rate of amino acid addition in ribosomes?
Eukaryotic- 2 amino acids per secondBacterial- 20 amino acids per second
How many binding sites are there in a ribosome?
4 - one for the mRNA, three for the tRNA (A, P, E)
Do tRNA genes have introns?
Some eukaryotic tRNAs have introns, however bacterial tRNAs do not.
Why is wobble base pairing stable?
The binding of the first 2 bases provides enough binding strength to allow the wobble at the 3rd codon position.
Describe the secondary structure of tRNA.
CCA at the 3’end- A involved in charging.
TWC loop- 5bp stem, contains pseudouridine
Variable arm- 3-21nts, contains modified bases
Anticodon arm- 5bp stem, contains anticodon
D-loop - 3/4 bp stem, contains dihydrouridine.
Acceptor stem- 7bp stem, containing G-U base pair.
Self complementary regions in each stem allow clover structure.
How is translation fidelity increased in the anticodon arm?
Modified bases found either side of the anticodon strengthens anticodon-codon base pairing.
Describe the tertiary structure of tRNA.
L- shaped, A-form RNA like helix. Maintained by hydrogen bonding and stacking interactions. Keeps most bases inaccessible to solvent (apart from anticodon and CCA).
What is the stereochemical theory for genetic code evolution?
Thought that amino acids may have originally bound directly to mRNA codons by binding complementarity and similar hydrophobicity. Therefore the genetic code was based on this complementarity between codons and amino acids.
What makes up the prokaryotic 50S ribosomal subunit?
Proteins L1-L31, 5S and 23S rRNA
What makes up the prokaryotic 30S ribosomal subunit?
Proteins S1-S21 and 16S rRNA.
What does the eukaryotic 60S ribosomal subunit consist of?
Proteins L1-L50, 28S, 5.8S and 5S rRNA
What does the eukaryotic 40S ribosomal subunit consist of?
Proteins S1-S33 and 18S rRNA.
How are rRNA genes expressed in prokaryotes?
As polycistronic transcripts that are cleaved by RNases and methylated to give mature rRNAs.
How are rRNA genes processed in eukaryotes?
Can be self splicing
Describe the ribosomal proteins.
Rich in basic amino acids (allows binding to nucleic acids). Contain the RNA recognition motif (RRM).
Describe polyribosomes.
There is approximately 1 ribosome for every 80nts of mRNA. Once an active ribosome has passed its initiation site, another ribosome can also initiate translation- more efficient protein production.
