Translation Flashcards
tRNA
A type of RNA that transports amino acids to the ribosome for protein assembly
An aminoacyl-tRNA-synthetase combines the tRNA and the amino acid at the 3’ end of the tRNA. At least one synthetase enzyme exists for each amino acid.
The tRNA then bonds to the appropriate codon (nucleotide triplet) on mRNA
How many codons are there?
Four different nucleotides taken in triplets
4^3 = 64
What is the start codon?
AUG
AUG also codes for methionine (contains sulfur)
Describe the structure of a ribosome
A ribosome has an mRNA binding site and 3 tRNA binding sites (A, P, E)
Polypeptide synthesis takes place at the peptidyl and aminoacyl sites (the P and A sites)
A site: holds the tRNA with its associated amino acid to the polypeptide chain. Proofreading occurs here and only a tRNA with a matching anticodon can bind to the ribosome
P site: holds the tRNA attached to the growing polypeptide chain. As the amino acid attached to the tRNA in the A site bonds to the polypeptide chain, it moves to the P site and the tRNA in the P site moves to the E site.
E site: tRNA is discharged here
Where is rRNA made?
In the nucleolus
Tetracycline
A drug that inhibits prokaryotic ribosome function. Since prokaryotic and eukaryotic ribosomes differ in structure and proteins, they can be targeted.
What are the steps of translation?
- Initiation: The initiator tRNA is loaded onto the the small ribosomal subunit, the anti-codon on the tRNA matches up with the start codon (AUG) on the mRNA. This codon also codes for Methionine and is placed in the P site to start. The A site is now ready for another tRNA carrying an amino acid. The Large subunit binds with the small subunit and elongation can now occur.
- Elongation: A tRNA matching the next codon enters the A site and a peptide bond is formed between the amino acids in the A and P sites. The tRNA’s move sites each shifting one over. The tRNA in the A site moves to the P site, and the tRNA in the P site moves to the E site for release. This is called translocation and it requires GTP. The process continues until it hits a stop codon.
- Termination: A stop codon is a codon that doesn’t code for any amino acid. Instead a release factor binds to the stop codon and ribosome initiating release of the mRNA from the ribosome. The finishing polypeptide enters the cytoplasm or into the Rough ER to finish any post-translational modifications.
What is the wobble hypothesis?
The third base of a codon and the third base of the anticodon may not always be complimentary. It’s pairing is not as strict as DNA
Point Mutation
One single base pair is mutated to the wrong base pairing
Sickle cell anemia is a type of this. A glutamic acid is replaced by a Valine on the beta chain of the amino acid, making the red blood cell more positively charged and bent.
Base Pair Substitution
A type of point mutation where one nucleotide and it’s complimentary base in the DNA have been changed to another.
Missense mutation
When a base pair substitution occurs causing an incorrect amino acid to be coded for.
Nonsense mutation
When a base pair substitution occurs causing a stop codon to be made prematurely.
Frameshift mutation
A type of point mutation where a base pair is deleted or inserted into the DNA sequence.
This spells disaster for the protein coding.
Transposons
jumping genes that jump around the genome.
Seen in both Eukaryotes and Prokaryotes.
The genes are cut and pasted to a new part of the DNA by enzymes and sometimes with the help of RNA.
Sometimes this helps repair DNA, sometimes this causes cancer or other diseases.
Physical Mutagens
Includes X-rays and UV radiation are potential mutagens. Gamma radiation is also a mutagen.
Xrays cause the formation of free radicals, and the repair enzymes often can’t restore proper function
UV radiation can cause Thymines that are next to each other to dimerize. Cells with a lot of thymine dimers will either self destruct or become malignant.
