(Dr. Heinemann) (Unit C) Topic 21 Flashcards
How does mRNA code for amino acids?
Codons
* 3 bases per amino acid
Characteristics of the genetic code
- Universal (in all organisms, exceptions occur)
- Nonoverlapping
- No gaps
- 61 codons for 20 amino acids (redundant)
- 3 stop codons, 1 start codon
Where does redundancy occur?
At the 3rd position of the codon (wobble)
Why do more common amino acids have more codons?
More codons means the greater chance of coding for those amino acids (hence, increasing how prevalent they are)
Why do related amino acids have similar codons?
As they are related (similar), the codons would also be related/similar
tRNA
- Required for translation
- Acts as the vehicle that bring amino acids to the growing peptide chain
- ~80 nucelotides in length
- Cloverleaf secondary structure
What part of the tRNA hybridizes with the codon?
Anticodon
Where is the amino acid linked to on the tRNA?
3’ end
What is wobble? How does it happen?
Accurate base pairing for some tRNAs only requires matching at the first 2 position of the codon
* If the 3rd position doesn’t exactly match, it “wobbles”
Aminoacyl tRNA synthetases
Amino acid activation
* Uses ATP as energy source
* Transfers energy to a high-energy bond between amino acid and 3’ end of tRNA
What is amino acid activation by aminoacyl tRNA synthetases important for?
- Providing an energy source for later peptide bond formation
- Providing specificity by matching the correct amino acid to the specific tRNA
- Some synthetases have proofreading activity (ensures correct amino acid is ligated to the tRNA)
Ribosomes
Large multimeric protein RNA complexes
Structure of Ribosomes
Two subunits
* Large and small
* Both composed of RNA (rRNA) and protein)
Composition of large and small subunits of ribosomes in eukaryotes
- Large: 49 proteins and 3 RNAs
- Small: 33 proteins and 1 RNA
Functions of small and large subunits in ribosomes
- Small: Matches tRNAs to the codons
- Large: Catalyzes the formation of peptide bonds
What are the sites for tRNA in ribosomes?
3 sites
* A (aminoacyl tRNA) site
* P (peptidyl tRNA) site
* E (exit) site
How many sites of the ribosome is occupied at any one time?
2
How is mRNA read?
5’ to 3’
Where does translation begin?
Start codon (AUG, for methionine)
How does initiation of translation work in eukaryotes?
- Initiator tRNA loaded onto small subunit with initiation factors (proteins)
- Small subunit recognizes 5’ cap, moves in 5’ to 3’ direction until it finds AUG
- Initiator factors dissociate, large subunit binds to small subunit (initiator tRNA is positioned at the P site)
In initiation, where is initiator tRNA located in the ribosome?
P site
How does translation initiation work in bacteria?
Ribosome binding sites recruit the ribosome
* Bacteria have open reading frames which means multiple proteins can be translated from RNA
Where is the growing polypeptide attached to in the ribosome?
tRNA in the P Site
How does the polypeptide chain shift from one tRNA to the next?
The energy in the aminoacyl bond between polypeptide chain and tRNA is used to form a peptide bond between the polypeptide chain and the amino acid of the tRNA in the A site
During the formation of the peptide bond, what else happens?
- E site exits
- P site moves to E site
- A site moves to P site
- Recruits new A site
What is the peptide bond formation catalyzed by?
Peptidyl transferase
What causes the large subunit to move forward and thus moving the small subunit forward?
The peptide bond formation reaction causes conformational shift that causes the large subunit to move forward
How is new incoming aminoacyl-tRNA delivered to the ribosome?
via Elongation Factor EF-Tu (in bacteria)
How does EF-Tu release the amino acid to the tRNA?
Only if it pairs
* GTP is actually complexed with the EF-Tu
* GTP is hydrolyzed into GDP
* The energy allows bond to form between amino acid and tRNA
How does translation termination work in bacteria?
Specific termination factors (proteins) known as release factors (RF)
Describe the mechanism of termination of translation in bacteria
- RF is slow at attaching
- When the ribosome reaches a stop codon, it cannot translate
- This slowdown gives RF time to attach
- The addition of a water molecule at the end of the chain allows for the polypeptide chain to detach
- GTP hydrolysis allows the RF to detach
