13 Questions
Which release factor recognizes the stop codons UAA and UAG?
RF-1
What do release factors cause to be released from the P site tRNA?
The peptide chain
Which stop codons are recognized by eRF-1?
All three stop codons (UGA, UAA, UAG)
Where must release factors act to perform their function?
A site
Which of the following statements is correct regarding stop codons and tRNAs?
Stop codons are not recognized by any tRNA
What is guided to the 30S ribosomal subunit by the Shine-Dalgarno sequence?
mRNA
Which initiation factor mediates the binding of formylmethionine tRNA (fMet) to the peptidyl site?
IF-2
Which of the following components is part of the 30S subunit complex?
mRNA
Where is the Shine-Dalgarno sequence generally located relative to the AUG start codon?
8 bp upstream
During elongation, what happens when the A site is filled?
The E site is emptied
How many sites are there in a ribosome for elongation?
3
Which of the following binds to the P site during the initiation stage?
Formylmethionine tRNA (fMet)
What is recycled by the third elongation factor (EF) during translocation?
First elongation factor
Study Notes
Coupled Transcription and Translation in Bacteria
Termination
- STOP codons: UGA, UAA, and UAG have no corresponding tRNAs
- Release factors (RF) recognize STOP codons:
- RF-1 recognizes UAA and UAG
- RF-2 recognizes UGA and UAA
- eRF-1 recognizes all three STOP codons
- RFs act at the A site and require an occupied P site
- RFs cause the polypeptide chain to be released from the P site tRNA
- RFs then escape from the complex
Translation Initiation in Prokaryotes
Stage 2: Initiation
- Ingredients:
- 30S subunit of the ribosome
- Initiator tRNA (tRNAMet)
- mRNA
- Initiation Factors IF-1, IF-2, and IF-3
- One molecule of GTP
- 50S subunit of the ribosome
Formation of the Initiation Complex
- mRNA is guided to the 30S ribosomal subunit by the Shine-Dalgarno sequence
- Shine-Dalgarno sequence is complementary to a sequence in ribosomal RNA and is a ribosomal binding site in the mRNA
- Generally located 8 bp upstream of the AUG start codon
Formation of the Initiation Complex
- IF-1, IF-2, and IF-3 are initiation factors
- Sites on the ribosome:
- P site (peptidyl site)
- A site (aminoacyl site)
- E site (exit site)
Formation of the Initiation Complex
- Large 50S subunit combines with the 30S subunit to form the initiation complex
- Formylmethionine tRNA (fMet) binds to the peptidyl site (P site) mediated by IF-2-GTP
Translocation
- Elongation requires another elongation factor for translocation
- All amino acids move along one space, resulting in an empty A site
- When the A site is filled, the E site is emptied
- The 3rd EF recycles the first elongation factor
- This process continues until a STOP codon is reached
Elongation (BOTH eukaryotes & prokaryotes)
- All ribosomes have 3 sites: A, P, and E
- Initiator tRNA is in the P site
- Next amino acid is escorted to the A site
- The small subunit and base pairing "check" that the amino acid is correct
- Correct amino acid causes a conformational change and hydrolysis of bound GTP
- EFs are released, and a peptide bond is formed
This quiz covers the process of coupled transcription and translation in bacteria, including termination and initiation stages. It focuses on the role of release factors and STOP codons in polypeptide chain release.
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