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Questions and Answers
Which site on the ribosome does the initiator tRNA occupy?
What happens to the amino acids during elongation translocation?
Which of these release factors recognizes all three stop codons?
Which elongation factor is responsible for recycling the first elongation factor?
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What is required for release factors (RF) to act at the A site?
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What process occurs when the small subunit and base pairing 'check' if the amino acid is correct?
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What is the role of the 40S ribosomal subunit in eukaryotic translation initiation?
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What differentiates translation initiation in eukaryotes from prokaryotes?
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According to Kozak's rules for efficient translation initiation in eukaryotes, what is the optimal consensus sequence around the AUG start codon?
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What is the function of the eIF4 family and Poly A Binding Protein (PABP) in eukaryotic translation initiation?
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Which component binds to the 5' CAP during eukaryotic translation initiation?
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What happens immediately before the 60S subunit binds in eukaryotic translation initiation?
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Study Notes
Protein Synthesis
- All ribosomes have 3 sites: A, P, and E
- Initiator tRNA is in P site, and next amino acid is escorted to A site
- The small subunit and base pairing "check" the amino acid is correct
Elongation
- Correct amino acids cause conformational change and hydrolysis of bound GTP
- EF's released and peptide bond formed
- Elongation needs 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
Translocation
- The 3rd EF recycles the first elongation factor
- This continues until a STOP codon is reached
Stage 3: Termination
- STOP codons: UGA, UAA, and UAG
- These have no tRNAs
- Are recognised by release factors (RF)
- RF-1 for UAA and UAG
- RF-2 for UGA and UAA
- eRF-1 recognises all 3 codons
- Act at A site and need an occupied P site
- RF's cause the chain to be released from the P site tRNA
- The RFs then escape from the complex
Coupled Transcription and Translation in Bacteria
- DNA duplex
- RNA polymerase
- Ribosome
- mRNA
- Direction of transcription
- Direction of translation
The Polysome
- Incoming ribosomal subunits bind to the 5' end of the mRNA
- The 40S subunit binds to the mRNA
- The 60S subunit binds to the 40S subunit
- The polypeptide chain is synthesized
Translation Initiation in Eukaryotes
- The amino group of the methionine on the initiator tRNA is NOT formylated (i.e. Met, not fMet)
- The initiation complex forms at the 5' terminus of the mRNA
- NO zone of Shine-Dalgarno-AUG start seqs
- The initiation complex scans the mRNA for an AUG initiation codon
- Translation usually begins at the 1st AUG found
- Kozak's Rules describe the optimal sequence for efficient translation initiation in eukaryotes
Kozak Consensus Sequence
- The sequence logo shows the most conserved bases around the AUG start codon from all human mRNAs
- The consensus sequence is ACCAUGG
Stage 2: Initiation in Eukaryotes
- Prepare the ribosome
- 40S subunit binds initiation factors (eIF1, 1A, 3, and 5)
- Prepare the 1st amino acid
- tRNA-Met binds to eIF2-GTP
- Prepare the RNA
- eIF4 family + Poly A Binding Protein (PABP) check ends of mRNA
- All merge, ATP-> ADP, all factors released
- 60S subunit binds
40S Ribosomal Subunit
- The ribosome (40S subunit) and mRNA interact indirectly
- BOTH ends of mRNA are bound
- EIF4F complex (containing EIFs 4E, G, and A) binds to 5' CAP
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Description
This quiz covers the process of elongation and translocation in protein synthesis, including the role of ribosomes, initiator tRNA, and elongation factors.
