Podcast
Questions and Answers
What does 16S rRNA do in the small ribosomal subunit?
What does 16S rRNA do in the small ribosomal subunit?
It interacts with the RBS to position the AUG in the P-site.
What is the Shine-Dalgarno sequence?
What is the Shine-Dalgarno sequence?
It is a sequence element that acts as a ribosome binding site found just upstream of the start codon in many prokaryotic genes.
What binds to the Shine-Dalgarno sequence during translation?
What binds to the Shine-Dalgarno sequence during translation?
The small ribosomal subunit recognizes the sequence, specifically a region of the 16S RNA.
Describe how the Shine-Dalgarno sequence can vary.
Describe how the Shine-Dalgarno sequence can vary.
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What is the role of the 5′ cap during eukaryotic translation?
What is the role of the 5′ cap during eukaryotic translation?
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What happens to mRNA molecules that are missing a cap?
What happens to mRNA molecules that are missing a cap?
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What are the steps by which aminoacyl tRNA synthetases charge tRNAs?
What are the steps by which aminoacyl tRNA synthetases charge tRNAs?
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How can some organisms have fewer than 20 synthetases and still charge tRNAs with all 20 amino acids?
How can some organisms have fewer than 20 synthetases and still charge tRNAs with all 20 amino acids?
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Explain how EF-Tu escorts charged tRNA to the A-site.
Explain how EF-Tu escorts charged tRNA to the A-site.
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How do eukaryotic cells recognize the stop codon during translation?
How do eukaryotic cells recognize the stop codon during translation?
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How does recognition of the codon lead to the termination of protein synthesis?
How does recognition of the codon lead to the termination of protein synthesis?
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How does the degree of translation accuracy compare to DNA replication and transcription?
How does the degree of translation accuracy compare to DNA replication and transcription?
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What three key events must occur for the correct addition of an amino acid during translation elongation?
What three key events must occur for the correct addition of an amino acid during translation elongation?
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Is translation elongation identical between prokaryotes and eukaryotes?
Is translation elongation identical between prokaryotes and eukaryotes?
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To which of the three tRNA binding sites does the initiator tRNA bind?
To which of the three tRNA binding sites does the initiator tRNA bind?
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What are the four components of the translation machinery?
What are the four components of the translation machinery?
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What is meant by ORF?
What is meant by ORF?
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Explain the structure of tRNA.
Explain the structure of tRNA.
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What type of 3D structure does tRNA have?
What type of 3D structure does tRNA have?
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What is the size of the ribosome for prokaryotes and eukaryotes?
What is the size of the ribosome for prokaryotes and eukaryotes?
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The large and small subunits undergo association and dissociation during each cycle of translation.
The large and small subunits undergo association and dissociation during each cycle of translation.
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What are the names of the different sites in the ribosome?
What are the names of the different sites in the ribosome?
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What does it mean for tRNA to be charged?
What does it mean for tRNA to be charged?
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What is the function of each subunit in the ribosome?
What is the function of each subunit in the ribosome?
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What is the error rate of translation?
What is the error rate of translation?
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What type of catalysis occurs when forming a peptide bond?
What type of catalysis occurs when forming a peptide bond?
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What catalyzes the peptidyl transferase reaction?
What catalyzes the peptidyl transferase reaction?
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What mechanisms ensure that the correct aminoacyl-tRNA is inserted?
What mechanisms ensure that the correct aminoacyl-tRNA is inserted?
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Study Notes
16S rRNA and Shine-Dalgarno Sequence
- 16S rRNA helps position the AUG start codon in the P-site of the ribosome by interacting with the ribosome binding site (RBS).
- The Shine-Dalgarno sequence (5′-GGAGG-3′) is crucial for translation initiation in prokaryotes, typically found 3 to 9 nucleotides upstream of the start codon.
- Recognition of the Shine-Dalgarno sequence by the small ribosomal subunit is facilitated by complementary sequences in 16S RNA.
Variability of the Shine-Dalgarno Sequence
- Variations in the Shine-Dalgarno sequence can affect translation efficiency.
- Sequences closely matching the ideal Shine-Dalgarno sequence display higher activity, while deviations generally reduce activity.
- Proper distance from the start codon also influences translation efficiency.
Eukaryotic Translation Initiation
- The 5′ cap on mRNA is recognized by eIFs, recruiting the small ribosomal subunit and assisting in scanning for the start codon.
- mRNA lacking a cap is vulnerable to degradation by exonucleases, underscoring the protective role of the cap.
Charging tRNA by Aminoacyl tRNA Synthetases
- Aminoacyl tRNA synthetases charge tRNAs in two steps: adenylation (binding amino acid to AMP) and tRNA charging (transferring the amino acid to tRNA).
- Some organisms have fewer than 20 synthetases yet can charge all amino acids by modifying others.
Role of EF-Tu in Translation
- Charged tRNA is bound to EF-Tu-GTP, which delivers it to the ribosome's A site.
- Upon correct codon-anticodon pairing, EF-Tu hydrolyzes GTP, releasing tRNA into the ribosome.
Stop Codon Recognition in Eukaryotes
- Stop codons are recognized by class I release factors RF1 and RF2 using a peptide anticodon that specifically binds to the codon.
- This recognition leads to the termination of protein synthesis by hydrolyzing the bond between the polypeptide and tRNA.
Termination of Translation
- Upon stop codon recognition, release factors facilitate polypeptide release and ribosome disassembly through a series of interactions, involving RF3 and RRF.
- EF-G helps displace deacylated tRNAs from the ribosome.
Translation Accuracy
- The error rate for translation (10^-3 to 10^-4) is higher than that for transcription (10^-4 to 10^-5) and DNA replication (as low as 10^-10).
- Higher error rates in translation are acceptable since they only affect individual proteins, unlike transcription which influences multiple proteins from a single mRNA.
Translation Elongation Events
- Key events for amino acid addition during elongation include the insertion of the correct aminoacyl-tRNA into the ribosome’s A site, peptidyl bond formation, and translocation of peptidyl-tRNA to the P site.
Differences in Translation between Prokaryotes and Eukaryotes
- While the processes are similar, distinct differences exist in translation elongation between prokaryotes and eukaryotes.
Ribosome Structure and Function
- Prokaryotic ribosome: 70S total (30S + 50S), Eukaryotic ribosome: 80S total (40S + 60S).
- The ribosome has three tRNA binding sites: A (aminoacyl), P (peptidyl), and E (exit).
Understanding tRNA
- Charged tRNA is one that has an amino acid attached, crucial for protein synthesis.
- tRNA structure features a 3' terminus with a CCA sequence and contains an anticodon that recognizes mRNA codons.
Peptidyl Transferase Reaction
- Catalyzed by the 23S rRNA of the large subunit, the reaction facilitates peptidyl bond formation.
Mechanisms Ensuring Accurate tRNA Insertion
- Hydrogen bonds between adenine residues in 16S rRNA and the anticodon-codon pair enhance specificity.
- Correct pairing allows for EF-Tu interaction with the ribosome, leading to subsequent peptide bond formation through tRNA accommodation.
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Test your knowledge on the roles of 16S rRNA and the Shine-Dalgarno sequence in translation. These flashcards cover key concepts important for understanding protein synthesis in prokaryotes. Ideal for biology students looking to reinforce their learning.