Translation Processes in Molecular Biology
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Questions and Answers

What is the role of eIF2 in the initiation of translation in eukaryotes?

  • It forms a closed loop with the cap and polyA tail.
  • It blocks the P site for incoming tRNA.
  • It recruits aminoacyl-tRNA to the ribosome.
  • It hydrolyzes GTP when the start site is recognized. (correct)

Which factor is primarily responsible for blocking the A site during the initiation process in eukaryotes?

  • eIF1 (correct)
  • eIF3
  • eIF2
  • eIF5b

What initiates the peptide bond formation during elongation in the ribosome?

  • A dependent transfer from the P-site tRNA
  • Interaction of rRNA with the tRNA
  • GTP hydrolysis by eEF-Tu
  • A 2'OH nucleophile attack on the aminoacyl ester bond (correct)

What triggers the termination of translation?

<p>Recognition of a stop codon by release factors (D)</p> Signup and view all the answers

Which process involves the degradation of mRNA containing premature stop codons?

<p>Nonsense mediated decay (B)</p> Signup and view all the answers

How does antibiotic resistance arise during elongation?

<p>Mutations in the rRNA gene and r-proteins (C)</p> Signup and view all the answers

What is the primary role of polypurine tracts in prokaryotic translation initiation?

<p>They enhance binding of the small ribosomal subunit. (D)</p> Signup and view all the answers

Which factor is equivalent to IF2 in prokaryotes during large subunit joining in eukaryotes?

<p>eIF5b (D)</p> Signup and view all the answers

What is the role of elongation factors (EFs) during protein synthesis?

<p>They facilitate the formation of peptide bonds between amino acids. (B), They promote the hydrolysis of pyrophosphate to drive amino acid transfer. (C)</p> Signup and view all the answers

Which statement accurately describes termination in protein synthesis?

<p>The process of termination correctly identifies a stop codon and releases the polypeptide chain. (B)</p> Signup and view all the answers

What is the main function of polysome profiling in the study of translation?

<p>To analyze the distribution of ribosomes on mRNA molecules. (C)</p> Signup and view all the answers

What is nonsense-mediated decay (NMD)?

<p>A cellular mechanism that degrades mRNA containing premature stop codons. (B)</p> Signup and view all the answers

Which component is responsible for charging tRNA with its respective amino acid?

<p>Aminoacyl-tRNA synthetase (B)</p> Signup and view all the answers

During ribosome recycling, what happens after the polypeptide chain is released?

<p>The ribosome disassembles into its large and small subunits. (B)</p> Signup and view all the answers

What occurs at the A site of the ribosome during translation elongation?

<p>Incoming tRNA carries the next amino acid to be added to the polypeptide. (B)</p> Signup and view all the answers

Which of the following statements regarding the fidelity of protein synthesis is true?

<p>Fidelity ranges from $10^{-2}$ to $10^{-5}$ based on various factors. (A)</p> Signup and view all the answers

What is the primary function of EF-G during translation?

<p>To facilitate the movement of the mRNA:tRNA complex (A)</p> Signup and view all the answers

Which release factor in prokaryotes is responsible for recognizing the UAG and UAA stop codons?

<p>RF1 (B)</p> Signup and view all the answers

What defines the main distinction between polysome profiling and ribosome profiling?

<p>Ribosome profiling focuses on mRNA protected by ribosomes (C)</p> Signup and view all the answers

What happens during nonsense-mediated decay (NMD)?

<p>NMD eliminates mRNAs that incorrectly terminate translation (C)</p> Signup and view all the answers

Which molecule hydrolyzes GTP to release the polypeptide chain during termination in eukaryotes?

<p>eRF3 (A)</p> Signup and view all the answers

What is the role of RRF in prokaryotic ribosome recycling?

<p>To facilitate the separation of ribosomal subunits (A)</p> Signup and view all the answers

What represents a significant feature of polysome profiling?

<p>It measures RNA absorbance at various fractions (D)</p> Signup and view all the answers

What is the significance of the pioneer round of translation in relation to NMD?

<p>It identifies mRNAs with incorrect codons early (C)</p> Signup and view all the answers

Flashcards

Translocation (EF-G)

The movement of the mRNA-tRNA complex on the small ribosomal subunit, shifting from the A site to the P site by three nucleotides.

Termination (bacteria)

The phase of protein synthesis where RF1 or RF2 recognize a stop codon, leading to polypeptide chain release.

Termination (eukaryotes)

eRF1 recognizes all stop codons, eRF3 hydrolyzes GTP to releases the polypeptide chain.

Ribosome recycling

The process of splitting the ribosomal subunits to prepare the ribosome for a new round of translation.

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Polysomes

Multiple ribosomes translating the same mRNA simultaneously.

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Nonsense-Mediated Decay (NMD)

A cellular mechanism that eliminates mRNAs with premature stop codons.

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Pioneer round of translation

The first round of translation of an mRNA molecule, during which NMD takes place.

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Polysome profiling

A technique used to study translation dynamics by measuring RNA abundance on a gradient.

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Protein Synthesis Speed

The rate at which proteins are made, ranging from 20 amino acids per second in E. coli to 5-10 per second in S. cerevisiae.

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tRNA Wobble Pairing

Flexible base pairing rules in the third codon position. Allows one tRNA to recognize multiple codons.

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Aminoacyl-tRNA Synthetases

Enzymes that attach the correct amino acid to its corresponding tRNA.

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tRNA Structure

tRNA has specific loops (D and T loops) for 3D structure, an anticodon for codon recognition, and an acceptor end for amino acid attachment.

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Start Codon

AUG, the codon that signals the start of protein synthesis.

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Ribosome Sites

A site (aminoacyl), P site (peptidyl), and E site (exit) are locations on the ribosome where tRNAs bind during protein synthesis.

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Translation Fidelity

The accuracy of protein synthesis, ranging from 10^-2 to 10^-5 error rate.

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Aminoacylation Site

The part of an aminoacyl-tRNA synthetase where the amino acid is attached to its tRNA.

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Polypurine Tract

A sequence of 6-8 purines (A or G) found upstream of the start codon (AUG) in bacterial mRNA. It helps guide the small ribosomal subunit to the correct start codon by pairing with a complementary polypyrimidine tract (rich in C or U) on the 16S rRNA.

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Shine-Dalgarno Sequence

A polypurine tract in bacterial mRNA that pairs with a complementary polypyrimidine tract (anti-SD) on the 16S rRNA to position the ribosome for initiation.

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Initiation Factors (IFs)

Proteins that help assemble the ribosome and initiate translation in prokaryotes. They bind to the small ribosomal subunit and help guide the initiator tRNA to the start codon.

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IF2

An initiation factor in prokaryotes that is a GTPase. It promotes the joining of the large ribosomal subunit to the small subunit, hydrolyzing GTP upon recognition of the start codon.

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Formyl-Methionine (f-Met)

A modified methionine amino acid that is the first amino acid incorporated into a polypeptide chain in bacteria. It is attached to the initiator tRNA.

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Cap and PolyA Tail (eukaryotes)

A 5' cap and a 3' polyA tail are modifications found on eukaryotic mRNAs. These structures are critical for initiating translation, as they form a closed loop structure that helps the ribosome recognize the mRNA.

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eIF4E

An eukaryotic initiation factor that binds to the 5' cap of mRNA. It is part of the complex that forms the closed loop for initiation of translation.

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Kozak Consensus Sequence

A sequence of nucleotides surrounding the start codon (AUG) in eukaryotic mRNA. It provides a favorable context for initiation of translation but does not directly bind to the small ribosomal subunit.

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Study Notes

Translation: Initiation, Elongation, and Termination

  • Translation speed varies, 20 amino acids per second in E. coli, 5-10 in S. cerevisiae.
  • Fidelity is between 10−2 and 10−5.
  • Start codons include AUG.
  • Stop codons are UAG, UAA, and UGA.

tRNA Structure

  • Has a 3' CCA tail for amino acid attachment.
  • Contains a D-loop and T-loop, aiding 3D structure.
  • Has an anticodon loop, interacting with codons for specificity.
  • Anticodon doesn't recognize stop codons.
  • Acceptor end interacts with EF-Tu and ribosomal tRNA binding sites.

Aminoacyl-tRNA Synthetases

  • There's one synthetase for each amino acid.
  • Accuracy (adding correct amino acids to tRNA) is crucial.
  • Editing site removes incorrect amino acids.
  • Aminoacylation site adds amino acids to tRNA using ATP.
  • Two-step loading—activation and transfer.

Ribosomes

  • Bacterial ribosomes are 70S, composed of 50S and 30S subunits.
  • Eukaryotic ribosomes are 80S, composed of 60S and 40S subunits.
  • Ribosomes consist of RNA (majority) and proteins.
  • rRNA highly conserved across species, crucial for function.
  • rRNA sequences mediate interactions between large and small subunits.
  • Ribosome's active site is solely RNA (ribozyme), not protein, for catalysis
  • Important regions of rRNA include the large-small subunit interface and tRNA-binding sites. This provides the site for folding of the rRNA.

Translation Initiation

  • IFs (initiation factors) are critical.
  • IF1 blocks A site, IF3 blocks E site.
  • IF2 is involved in large subunit joining, GTP hydrolysis occurs upon the finding of AUG.
  • Bacteria use formyl-methionine (f-Met) as the initiator.
  • Eukaryotes use methionine (Met).
  • Initiation complex involves initiator tRNA, and initiation factors, then associates with the mRNA and small ribosomal subunit.

Translation Elongation

  • EF-Tu/eEF-1a (GTPases) load tRNAs.
  • Correct codon-anticodon matches are critical (faster GTP hydrolysis if correct).
  • Peptide bond formation is a ribozyme-catalyzed reaction.
  • Translocation moves the mRNA-tRNA complex.
  • EF-G/ eEF-2 (GTPases) is responsible for translocation.
  • GTP hydrolysis promotes translocation.

Translation Termination

  • Release factors (RFs) recognize stop codons.
  • RF1 recognizes UAG/UAA.
  • RF2 recognizes UGA/UAA.
  • RF3 stimulates RF1/RF2 activity.
  • GTP hydrolysis releases the polypeptide chain.
  • Requires recycling of ribosomes (separating small and large subunits).

Polysomes

  • Multiple ribosomes translate the same mRNA simultaneously.
  • Studying polysomes helps understand translation dynamics.

Nonsense-Mediated Decay (NMD)

  • NMD eliminates mRNAs with premature stop codons.
  • NMD occurs in the pioneer round of translation in mammalian cells.
  • Exon junctions in mRNA help identify premature stop codons.
  • NMD may use the EJC to identify premature termination codons (PTCs).
  • eRF1 and eRF3 identify the stop codon, and lead to the degradation of the mRNA.

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Lecture 10: Molecular Biology - Translation I (2021)

Description

This quiz covers key concepts in the translation process, including initiation, elongation, and termination of protein synthesis. It examines the structures of tRNA and the functions of aminoacyl-tRNA synthetases, as well as the differences between bacterial and eukaryotic ribosomes. Test your understanding of these fundamental processes in molecular biology.

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