The Central Dogma: Translation and Modifications

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Questions and Answers

Which of the following is NOT a stop codon in most species?

  • UGU (correct)
  • UGA
  • UAA
  • UAG

Which release factor in bacteria recognizes both UAA and UGA stop codons?

  • RF2 (correct)
  • RF1
  • RF3
  • eRF

What is the primary function of RF3 in bacteria?

  • Recognizing UAA and UGA stop codons
  • Recognizing UAA and UAG stop codons
  • Recognizing all three stop codons
  • Facilitating the termination process by binding GTP (correct)

Which of the following is an example of a post-translational modification?

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

What is the significance of the N-terminal and C-terminal ends of a polypeptide chain?

<p>They indicate the directionality of the chain, with the N-terminal being the start and the C-terminal being the end. (B)</p> Signup and view all the answers

What is the name of the modified methionine carried by the initiator tRNA in bacteria?

<p>N-formylmethionine (B)</p> Signup and view all the answers

What is the purpose of the Shine-Dalgarno sequence in the mRNA?

<p>It facilitates the binding of the mRNA to the 30S ribosomal subunit. (D)</p> Signup and view all the answers

Which of the following is NOT a requirement for the formation of the initiation complex in translation?

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

What happens to the GTP molecule during the initiation of translation?

<p>It is hydrolyzed to GDP. (D)</p> Signup and view all the answers

Which ribosomal site does the charged initiator tRNA enter during the initiation of translation?

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

Which of the following is TRUE about the initiation of translation in eukaryotes compared to bacteria?

<p>Eukaryotes require additional factors. (B)</p> Signup and view all the answers

What is the length of the Shine-Dalgarno sequence in the mRNA?

<p>9 nucleotides (C)</p> Signup and view all the answers

What is the function of the ribosomal-binding site?

<p>To facilitate the binding of the mRNA to the ribosomal subunit (C)</p> Signup and view all the answers

What molecule facilitates the entry of a charged tRNA into the A site during translation elongation?

<p>EF-Tu (B)</p> Signup and view all the answers

Which ribosomal subunit is responsible for the formation of the peptidyl transferase activity?

<p>60S subunit (A)</p> Signup and view all the answers

What is the primary role of 16S rRNA in translation?

<p>Checking for correct codon-anticodon pairing (A)</p> Signup and view all the answers

What is the approximate rate of translation elongation in eukaryotes?

<p>6 amino acids per second (C)</p> Signup and view all the answers

Which of the following is NOT involved in the initiation of protein synthesis in eukaryotes?

<p>EF-Tu (D)</p> Signup and view all the answers

What happens when a stop codon is encountered during translation?

<p>Translation terminates and the newly synthesized polypeptide is released (D)</p> Signup and view all the answers

Which of the following correctly describes the movement of the ribosome during translation elongation?

<p>The ribosome moves one codon to the right along the mRNA (B)</p> Signup and view all the answers

What is the role of the 60S ribosomal subunit in translation elongation?

<p>Performing the peptidyl transferase reaction (A)</p> Signup and view all the answers

Flashcards

Stop Codons

Nonsense codons UAG, UAA, and UGA that signal termination of protein synthesis.

Release Factors

Proteins that recognize stop codons and promote termination of translation.

Bacterial Release Factors

Three specific proteins in bacteria: RF1, RF2, and RF3 that recognize specific stop codons.

Eukaryotic Release Factor

A single release factor (eRF) that recognizes all three stop codons in eukaryotes.

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Post-translational Modifications

Chemical changes after translation that enhance protein function and diversity.

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Central Dogma

The process of genetic information flow: DNA to RNA to Protein.

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Eukaryotic Initiation Factors (eIF)

Proteins that bind to the 5' cap of mRNA and assist in translation initiation.

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80S initiation complex

The structure formed by the assembly of the 40S subunit, tRNAmet, and 60S subunit during translation initiation.

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Stages of Translation

Translation occurs in three stages: Initiation, Elongation, Termination.

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Translation elongation

The stage in protein synthesis where amino acids are added to the growing polypeptide chain one by one.

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Initiation Complex

Formation of a complex involving mRNA, initiator tRNA, and ribosomal subunits.

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Start Codon

The codon in mRNA that signals the start of protein synthesis, typically AUG.

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Codon-anticodon recognition

The process by which the ribosome ensures correct pairing of tRNA and mRNA codons during translation.

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tRNAfmet

Initiator tRNA in bacteria that carries N-formylmethionine.

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Peptidyl transferase

An enzymatic function of ribosomal RNA that facilitates the formation of peptide bonds between amino acids.

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Translocation in translation

The movement of the ribosome along mRNA to position the next codon for translation.

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Shine-Dalgarno Sequence

A ribosomal-binding site in bacterial mRNA that aligns with 16S rRNA.

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Initiation Factors

Proteins required to assist the formation of the initiation complex.

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EF-Tu

A protein that facilitates the entry of charged tRNA into the A site of the ribosome.

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Differences in Eukaryotes

Eukaryotic translation initiation requires additional factors compared to bacteria.

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Study Notes

The Central Dogma: Translation of mRNA and Post-translational Modifications

  • Translation occurs in three stages: initiation, elongation, and termination.
  • Initiation involves the association of mRNA, initiator tRNA, and ribosomal subunits to form an initiation complex. Initiator tRNA recognizes the start codon (AUG, GUG, or UUG). In bacteria, this tRNA is tRNAfMet, carrying N-formylmethionine.
  • The binding of mRNA to the 30S ribosomal subunit is aided by the Shine-Dalgarno sequence on mRNA which is complementary to a sequence in the 16S rRNA.
  • IF3 promotes mRNA binding to the 30S subunit.
  • IF2 promotes the binding of the initiator tRNA.
  • The 50S ribosomal subunit joins.
  • This forms the 70S initiation complex.
  • Elongation involves adding amino acids to the polypeptide chain one at a time. This process is remarkably fast, ~15-18 amino acids per second in bacteria, and ~6 per second in eukaryotes.
    • A charged tRNA binds to the A site.
    • EF-Tu facilitates tRNA entry.
    • Peptidyl transferase catalyzes bond formation between the polypeptide chain and the amino acid in the A site.
    • The polypeptide is transferred to the A site.
    • The ribosome translocates one codon to the right.
  • Termination occurs when a stop codon (UAG, UAA, or UGA) is reached.
    • Release factors (RFs) recognize the stop codons and facilitate the release of the polypeptide chain.
      • Bacteria have three release factors: RF1 (recognizes UAA and UAG), RF2 (recognizes UAA and UGA), and RF3 (does not recognize any of the stop codons but helps to facilitate the termination process).
      • Eukaryotes have one release factor (eRF).

Prokaryotic vs. Eukaryotic Translation

  • Prokaryotic transcription and translation are simultaneous.
  • Eukaryotic transcription and translation are discontinuous.
  • Prokaryotic ribosomes are 70S (30S + 50S subunits). mRNAs are unstable, living for a few seconds to two minutes.
  • Eukaryotic ribosomes are 80S (40S + 60S subunits). mRNAs are more stable, living for several hours to days. Transcription occurs in the nucleus, translation occurs in the cytoplasm.
  • Initiation in prokaryotes involves three initiation factors (IFs), while initiation in eukaryotes involves nine initiation factors (eIFs).
  • Prokaryotic initiation is cap-independent.
  • Eukaryotic initiation is cap-dependent (and cap-independent in some cases).

Post-Translational Modifications (PTMs)

  • Protein PTMs increase the functional diversity of the proteome.
  • PTMs include phosphorylation, glycosylation, ubiquitination, nitrosylation, methylation, acetylation, lipidation, and proteolysis.

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