Bacterial Translation and Transcription

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Questions and Answers

Which release factor recognizes the stop codons UAA and UAG?

  • RF-2
  • RF-1 (correct)
  • eRF-1
  • None of the above

What do release factors cause to be released from the P site tRNA?

  • The RNA polymerase
  • The peptide chain (correct)
  • The release factors themselves
  • The ribosome

Which stop codons are recognized by eRF-1?

  • UAA and UAG only
  • All three stop codons (UGA, UAA, UAG) (correct)
  • None of these
  • UGA only

Where must release factors act to perform their function?

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

Which of the following statements is correct regarding stop codons and tRNAs?

<p>Stop codons are not recognized by any tRNA (D)</p> Signup and view all the answers

What is guided to the 30S ribosomal subunit by the Shine-Dalgarno sequence?

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

Which initiation factor mediates the binding of formylmethionine tRNA (fMet) to the peptidyl site?

<p>IF-2 (C)</p> Signup and view all the answers

Which of the following components is part of the 30S subunit complex?

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

Where is the Shine-Dalgarno sequence generally located relative to the AUG start codon?

<p>8 bp upstream (B)</p> Signup and view all the answers

During elongation, what happens when the A site is filled?

<p>The E site is emptied (B)</p> Signup and view all the answers

How many sites are there in a ribosome for elongation?

<p>3 (B)</p> Signup and view all the answers

Which of the following binds to the P site during the initiation stage?

<p>Formylmethionine tRNA (fMet) (A)</p> Signup and view all the answers

What is recycled by the third elongation factor (EF) during translocation?

<p>First elongation factor (B)</p> Signup and view all the answers

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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

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