Protein Biosynthesis: Stages of Translation

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Questions and Answers

In protein synthesis, what role do elongation factors play?

  • They are involved in the termination of translation.
  • They are responsible for unwinding the DNA double helix.
  • They facilitate the binding of aminoacyl-tRNA to the ribosome. (correct)
  • They ensure the correct positioning of the ribosome at the start codon.

During translation initiation in prokaryotes, what sequence on mRNA plays a critical role in recognizing and binding to the small ribosomal subunit?

  • Shine-Dalgarno sequence (correct)
  • Poly-A tail
  • Kozak sequence
  • 5' cap

Which of the following is NOT a difference between prokaryotic and eukaryotic translation initiation?

  • Prokaryotic mRNA does not undergo extensive processing before translation.
  • Eukaryotic translation initiation requires a 5' cap, while prokaryotic translation does not.
  • Ribosomes in both prokaryotes and eukaryotes have the same size and composition. (correct)
  • Eukaryotes use the Kozak sequence, while prokaryotes use the Shine-Dalgarno sequence.

Which of the following events involves the hydrolysis of GTP (guanosine triphosphate) during translation?

<p>All of the above (D)</p> Signup and view all the answers

Termination of translation is signaled by the presence of what in the mRNA sequence?

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

In the initiation stage of translation, what is the role of the small ribosomal subunit?

<p>It brings together the initiator tRNA and the start codon on the mRNA. (A)</p> Signup and view all the answers

During elongation, what occurs after the ribosome translocates along the mRNA?

<p>A new codon in the A site is exposed for the next tRNA to bind. (A)</p> Signup and view all the answers

What is the function of the peptidyl transferase activity of the ribosome?

<p>Catalyzing the formation of peptide bonds between amino acids. (A)</p> Signup and view all the answers

What happens to the tRNA in the E site after the ribosome translocates during elongation?

<p>It is released from the ribosome and is free to bind to another amino acid. (D)</p> Signup and view all the answers

Which of the following is NOT a characteristic of the termination stage of translation?

<p>A new tRNA carrying the next amino acid enters the A site. (D)</p> Signup and view all the answers

What is the role of release factors in translation termination?

<p>They bind to the stop codon in the A site and hydrolyze the bond between the polypeptide chain and the tRNA. (D)</p> Signup and view all the answers

Which of the following statements is TRUE about the Shine-Dalgarno sequence?

<p>It is found in prokaryotic mRNA and facilitates the binding of the small ribosomal subunit. (A)</p> Signup and view all the answers

In what direction does the ribosome move along the mRNA during elongation?

<p>From the 5' end to the 3' end. (D)</p> Signup and view all the answers

Flashcards

Initiation

The first stage of protein translation where ribosomal subunits assemble on mRNA.

Ribosome Binding Sites

Specific sequences where ribosomal subunits attach to mRNA (Shine-Dalgarno in prokaryotes, Kozak in eukaryotes).

Initiator tRNA

tRNA that binds to the start codon AUG, carrying methionine or formylmethionine.

Elongation

The stage where amino acids are added to the growing polypeptide chain.

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Peptide Bond

A bond formed between amino acids during translation, catalyzed by the ribosome.

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Termination

The final stage where the ribosome encounters a stop codon and releases the polypeptide.

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Release Factors

Proteins that bind to the stop codon and trigger the release of the polypeptide chain.

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Ribosome Sites

The three functional sites in a ribosome: A site, P site, and E site for tRNA binding.

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Enzymes in Translation

Proteins that assist in the accuracy and efficiency of protein synthesis: aminoacyl-tRNA synthetases, initiation, elongation, and termination factors.

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GTP Hydrolysis

The process that provides energy for translation steps like translocation and tRNA binding.

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Shine-Dalgarno vs Kozak Sequence

Prokaryotes use the Shine-Dalgarno sequence for ribosome binding; eukaryotes use the Kozak sequence.

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mRNA Processing

Eukaryotic mRNA undergoes capping, splicing, and polyadenylation, while prokaryotic mRNA is usually translated directly.

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Study Notes

Protein Biosynthesis: Stages of Translation

  • Initiation: The process starts with assembling protein synthesis components.

    • Ribosomal subunits (small and large) attach to the mRNA molecule. The small subunit specifically binds to the mRNA's ribosome-binding site (Shine-Dalgarno sequence in prokaryotes, Kozak sequence in eukaryotes).
    • The initiator tRNA (carrying methionine in eukaryotes, formylmethionine in prokaryotes) binds to the start codon (AUG) on the mRNA, placed in the P site of the ribosome.
    • The large ribosomal subunit joins the complex, creating the initiation complex.

  • Elongation: Amino acids are added to the polypeptide chain in a stepwise manner.

    • Aminoacyl-tRNAs are sequentially added to the A site of the ribosome in each cycle.
    • The ribosome moves along the mRNA in a 5' to 3' direction.
    • A peptide bond forms between the amino acid in the A site and growing polypeptide in the P site. This reaction is catalyzed by peptidyl transferase activity of the ribosome.
    • The ribosome translocates, shifting the tRNA in the P site to the E site, the tRNA in the A site to the P site, and exposing a new codon in the A site.
    • This repeats until a stop codon is encountered.

  • Termination: The process ends when a stop codon (UAA, UAG, or UGA) is reached in the A site.

    • Release factors (proteins) recognize the stop codon.
    • Release factors bind to the A site, promoting water molecule addition to the polypeptide chain. This reaction hydrolyzes the polypeptide-tRNA bond.
    • The polypeptide is released.
    • The ribosome subunits detach from the mRNA.

Key Aspects and Factors

  • tRNAs: Transfer RNA molecules carry amino acids to the ribosome, recognizing specific mRNA codons. Each tRNA is specific for one amino acid.
  • mRNA: Messenger RNA carries the genetic code from DNA to the ribosome. mRNA codons determine the amino acid sequence of the polypeptide chain.
  • Ribosomes: Ribosomes are the sites for peptide bond formation between amino acids. The ribosome has three sites: A (aminoacyl-tRNA), P (peptidyl-tRNA), and E (exit).
  • Enzymes: Various enzymes are involved, including aminoacyl-tRNA synthetases (activating amino acids), initiation, elongation, and termination factors. These ensure the accuracy and efficiency of protein synthesis.
  • Energy: GTP hydrolysis powers various translation steps, including translocation, tRNA binding, and initiation complex assembly.

Differences in Prokaryotic and Eukaryotic Translation

  • Ribosome size: Prokaryotic ribosomes are smaller (70S) than eukaryotic ribosomes (80S).
  • Initiation factors: Different initiation factors are used in prokaryotes and eukaryotes. Prokaryotes use the Shine-Dalgarno sequence, while eukaryotes use the Kozak sequence to increase initiation complex efficiency.
  • mRNA processing: Eukaryotic mRNA undergoes capping, splicing, and polyadenylation; prokaryotic mRNA is often directly translated after transcription.
  • Location: Prokaryotic translation occurs in the cytoplasm; eukaryotic transcription occurs in the nucleus and translation in the cytoplasm.

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