Question

During the elongation stage of eukaryotic protein synthesis, A. the incoming aminoacyl-tRNA binds to the P site. B. a new peptide bond synthesized by peptidyl transferase requires GTP hydrolysis. C. the peptidyl- tRNA is translocated to a different site on the ribosome D. streptomycin can cause premature release of the incomplete peptide. E. peptide bond formation occurs by the attack of the carboxyl group of the incoming amino acyl tRNA on the amino group of the growing peptide chain.

Short answer

Answer: The peptidyl-tRNA is translocated to a different site on the ribosome.

Step-by-step solution

Understanding the elongation stage of protein synthesis

In the elongation stage, the ribosome moves along the mRNA strand, synthesizing the polypeptide chain while reading codons in the mRNA. There are three primary steps in this stage: aminoacyl-tRNA binding, peptide bond formation, and translocation.

Analyze each statement

Now, let's analyze each statement and check if it accurately describes the elongation stage of eukaryotic protein synthesis: A. The incoming aminoacyl-tRNA binds to the P site. This statement is incorrect. The incoming aminoacyl-tRNA binds to the A site, not the P site, on the ribosome during elongation. B. A new peptide bond synthesized by peptidyl transferase requires GTP hydrolysis. This statement is incorrect. Peptide bond formation is facilitated by the peptidyl transferase enzyme, but GTP hydrolysis is used to drive translocation, not peptide bond formation. C. The peptidyl-tRNA is translocated to a different site on the ribosome. This statement is correct. During the translocation step, the ribosome moves one codon forward on the mRNA, and the peptidyl-tRNA is translocated from the A site to the P site. D. Streptomycin can cause premature release of the incomplete peptide. This statement is incorrect. Streptomycin is an antibiotic that affects prokaryotic ribosomes, not eukaryotic ribosomes. E. Peptide bond formation occurs by the attack of the carboxyl group of the incoming amino acyl tRNA on the amino group of the growing peptide chain. This statement is incorrect. The peptide bond formation occurs by the attack of the amino group of the incoming aminoacyl-tRNA on the carboxyl group of the growing peptide chain, not the other way around.

Identifying the correct statement

After analyzing each statement, we can conclude that statement C is the correct answer. The peptidyl-tRNA is translocated to a different site on the ribosome during the elongation stage of eukaryotic protein synthesis.

Key concepts

Elongation Stage of Protein Synthesis

The elongation stage is a crucial phase in eukaryotic protein synthesis, where the existing polypeptide chain is extended by the addition of new amino acids. During this stage, the ribosome progresses along the messenger RNA (mRNA) strand in a 5' to 3' direction, decoding the genetic information for proper amino acid incorporation.

During each cycle of elongation, three main events occur: the binding of aminoacyl-tRNA to the ribosome, the formation of a peptide bond between the new amino acid and the growing polypeptide chain, and the translocation of the ribosome along the mRNA strand. It's essential to remember that accuracy is key during this process, as mistakes can lead to nonfunctional or harmful proteins.

Aminoacyl-tRNA Binding

A critical step in the elongation phase is the binding of an aminoacyl-tRNA to the ribosome. Each tRNA molecule carries a specific amino acid and a set of three nucleotides called an anticodon, which matches the codon on the mRNA strand. Instead of binding to the P site, the incoming aminoacyl-tRNA initially binds to the A site (aminoacyl site) of the ribosome. This binding is facilitated by elongation factors and requires energy in the form of GTP hydrolysis, ensuring that the process is highly controlled and efficient.

Once the correct match between the mRNA codon and the tRNA anticodon is achieved, the ribosome is ready for the next step—the formation of a peptide bond.

Peptidyl Transferase

Peptidyl transferase is an enzyme integral to the ribosome's large subunit that catalyzes the formation of peptide bonds. This activity is intimately tied to the ribosome’s ribozyme function — its capacity to act as an enzyme. Contrary to statement B from the original exercise, peptidyl transferase does not require GTP hydrolysis to form a new peptide bond. Instead, the enzyme aids in positioning the amino group of the incoming amino acid to attack the carboxyl end of the existing polypeptide chain, resulting in a newly formed bond and one elongated polypeptide.

• Peptide bond formation is fundamental as it links amino acids together to form a protein.
• This enzymatic reaction centers on the ribosomal RNA (rRNA), highlighting the pivotal role of rRNA in protein synthesis.

Ribosome Translocation

Translocation is the process by which the ribosome moves one codon forward on the mRNA following peptide bond formation. This precise motion requires the input of energy, which, as per statement B of the exercise, is delivered through GTP hydrolysis—this is separate from the action of peptidyl transferase. After the creation of the new peptide bond, the tRNA at the P site (peptidyl site), now without an amino acid, exits the ribosome. Concurrently, the peptidyl-tRNA at the A site, with the newly elongated peptide chain, is shifted to the P site.

This step is pivotal as it ensures that the A site is vacated and ready to accept the next aminoacyl-tRNA, perpetuating the cycle of elongation. Translocation ensures continuity and is essential for creating the correct polypeptide sequence.