Examine the following steps involved in protein synthesis.

A) The elongation reaction transfers the peptide chain from the peptidyl-tRNA in the P-site to the aminoacyl-tRNA in the A-site.

B) The incoming aminoacyl-tRNA binds to the A site in an energy requiring process involving the hydrolysis of GTP.

C) The aminoacyl-tRNA dissociated from the E site.

D) The incoming aminoacyl-tRNA binds to an elongation factor bound to GTP.

E) The new longer peptidyl-tRNA moves from the A site into the P site as the ribosomes moves further along the mRNA in an energy requiring process involving hydrolysis of GTP.

Which of the following is the appropriate order for the basic steps of protein synthesis?

Question

Examine the following steps involved in protein synthesis.

A) The elongation reaction transfers the peptide chain from the peptidyl-tRNA in the P-site to the aminoacyl-tRNA in the A-site.

B) The incoming aminoacyl-tRNA binds to the A site in an energy requiring process involving the hydrolysis of GTP.

C) The aminoacyl-tRNA dissociated from the E site.

D) The incoming aminoacyl-tRNA binds to an elongation factor bound to GTP.

E) The new longer peptidyl-tRNA moves from the A site into the P site as the ribosomes moves further along the mRNA in an energy requiring process involving hydrolysis of GTP.

Which of the following is the appropriate order for the basic steps of protein synthesis?

BlackTom AI · Accepted Answer

Question restatement: We are asked to determine the appropriate order for the basic steps of protein synthesis among the given steps A–E and options of sequences.

Option A: B, D, A, C, E
- The sequence starts with B, which describes the incoming aminoacyl-tRNA binding to the A site with GTP hydrolysis. In the established elongation cycle, the delivery of the aminoacyl-tRNA to the A site (via an elongation factor and GTP) should occur before any peptide bond formation, so starting with B could be plausible. However, the next step D describes the aminoacyl-tRNA binding to the A site bound to a GTP factor, which overlaps conceptually with B. This redundancy and the mismatch with the canonical order (peptide bond formation after tRNA arrival) make this ordering suspect. The rest of the sequence (A, C, E) also misalign: A is peptide bond transfer from P-site tRNA to A-site tRNA, which should occur after A-site occupancy, not after D if D already described A-site binding. C, the dissociation of tRNA from the E site, occurs after translocation, not before E. Overall, this option does not fit the standard sequence cleanly.

Option B: C, B, D, A, E
- This starts with C, which states the aminoacyl-tRNA dissociates from the E site. Dissociation from the E site occurs after translocation, not before the tRNA has bound and the peptide bond has formed. Placing C at the very beginning contradicts the core elongation cycle. The subsequent steps (B, D, A, E) cannot salvage the initial misplacement. Therefore this option is incorrect due to an impossible starting event.

Option C: D, B, A, C, E
- This sequence begins with D: the incoming aminoacyl-tRNA binds to an elongation factor bound to GTP. This reflects the delivery of the aminoacyl-tRNA to the A site via the EF-Tu–GTP complex, which is a correct initial step. Next, B describes the energy-dependent binding with GTP hydrolysis, which aligns with the proofreading/confirmatory step after correct codon–anticodon pairing. Then A corresponds to the peptide bond formation transferring the growing peptide from the P-site tRNA to the A-site tRNA, which is the subsequent chemical step. After that, C states the aminoacyl-tRNA dissociates from the E site, but dissociation from the E site typically occurs after translocation has moved the tRNAs to the P and E sites; placing C here implies dissociation before translocation, which is inconsistent with the standard order. Finally, E describes translocation and groove-moving of the peptidyl-tRNA from A to P with GTP hydrolysis, which should occur before E-site tRNA release. Because C is out of the correct order (E-site dissociation should occur after translocation), this option is not fully correct.

Option D: D, B, A, E, C
- Starting with D (A-site delivery via elongation factor–GTP) is correct as the first step. Then B (GTP hydrolysis during A-site tRNA accommodation) follows logically as the next step in the elongation cycle. Next, A (peptidyl transfer from P-site to A-site tRNA) is the subsequent chemical step, which is appropriate after the A-site tRNA is properly positioned. After peptide bond formation, E describes the translocation of the ribosome, moving the peptidyl-tRNA from the A site to the P site as the ribosome advances along the mRNA with GTP hydrolysis. Finally, C states the aminoacyl-tRNA dissociates from the E site. Since translocation has already occurred, the E-site tRNA can dissociate, making this placement consistent with the cycle. This ordering matches the canonical sequence of events in the elongation cycle: delivery (D) → GTP hydrolysis during loading (B) → peptide bond formation (A) → translocation (E) → E-site tRNA release (C).

Summary of why other options are incorrect:
- Option A places steps in a sequence that misaligns A and C relative to the actual translocation and E-site release, causing logical inconsistency.
- Option B starts with C, which asserts E-site dissociation before any tRNA has properly bound and peptide bond formation has occurred, an impossibility in the cycle.
- Option C moves C (E-site dissociation) too early, before translocation, contradicting the requirement that E-site release follows translocation.
- Only Option D aligns with the established order of elongation steps, correctly sequencing delivery, GTP-assisted accommodation, peptide bond formation, translocation, and E-site release in a coherent, energy-dependent cycle.

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