AP Biology: Unit 6, Translation (Drill 25)

Question 1. Using the mRNA sequence provided, how many amino acids are incorporated into the polypeptide before translation terminates?

• A) 2
• B) 3 ✓
• C) 4
• D) 6

Explanation: Reading 5' to 3' in triplets: AUG (Met) -- UUC (Phe) -- CCG (Pro) -- UAG (stop) -- CGU (not translated). Three sense codons are read before the UAG stop codon, producing a tripeptide: Met-Phe-Pro. A is incorrect because the student stops after UUC, missing the Pro codon. C is incorrect because the student counts CGU after the stop codon as a translated codon. D is incorrect because the student counts all six possible triplets without recognizing the stop codon.

Question 2. A student claims that charged tRNA is required for peptide bond formation during elongation. Which condition in the table provides the strongest evidence for this claim?

• A) Condition 1, because all components are present and a polypeptide is produced.
• B) Condition 2, because the ribosome is active but no polypeptide is produced without charged tRNA. ✓
• C) Condition 3, because the ribosomes themselves charge the tRNAs before they enter the A site according to this view.
• D) Conditions 1 and 3 together, because they show that both tRNA and ribosomes are required.

Explanation: Condition 2 isolates the variable of charged tRNA -- the ribosome is present and active, the mRNA is present, but no polypeptide is produced. The data are consistent with the claim that charged tRNA is required for peptide bond formation. Condition 1 confirms translation works when all components are present but does not isolate tRNA as the critical variable. Condition 3 tests ribosome necessity, not tRNA necessity. D is partially correct in identifying both requirements but does not identify the single condition that most directly supports the specific claim about tRNA.

Question 3. During translation of the synthetic mRNA, at which ribosomal site does peptide bond formation occur?

• A) The A site, where the incoming aminoacyl-tRNA enters and immediately catalyzes the new peptide bond.
• B) The E site, where uncharged tRNA exits after transferring its amino acid.
• C) The mRNA entry channel, where the ribosome first contacts the Shine-Dalgarno sequence.
• D) The P site, where the peptidyl transferase center catalyzes transfer of the growing polypeptide to the incoming aminoacyl-tRNA. ✓

Explanation: Peptide bond formation occurs at the P site, where the peptidyl transferase center of the large ribosomal subunit catalyzes transfer of the growing polypeptide chain to the amino acid carried by the incoming tRNA in the A site. A is incorrect because the A site is where incoming aminoacyl-tRNA docks, not where peptide bond formation is completed. B is incorrect because the E site is the exit site for uncharged tRNA after it has already donated its amino acid. C is incorrect because the mRNA entry channel positions the mRNA but is not the site of peptide bond formation.

Question 4. The researchers repeat the experiment using a modified mRNA in which the AUG start codon is replaced with AUC. Which result is most likely?

• A) Translation initiates normally because AUC codes for isoleucine, which can substitute for methionine under these conditions.
• B) Translation initiates normally because AUC is recognized by the initiator tRNA as a start codon.
• C) Translation does not initiate because AUG is the primary start codon required for initiator tRNA recognition. ✓
• D) The ribosome binds the mRNA but stalls at the first codon without producing any product.

Explanation: AUG is the primary start codon recognized by the initiator tRNA (Met-tRNA) during translation initiation. Without AUG, the ribosome cannot properly assemble the initiation complex at the correct position. A is incorrect because although AUC codes for isoleucine, the initiator tRNA specifically recognizes AUG -- codon-amino acid compatibility alone does not substitute for start codon recognition. B describes leaky scanning, a phenomenon that can occur in eukaryotes under some conditions, but in this E. coli system with no downstream AUG in the short synthetic sequence, no initiation would occur. D is incorrect because without AUG recognition the initiation complex does not stably assemble -- the ribosome does not simply stall.

Question 5. In eukaryotic cells, translation of most mRNAs requires both a 5' cap and a poly-A tail. A researcher proposes that these two modifications act independently to promote translation. Which experimental finding would most directly challenge this proposal?

• A) Removing the poly-A tail reduces mRNA stability but does not affect translation initiation rate.
• B) Removing the 5' cap eliminates ribosome binding so that no protein is produced, which by itself shows the two modifications act independently.
• C) A protein complex is discovered that simultaneously binds both the 5' cap and the poly-A tail to circularize the mRNA, and disrupting this complex abolishes translation. ✓
• D) mRNAs with longer poly-A tails produce more protein over time than mRNAs with shorter tails.

Explanation: If a protein complex physically bridges the 5' cap and poly-A tail to circularize the mRNA, the two modifications are functionally interdependent, not independent -- directly challenging the proposal. A is consistent with the independence proposal (separate functions for cap and tail). B shows cap necessity but does not address whether cap and tail act independently or together. D shows a quantitative effect of tail length on output but does not address whether the cap and tail interact mechanistically.