A drug blocks EF-Tu's GTPase activity in bacteria. Which step of the elongation cycle would be most directly disrupted?
APeptide bond formation — EF-Tu catalyzes the transfer of the amino acid to the growing chain
BTranslocation — EF-Tu powers the movement of the ribosome along the mRNA
CAminoacyl-tRNA selection and delivery to the A site — EF-Tu uses GTP hydrolysis to verify correct codon-anticodon matching before releasing the tRNA
DTermination — EF-Tu recognizes stop codons and recruits release factors
EF-Tu (eEF1A in eukaryotes) delivers aminoacyl-tRNA to the A site and uses GTP hydrolysis as an accuracy checkpoint: if the tRNA's anticodon matches the mRNA codon, GTP hydrolysis proceeds and the tRNA is accepted; if not, the tRNA is rejected before incorporation. Blocking GTPase activity would prevent this proofreading step, jamming tRNA delivery. Translocation is driven by EF-G (not EF-Tu), and peptide bond formation is catalyzed by the ribosomal RNA itself, not EF-Tu.
Question 2 Multiple Choice
At the end of initiation, in which ribosomal site does the initiator tRNA sit, and what is the significance of this position?
AThe A site — this positions it to immediately accept a peptide bond from the incoming aminoacyl-tRNA
BThe P site — this positions the initiator tRNA (carrying the first amino acid) to donate to the first incoming aminoacyl-tRNA in the A site, beginning elongation
CThe E site — the initiator tRNA waits here until the large subunit joins
DNo specific site — the initiator tRNA floats freely until the first aminoacyl-tRNA arrives
After initiation, the initiator tRNA (carrying fMet in bacteria or Met in eukaryotes) occupies the P site (peptidyl site) of the assembled ribosome. This is critical: during elongation, the P site holds the growing polypeptide chain, and peptide bond formation transfers the peptide from the P-site tRNA to the amino acid on the A-site tRNA. Starting in the P site means the first elongation cycle immediately adds a second amino acid to fMet/Met, beginning the polypeptide chain.
Question 3 True / False
The peptidyl transferase activity that catalyzes peptide bond formation in the ribosome is provided by RNA (specifically the 23S rRNA in bacteria), not by any ribosomal protein.
TTrue
FFalse
Answer: True
This was a major discovery: the ribosome is a ribozyme — its catalytic activity resides in its RNA, not its proteins. The 23S rRNA (28S in eukaryotes) in the large subunit carries the peptidyl transferase center. Ribosomal proteins play structural and regulatory roles but do not directly catalyze the peptide bond. This supports the RNA World hypothesis — that RNA enzymes predated protein enzymes in early life.
Question 4 True / False
Stop codons (UAA, UAG, UGA) are recognized by specialized tRNA molecules with anticodons complementary to each stop codon, just as sense codons are recognized by aminoacyl-tRNAs.
TTrue
FFalse
Answer: False
Stop codons are recognized by release factors — proteins, not tRNAs. In bacteria, RF1 recognizes UAA and UAG; RF2 recognizes UAA and UGA; RF3 is a GTPase that assists. In eukaryotes, eRF1 recognizes all three stop codons. Release factors mimic the shape of a tRNA but trigger hydrolysis of the polypeptide from the final tRNA rather than peptide bond formation. The absence of aminoacyl-tRNAs for stop codons is why these codons terminate translation rather than incorporating an amino acid.
Question 5 Short Answer
Why is initiation the most regulated phase of translation, and what advantage does regulating this step provide to the cell?
Think about your answer, then reveal below.
Model answer: Initiation determines which mRNAs are translated and at what rate — it is the first committed step of protein synthesis. By regulating initiation factors (especially eIF2 and eIF4 in eukaryotes), cells can rapidly and globally modulate protein production in response to stress, nutrient availability, or developmental signals. Regulating elongation instead would be wasteful: the ribosome would be committed but stalled, tying up resources. Regulating initiation prevents ribosome commitment to unneeded mRNAs in the first place.
This is analogous to transcriptional regulation being preferred over post-transcriptional control when possible — intervening early is more efficient. Global translational shutdown via eIF2α phosphorylation (the integrated stress response) can halt most translation within minutes while allowing stress-response mRNAs with special 5' UTRs to escape the block. This kind of rapid, selective response would be impossible if elongation were the regulated step.