How are amino acids attached to tRNA by aminoacyl-tRNA synthetases?

• A. They attach a specific amino acid to its tRNA using ATP
• B. They catalyze peptide bonds between amino acids
• C. They unwind mRNA
• D. They edit tRNA to remove amino acids

Answer: (A)

The key process being tested is how amino acids are attached to their tRNA in an ATP-powered, two-step reaction. Aminoacyl-tRNA synthetases first use ATP to activate the specific amino acid, forming aminoacyl-AMP and releasing pyrophosphate. Then the activated amino acid is transferred to the 3' end of its corresponding tRNA, yielding aminoacyl-tRNA and AMP. This ATP-driven activation and transfer guarantees that each tRNA carries the correct amino acid before it participates in protein synthesis, and many synthetases include proofreading to hydrolyze incorrect charges, further increasing fidelity. Other roles described in the distractors don’t describe charging: peptide bonds are formed by the ribosome, unwinding mRNA is done by helicases, and removing amino acids from tRNA isn’t the primary charging function (though editing can occur as a proofreading step).