What is tRNA charging and what enzyme mediates it?

• A. Attaching amino acids to tRNA by aminoacyl-tRNA synthetases; uses ATP to form aminoacyl-tRNA.
• B. Adding poly-A tails to mRNA by poly(A) polymerase.
• C. Transferring amino acids to ribosome rRNA.
• D. Hydrolyzing ATP to provide energy for translation.

Answer: (A)

tRNA charging is the process of attaching the correct amino acid to its corresponding tRNA to form aminoacyl-tRNA, the charged carrier used in translation. This reaction is carried out by aminoacyl-tRNA synthetases, a family of enzymes specific for each amino acid. The amino acid is first activated by ATP to form aminoacyl-AMP, then the aminoacyl group is transferred to the 3' end of the tRNA, producing aminoacyl-tRNA and releasing AMP. This pairing ensures fidelity so the ribosome inserts the right amino acid when decoding the mRNA. The other options describe different steps: polyadenylation of mRNA, transfer of amino acids to ribosomal RNA (which does not occur), and general energy use for translation rather than the specific charging step.