Amino acid activation (tRNA charging): During this preparatory step for translation, what covalent attachment occurs?

Introduction / Context:
Before translation can begin, amino acids must be “activated” by attachment to their corresponding tRNAs. This charging step is critical because the ribosome relies on accurate tRNA–amino acid pairing to interpret codons correctly.

Given Data / Assumptions:

• The step in question precedes peptide bond formation.
• The enzymes involved are aminoacyl-tRNA synthetases.
• Energy is provided by ATP hydrolysis.

Concept / Approach:

Aminoacyl-tRNA synthetases catalyze a two-step reaction: formation of aminoacyl-AMP and transfer of the aminoacyl group to the 3′ end of the cognate tRNA. The product, aminoacyl-tRNA (charged tRNA), delivers its amino acid to the ribosomal A site during elongation.

Step-by-Step Solution:

Verification / Alternative check:

Many antibiotics and toxins exploit this step’s specificity; mischarging leads to misincorporation and defective proteins. Experiments detect charged tRNA via gel shifts or acid-urea PAGE, corroborating the covalent link between amino acid and tRNA.

Why Other Options Are Wrong:

• Amino acid to mRNA: no covalent linkage occurs there; decoding is via base pairing, not covalent bonds.
• Methylation of rRNA or amino acids: these are separate regulatory or modification events, not “amino acid activation.”

Common Pitfalls:

• Confusing activation (charging) with initiation (small subunit + initiator tRNA + mRNA).
• Assuming GTP is used here; ATP is used by synthetases.

Final Answer:

An amino acid is bound to tRNA.