NEET 2020 Botany Question 33 Solution

Polypeptide synthesis requires energetic priming because the formation of an amide peptide bond between free amino acids is thermodynamically unfavorable: 1. Activation of Amino Acid: The amino acid reacts with ATP in the presence of the enzyme aminoacyl-tRNA synthetase to form an enzyme-bound aminoacyl-AMP intermediate (aminoacyl-adenylate), releasing inorganic pyrophosphate ($PP_i$). 2. Transfer to tRNA (Charging): The amino acid is transferred to the $3'\text{-CCA}$ terminal ribose of its cognate tRNA, forming high-energy aminoacyl-tRNA (charged tRNA). 3. Sequential Significance: NCERT explicitly designates aminoacylation of tRNA as the first phase of translation. Without charged tRNAs carrying energized amino acids, ribosomal initiation complexes cannot proceed.

- Binding of mRNA to ribosome: Occurs during the initiation phase on the small 40S/30S ribosomal subunit, *after* tRNAs have been charged. - Recognition of an anti-codon: Occurs at the ribosomal A-site during the elongation phase.