The first step in biosynthesis of polypeptide is catalyzed by :

Question

Accepted Answer

The correct option is 4: Aminoacyl-tRNA synthetase.

The biosynthesis of a polypeptide, a process known as translation, cannot begin until the raw materials are prepared. The absolute first step in this entire process is known as **aminoacylation** or the "charging" of transfer RNA (tRNA) molecules. This vital step is catalyzed by the enzyme **Aminoacyl-tRNA synthetase (aaRS)**. This enzyme acts as the "interpreter" of the genetic code, ensuring that each of the 20 standard amino acids is covalently linked to its correct, or cognate, tRNA molecule, a product called an aminoacyl-tRNA (aa-tRNA). This highly accurate two-step reaction requires ATP to activate the amino acid, setting the stage for all subsequent translation events by providing the ribosome with correctly labeled building blocks. Without the correct aa-tRNA being produced by this enzyme, the entire process of polypeptide assembly would be flawed.

The incorrect option is 1: Terminal transferase.

Terminal transferase (TdT) is a specialized enzyme that is not involved in protein synthesis or translation at all. It is a type of DNA polymerase that uniquely catalyzes the template-independent addition of deoxyribonucleotides to the 3'-hydroxyl terminus of a DNA strand. Biologically, its most significant role is in the adaptive immune system, where it adds non-templated nucleotides (N-nucleotides) to the junctions of V, D, and J gene segments during V(D)J recombination, thereby increasing the diversity of antibodies and T-cell receptors. This function is restricted to DNA modification, making it irrelevant to the initial steps of polypeptide production.

The incorrect option is 2: Peptidyl transferase.

Peptidyl transferase is a crucial enzyme, or more accurately, a **ribozyme** (a catalytic RNA component of the large ribosomal subunit). Its function is to catalyze the formation of the **peptide bond** between the growing polypeptide chain, which is attached to the tRNA in the P-site, and the new amino acid, which is carried by the tRNA in the A-site. This reaction is a key component of the **elongation** phase of protein synthesis, where the chain is actively being built. Therefore, while essential for polypeptide formation, it acts much later in the overall process, specifically after the tRNAs have already been charged by the aaRS.

The incorrect option is 3: Initiation factor (IFs).

Initiation factors (IFs) are a group of essential protein molecules that play a regulatory and structural role in the very early moments of translation. Their primary job is to help the small and large ribosomal subunits, the mRNA template, and the initiator tRNA correctly assemble at the start codon (usually AUG). While they are involved in the process of **initiation**, they are not the *catalyst* for the fundamental chemical step that prepares the primary building blocks. The chemical reaction that charges the tRNAs must be complete before the initiation factors can begin to form the functional ribosome complex.