Monomer of a polypeptide.

• A. Codon
• B. tRNA
• C. Amino acid
• D. Ribosome

Answer: (C)

Amino acids are the building blocks that form polypeptides. Proteins are polymers made when amino acids join end to end via peptide bonds, creating a chain. Each amino acid has an amino group and a carboxyl group; linking the carboxyl of one amino acid to the amino of the next releases a molecule of water in a condensation reaction, appending one residue at a time to the growing polypeptide. The sequence of these amino acids—their side chains or R groups—determines the protein’s properties and structure. While mRNA codons tell the cell which amino acid to add, and tRNA brings the amino acid to the ribosome and the ribosome catalyzes the bond formation, those elements are parts of the decoding and synthesis machinery, not the monomer itself. Therefore, the monomer of a polypeptide is the amino acid.