Q.Amino acids in a polypeptide are joined by --------------- bond.

Two amino acids condense, losing water, to form an amide (peptide) bond linking them.

A peptide (amide) linkage is formed by the condensation reaction between the carboxyl group (–COOH) of one amino acid and the amino group (–NH₂) of another amino acid, with elimination of a water molecule:

$H_2N-CH_2-COOH\ (glycine) + H_2N-CH(CH_3)-COOH\ (alanine) \rightarrow H_2N-CH_2-CO-NH-CH(CH_3)-COOH + H_2O$

The resulting $-CO-NH-$ bond is the peptide linkage, and the product (glycylalanine, a dipeptide) can continue to react with further amino acid molecules at either end to build up longer chains (polypeptides/proteins).

Amino acids condense, losing water, to form an amide (peptide) linkage between the $-COOH$ of one and the $-NH_2$ of the next.

Amino acids are organic compounds that contain both a basic amino group ($-NH_2$) and an acidic carboxyl group ($-COOH$) attached to the same carbon skeleton (typically both on the $\alpha$-carbon, giving $\alpha$-amino acids). They are the building-block monomers from which proteins are constructed.

Peptide bond formation: when the carboxyl group of one amino acid reacts with the amino group of a second amino acid, a molecule of water is eliminated and an amide linkage — called a peptide bond ($-CO-NH-$) — is formed between them:

$H_2N-CHR_1-COOH + H_2N-CHR_2-COOH \rightarrow H_2N-CHR_1-CO-NH-CHR_2-COOH + H_2O$

The resulting compound (a dipeptide here) still has a free $-NH_2$ at one end and a free $-COOH$ at the other, so the process can repeat with further amino acid units to build up a long polypeptide/protein chain.