What type of side chain does cysteine have?

Cysteine is an amino acid that is distinct for having a thiol group in its side chain. The thiol group, which consists of a sulfur atom bonded to a hydrogen atom (–SH), is responsible for the unique properties of cysteine, particularly its ability to form disulfide bonds with other cysteine residues. These disulfide bridges can stabilize the three-dimensional structure of proteins by linking different parts of a polypeptide chain or even different polypeptide chains together.

In the context of the other choices, the amino group is a fundamental part of all amino acids, but it is not specific to cysteine's side chain. The hydroxyl group is characteristic of serine and threonine, which are also amino acids, while the methyl group is a simple alkyl group found in amino acids like alanine. However, cysteine's defining feature is the thiol group, making it crucial for maintaining the integrity and functionality of many proteins.