Which amino acid is known for joining together two peptide strands?

Cystine is the correct answer because it is formed when two cysteine amino acids undergo oxidation, creating a disulfide bond. This bond is crucial in protein structure, as it helps stabilize the three-dimensional shape of proteins by linking different parts of a peptide chain or even linking separate peptide strands together. This connection between strands is vital for the overall stability and function of many proteins, particularly those found in hair and other keratin-rich structures.

The other amino acids listed have different roles in protein structure and function. Lysine and arginine are basic amino acids that play significant roles in protein interactions and modifications but do not specifically form disulfide bonds. Glycine, the simplest amino acid, offers flexibility but does not contribute to the formation of disulfide bonds like cystine does. Understanding the unique properties and functions of each amino acid helps clarify their importance in biochemical processes, particularly in the context of protein architecture and stability.