What determines the primary structure of proteins?

The primary structure of proteins is defined by the specific sequence of amino acids that are linked together in a polypeptide chain. This sequence is determined by the genetic information encoded in DNA, where each set of three nucleotides, known as a codon, corresponds to a specific amino acid. Once the amino acids are sequentially joined through peptide bonds, they form a linear chain which constitutes the protein's primary structure. The uniqueness of each protein is derived from its distinct sequence of amino acids, with different sequences leading to different proteins with various functions and properties.

In contrast, the other options describe aspects that are related to protein structure but do not define its primary structure. For example, while the arrangement of nucleotides in DNA influences the amino acid sequence, it is not the sequence itself but rather the intermediary step that leads to protein synthesis. Disulfide bridges are important for stabilizing the tertiary and quaternary structures of proteins, representing interactions that occur after the primary structure has been established. Additionally, hydrogen bonds contribute to secondary structures such as alpha-helices and beta-pleated sheets, which come into play only after the primary structure is formed.