The four different types of nucleotide bases are guanine, adenine, thymine, and cytosine. One DNA strand of each gene acts as a code that specifies the construction of a specific protein. Along the DNA strand of a gene, every three nucleotides code for a single amino acid in a protein. An alteration in the nucleotide sequence of a gene can change the structure of the protein for which it codes. The prion protein contains about 250 amino acids.
Amino acids differ in the type of side chains they contain.
The particular type of side chain gives each amino acid certain specific, characteristic chemical properties. The sequence of amino acids that compromises a protein does not usually retain a linear shape. Instead, once it is formed, the protein often assumes a globular shape in which the amino acid chain folds back on itself in a variety of complex ways. Consequently, the protein folds into a shape that best accommodates these different compatibilities within the chain, and with the water around it. The shape of a protein determines its function because its shape specifies the other molecules with which it can interact.