51.1 Genes as determinants of the inherent properties of speciesproteins an organism can synthesize, as well as the tim-ing and amount of production of each protein, is an ex-tremely important determinant of the structure andphysiology of organisms. A protein generally has one oftwo basic functions, depending on the gene. First, theprotein may be a structural component, contributing tothe physical properties of cells or organisms. Examplesof structural proteins are microtubule, muscle, and hairproteins. Second, the protein may be an active agent incellular processes—such as an active-transport proteinor an enzymethat catalyzes one of the chemical reac-tions of the cell.The primary structure of a protein is a linear chainof amino acids, called a polypeptide.The sequence ofamino acids in the primary chain is speci±ed by the se-quence of nucleotides in the gene. The completed pri-mary chain is coiled and folded—and in some cases, as-sociated with other chains or small molecules—to forma functional protein. A given amino acid sequence may
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