DNA Is the Repository of Genetic Information

The instructions for constructing proteins is contained in extremely long, double-stranded molecules called DNA (deoxyribonucleic acid). DNA molecules are composed of a linear array of subunits called nucleotides. How information is stored can be understood by likening DNA to a book (see Box A-1). The nucleotides are arranged in sets called genes. In general, one gene contains the information for making one protein. DNA molecules are long enough to contain genes for the many thousands of different proteins that form and run cells.

Every organism on earth contains its own DNA molecules. It is the nucleotide sequence, in some cases billions of nucleotides long, that determines the properties of each organism. Changes in the nucleotide sequence of a gene can change the amino acid sequence in the protein specified by the gene. If that protein is involved in antibiotic action, the DNA change may cause antibiotic resistance. Thus, changes in the information in DNA are the molecular basis underlying the emergence of antibiotic resistance. A microbe or virus with such a change is called an antibiotic-resistant mutant.