DNA can serve as the genetic material because it is unique among cellular molecules in being able to specify exact copies of itself. This copying process, known as replication, discussed in detail in Chapter 12, allows the genetic information from one DNA molecule to be copied into that of another DNA molecule. Faithful replication is critical in that it allows DNA to pass genetic information from cell to cell and from parent to offspring. The copying must reproduce the sequence of subunits almost exactly because, as we will see in Chapter 14, mistakes in DNA replication that go unrepaired may be harmful to the cell or organism.

An unrepaired error in DNA replication results in a mutation, which is a change in the genetic information in DNA. A mutation in DNA causes the genetic difference between virulent and nonvirulent Streptococcus pneumoniae. While most mutations in genes are harmful, rare favorable mutations are essential in the process of evolution because they allow populations of organisms to change through time and adapt to their environment.