Changes in Genetic Information
The amount of genetic information held within a set of human chromosomes is enormous, equal to twenty sets of Encyclopaedia Britannica. Because each of the trillions of cells in an adult body results from mitosis (except for egg and sperm), genetic information had to be replicated many times and with a high degree of accuracy. DNA can peruse itself for errors and correct them, a process termed DNA repair. Still, occasionally a replication mistake occurs or DNA is damaged, altering the genetic information. Such a change in DNA is called a mutation.
Some mutations can cause devastating medical conditions; occasionally, a mutation can confer an advantage. For example, up to one percent of the individuals of some populations have mutations that render their cells unable to become infected with HIV. These lucky people, thanks to their mutation, cannot contract AIDS.