Mutations defined as changes in the sequence of the bases, can occur by a number of means: for example, single base substitutions that might result from errors during DNA replication (copying of the DNA prior to cell division; see below), or deletion of larger pieces of a chromosome or translocation events (swapping pieces from one chromosome to another).
It is also important to note that mutations can be grouped into two general categories, germline and somatic.
Germline mutations are those that we inherit from our parents and will pass on to our children, while somatic mutations are those that occur during our lifetime and are not passed on to our descendants. Most mutations have no effect on our health and lifespan because they result in changes to DNA base sequences that do not alter the coded genetic information.
However, those mutations that make changes to the genetic information such that cells no longer can control their growth are the hallmark of cancer. The discovery of the structure of DNA in 1953 by James Watson and Francis Crick initiated a rapid increase in the study of how the information encoded in the double-helical fibers of DNA was copied and passed from generation to generation. It has been estimated that the human genome carries approximately 35,000 sequences whose codes are read to produce protein molecules with specific metabolic functions.
In cells that are growing and dividing, and even in those cells in a resting phase, mutations occur at an alarmingly high rate, both from normal cellular processes (e.g., copying of the DNA strands prior to cell division) and from exposure to environmental or chemical carcinogens [e.g., the ultraviolet rays in sunlight or especially cigarette smoke (at least 40 different carcinogens are found in a typical cigarette)]. However, all cells use remarkably effective strategies to either avoid mutations or to repair them when they occur.