Parent cells replicate using a process called mitosis. The purpose of mitosis is to replicate and separate the chromosomes, which contain the DNA, into separate nuclei for the daughter cells. Normally, this process occurs unhindered, passing exact copies of the chromosomes and thus, the DNA, down to the daughter cells. Of course, mutations can and do occur, but they are not the norm. Mutations are when the genetic code in the DNA is changed in some form. There are three mutations that can occur, that will change the existing genetic code: insertion, deletion, or substitution. In an insertion, one of the four existing bases that comprise the strand of DNA are inserted into the already existing genetic code. In a deletion, one of the bases in the existing genetic code is removed. In a substitution, one of the bases is removed, then replaced with one of the other three bases.