Genes are stretches of DNA that encode information about proteins.
Eukaryotic and Archaebacterial genes also contain introns , which are additional DNA sequences scattered through the coding sequence, that must be spliced out of the messenger RNA before it can be translated into protein. When the DNA is replicated, most of the genes are duplicated exactly. But occasionally, an incorrect base or two can be put into a new DNA strand, and the DNA sequence of the gene can be changed. When this happens, the two new DNA molecules don't have exactly the same sequence, and the two copies of the gene don't contain exactly the same information. We say that the DNA sequence has mutated , and that the new version of the gene is mutant .