Computer illustration of a DNA (deoxyribonucleic acid) molecule with three mutations. Near top is an insertion mutation, where an additional base has been added to the green strand. At centre is a deletion mutation, where a base had been deleted from the green strand. Near bottom is a substitution mutation, where a red base has been inserted instead of a yellow base. DNA consists of long strands forming a double helix of phosphates and sugars. These two helices are held together by pairs of nucleotide bases (sticks). There are four different nucleotide bases. It is the varying sequence of these four bases along a DNA helix that forms the genetic code for that individual. A typical DNA molecule can contain a sequence that is many millions of bases long. A mutation is the alteration of the nucleotide sequence.