DNA mutation, conceptual image. Computer illustration representing a mutation (yellow) in a gene on a DNA (deoxyribonucleic acid) molecule. 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.