DNA is a very long molecule (one cell contains about 2 metres of DNA!), meaning its coiled structure around proteins called histones allows it to fit into the small space of the nucleus. It also has a double helix structure, making it very stable, and the sugar-phosphate backbone protects the nitrogenous bases from damage (which could lead to mutation). Finally, each nucleotide is paired with a specific other: adenine - thymine, cytosine - guanine, which enables DNA to replicate itself easily and accurately, using the semi-conservative method.