DNA, computer artwork. DNA (deoxyribonucleic acid) consists of two strands of sugar phosphates forming a double helix (spiral). The strands are held together by interactions between nucleotide bases on each strand. Although there are four types of nucleotide base, only two base pairings are possible: adenine bonds to thymine, and cytosine bonds to guanine. This means each DNA strand is a complementary copy of the other, a vital part of DNA replication. The base sequence along the molecule is interpreted according to the genetic code, and controls growth and development. DNA is stored in cell nuclei.