DNA helices. Models showing the double helix and nucleotide base structure of DNA (deoxyribonucleic acid) molecules. The double helix is formed by two spiralling strands of sugar phosphates. Nucleotide bases (red, blue, yellow, green) are arrayed along these strands. The four bases can form two bonding pairs: adenine bonds to thymine and cytosine bonds to guanine. This complementary base pairing holds the two strands together to form the DNA 'ladder'. DNA is found in cell nuclei, and the sequence of its nucleotide bases forms the genetic code for protein synthesis and the functioning and growth of every living organism.