Each strand of the double helix of a DNA module is formed of molecular groups called nucleotides. A nucleotide is a combination of three smaller types of molecules: a sugar, one or more phosphate groups and a nitrogenous base. There are four types of bases called Adenine (A), Thymine (T), Guanine (G) and Cytosine (C), and the nucleotides containing these bases can appear in any order along a strand of DNA. While the nucleotides are bonded together with strong chemical bonds in a strand, the bases can also bond to other bases via a weaker so-called hydrogen bond, linking the two stands. These wind around each other to form a double helix. ‘A’ will only bond with its complementary base’ T’, and ‘C’ will only bond with ‘G’ — so for every ‘A’ in one strand there is an adjacent ‘C ‘in the other strand. Shown here is a segmentof DNA straightened for clarity.