The structure of DNA is a double helix. This means that there are two strands coiled around each other. The molecule is bonded together by the bases with hydrogen bonds. Guanine pairs with Cytosine by three hydrogen bonds while Adenine bonds with Thymine by two hydrogen bonds (see base pair).
Watson and Crick's insight into the double helical structure of the DNA molecule was based upon Erwin Chargaff noting that these pairs of bases were always in the same concentration.