DNA. Computer graphic of part of the molecule of DNA (Deoxyribonucleic Acid), showing its twisted double helix structure. DNA is the carrier of the genetic code. Its structure was discovered in 1953 by James Watson and Francis Crick, who recog- nised its helical form from X-ray crystallography data. Here, spherical atoms in the molecule are colour-coded: phosphate (yellow); oxygen (orange); carbon (pink); nitrogen (blue). The double helix of DNA contains complementary pairs of organic bases at its core: adenine and thymine; cytosine and guanine. It is the sequence of these bases which form the genetic codes of inheritance.