DNA. Computer generated model of a DNA (deoxyribonucleic acid) molecule. DNA is the carrier of the genetic code and controls the growth and development of all living things. Its double helix structure was discovered in 1953 by James Watson and Francis Crick. Human DNA is made up of a series of chemical building blocks represented by letters, A (adenine), T (thymine), G (guanine) and C (cytosine). The number and order of these letters, or bases, make up the genetic code which is unique to every individual. This code plays a large role in determining what we are, how we look, and the diseases to which we may be predisposed.