Transmitted light micrograph of liquid crystalline DNA (Deoxyribonucleic Acid), TLM. DNA is the carrier of the genetic code. Its 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, determine what we are, how we look, and the diseases to which we may be predisposed.