B-DNA. Computer generated model of B-DNA, showing its twisted double helix structure. B-DNA is a conformation of DNA (Deoxyribonucleic Acid), the carrier of the genetic code. Human DNA is made up of a series of chemical building blocks, or bases, represented by the letters, A (adenine), T (thymine), G (guanine) and C (cytosine). The number and order of these bases determine what we are, how we look, and the diseases to which we may be predisposed. B-DNA has a right-handed double helix with a wide and narrow groove. The bases of B-DNA are perpendicular to the helix axis.