A gene is a segment of DNA that contains instructions for building one or more molecules that help the body work. Deoxyribonucleic acid is shaped like a tightly coiled ladder, called a double helix. The two ladder rails are called backbones, and the rungs are pairs of four building blocks (adenine, thymine, guanine, and cytosine) called bases. The sequences of these bases provide the instructions for building molecules, most of which are proteins. It has a pentose sugar bonds and phosphate backbone