DNA molecule unwinding. Computer artwork of a DNA (deoxyribonucleic acid) molecule unwinding. DNA is composed of two sugar-phosphate strands twisted into a double helix. Between the strands are nucleotide bases (seen projecting from the unwound strands). There are four different bases: guanine, cytosine, thymine and adenine. The DNA is unwound by an enzyme called DNA helicase. Before DNA can be read, replicated or repaired it has to be unwound to allow access to the nucleotides by other molecules and enzymes. DNA contains sections called genes, which encode the body's genetic information.