The main structural molecule in DNA is deoxyribonucleic acid, which consists of two long strands forming a double helix. This helix is made up of nucleotide units that include a sugar, a phosphate group, and a nitrogenous base. The sequence of these bases encodes genetic information, dictating everything from inherited traits to cellular functions. DNA's structure allows it to replicate accurately during cell division, ensuring that genetic information is passed on to daughter cells.