DNA structure consists of two helical DNA chains coiled around the same axis to form a right-handed double helix. The hydrophilic backbones of alternating deoxyribose and negatively charged phosphate groups are on the outside of the double helix, facing the surrounding water.

The purine and pyrimidine bases of both strands are stacked inside the double helix, with their hydrophilic and nearly planar ring structures very close together and perpendicular to the long axis of helix. Each base of one strand is pared in the same plane with a base of the other strand. It is important to note the long that three hydrogen bonds can form between G and C, but only two can form between A and T. Other pairing of bases tend to destabilize the double-helical structure.

Many significant deviations form the Watson-Crick DNA structure, also referred to as B-form DNA, are found in cellular DNA. The B-form DNA is the most stable structure for a random sequence of DNA molecule under physiological conditions. Two other DNA structural variants that have been well characterized in crystal structures are the A and Z-forms. The first one is still arranged in a right-handed double-helix, but for a given DNA molecule, the A-form will be shorter and have a great diameter than the B-form. Z-form DNA is more radical departure from the B structure – the most obvious distinction is the left-handed helical rotation and it is longer than the other DNA forms.

The double-helical model of DNA structure immediately suggested a mechanism for the transmission of genetic information. The essential feature of the model is the complementarity of the two DNA strands. Making a copy of this structure (replication) could logically proceed by separating the two strands and synthesizing a complementary strand for each by joiing nucleotides in a sequence specified by the base-paring rules. Each preexisting strand could function as a template to guide the synthesis of the complementary strand.