DNA is a long molecule composed of two chains of smaller molecules called nucleotides, each which contain a region of nitrogen called the nitrogenous base, a carbon-based sugar molecule called deoxyribose and a region of phosphorus called the phosphate group. There are four types of nitrogenous bases: adenine (abbreviated as A), thymine (abbreviated as T), guanine (abbreviated as G) and cytosine (abbreviated as C).
The nitrogenous bases in DNA pair together, A with T and C with G, to form base pairs that appear as horizontal bars in DNA models. Each nitrogenous base also attaches to a deoxyribose and a phosphate group, which appear as twisting vertical strands called backbones in DNA models. In this way, nucleotides form a spiral called a double helix. The sequence of the bases in the double helix determines the information available to develop and maintain an organism, including its unique combination of traits.
DNA is the foundation of life because it can replicate, or make copies of, itself. When cells divide, a new cell contains an exact copy of the DNA from the old cell so it can function correctly. To create this copy, the new cell follows the pattern set in each strand of the double helix to duplicate the sequence of base pairs.