The double helix refers to the shape of the DNA, which can be likened to a spiral staircase or a twisted ladder. Using the ladder analogy, the outside rails are made of sugar and phosphate molecules, while the rungs are made of molecules known as "bases." An individual unit containing one sugar, one phosphate and one base is known as a nucleotide. Within each rung is a pair of bases linked together by a chemical bond.
DNA contains four specific bases only: adenine (A), thymine (T), guanine (G) and cytosine (C). The four bases only pair up in two ways: A with T, and G with C. Knowing the sequence of bases on one side (strand) of the molecule allows scientists to determine the sequence on the other side.
As was first observed once the structure of DNA was determined, DNA has an inherent ability to be copied. Because adenine always pairs with thymine, and guanine with cytosine, each strand can serve as a template to make identical copies of the molecule. What was not as obvious at the time was how a molecule with limited diversity - only four bases - could contain the information required to make extremely diverse molecules like proteins.