Bases are the components that make up DNA. There are 4 bases: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T). In a DNA molecule, the bases pair with each other to hold together the two strands in a double strand of DNA. A always pairs with T, and G always pairs with C. Base pairs are also used as a unit of measure to indicate a length of DNA. A piece of DNA that is 10bp long has 10 base pairs in it. Likewise, a gene that is 2Kb long has 2,000 base pairs. The human genome has around 3 billion base pairs.