Flexi Says: According to the National Human Genome Research Institute, the primary method used to sequence the human genetic code was map-based, or BAC-based, sequencing. BAC stands for "bacterial artificial chromosome." Human DNA is cut into smaller pieces between 150,000 and 200,000 base pairs. These fragments are cloned in bacteria in large enough amounts for sequencing. If carefully chosen to minimize overlap, it takes about 20,000 different BAC clones to contain the 3 billion pairs of bases of the human genome. A collection of BAC clones containing the entire human genome is called a "BAC library."