A bacterial artificial chromosome (BAC) is a type of vector used in molecular biology to introduce and replicate large DNA fragments (containing 100,000 to 200,000 bases) in bacteria. BACs are based on the structure of naturally occurring bacterial chromosomes, but they have been modified to contain additional genetic elements that allow scientists to easily insert, maintain, and manipulate large DNA fragments in bacteria.

BACs are commonly used in genome sequencing projects to facilitate the analysis of large DNA fragments, such as entire genes or even entire genomes. They are also used in gene expression studies, where they can be used to introduce specific genes into bacteria for the purpose of studying their function. In addition, BACs are used in the development of transgenic animals, where they can be used to introduce specific genes into the genome of an animal for research purposes. Overall, BACs are a useful tool in molecular biology and biotechnology, allowing scientists to study and manipulate large DNA fragments in a precise and efficient manner.