yeast artificial chromosome

Genetically engineered circular chromosomes that contain elements from chromosomes contributed by Saccharomyces and segments of foreign DNAs that can be much larger than those accepted by conventional cloning vectors (q.v.). As shown in the diagram below, YACs are generated from synthetic minichromosomes that contain a yeast centromere (C), a replication orgin (RO), and fused telomeres (Tℒ and Tr). In addition, the circular chromosome contains three marker genes (M1, M2, and M3), which when expressed, allow selection of the cells carrying the plasmid and sites 1 and 2, which allow specific restriction endonucleases (q.v.), to break the molecule. Cleavage at 1 opens the ring, while cleavage at 2 generates centric and acentric fragments with ends that will accept foreign DNA fragments. Once these are ligated, an artificial chromosome is generated with a short and a long arm. This contains the spliced segment of foreign DNA to be cloned. Such artificial chromosomes are distributed normally during subsequent yeast divisions, and so colonies containing YACs are generated. In cells possessing the insert, the M1 and M3 markers are expressed, but the damaged M2 is not. So religated YACs can be distinguished from unbroken plasmids. YACs can accept DNA inserts up to 1,000 kilobases long. Compare with bacterial artificial chromosomes (BACs) and P1 artificial chromosomes (PACs).

See Chronology, 1987, Burke, Carle, and Olson; DNA vector, kilobase, plasmid cloning vectors.

Yeast artificial chromosomes