The realization that the components of a eukaryotic chromosome that are required for stable replication and segregation, at least in the yeast Saccharomyces cerevisiae, consist of rather small and well-defined sequences has led to the construction of recombinant chromosomes (yeast artificial chromosomes; YACs). These were used initially for investigation of the maintenance of chromosomes, but latterly as vectors capable of carrying very large cloned fragments. The centromere, telomere and replication origin sequences have been isolated and combined on plasmids constructed in E. coli. The method of construction of the YAC clone is similar to that for cosmids, in that two end fragments are ligated with target DNA to yield the complete chromosome, which is then introduced (transfected) into yeast cells. YAC vectors can accommodate genomic DNA fragments of more than 1 Mb, and hence can be used to clone entire human genes, such as the cystic fibrosis gene, which is 250 kb in length. YACs have been invaluable in mapping the large-scale structure of large genomes, for example in the Human Genome Project
