3. Yeast artificial chromosome- Yeast artificial chromosome (YAC) is the vector of choice used to clone very large DNA fragment (~100kb) to prepare genomic library. YAC vector is like a chromosome as it has ARS sequences, centromere sequence and telomere at the two ends to give stability. A typical YAC plasmid (pYAC) is shown in Figure 19.2. It has an ampicillin resistance gene (Amp^r) for selection in e.coli and an e.coli origin of replication for propagation in bacteria. In addition, it has ARS for replication, CEN for centromere function, and URA3, TRP1 for selection in yeast. URA3 and TRP1 is the crucial gene of uracil biosynthesis and tryptophan biosynthesis pathway. For cloning, YAC is digested with SmaI/BamHI, alkaline phosphatase to generate a linear plasmid DNA, now foreign DNA is added for ligation. The recombinant DNA will allow a yeast (Ura-/Trp-) to grow on uracil and tryptophan deficient media.

Figure 19.2: Vector map of YAC plasmid (pYAC) and YAC cloning system.