Artificial Chromosome(YAC,MAC) Vectors for Cloning Large DNA Segments

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	Artificial Chromosome(YAC,MAC) Vectors for Cloning Large DNA Segments - We know that in plasmids, sequences upto 10-15 kbp; in lambda (A) phage sequences upto 22kbp and in cosmids sequences upto 40kbp can be cloned. Yeast artificial chromosome (Y AC) vectors, mentioned earlier allow cloning of sequences that are several hundred kilobase pairs (upto 1000 kbp = 1 Mbp) long. These long molecules of DNA which may represent whole chromosomes can be cloned in yeast by ligating them to vector sequences that allow their propagation as linear artificial chromosomes.
These long DNA molecules can be generated and allow construction of comprehensive libraries (with large DNA segments) in microbial hosts. With the isolation of mammalian telomere and centromere, mammalian artificial chromosomes (MACs) will also be produced in future. YACs have two disadvantages. (i) Cloning efficiency is low (1000 clones/ µg DNA as against 106 - 107 clones/µg DNA for cosmids), thus making it impractical to generate complete genomic libraries through the use of YACs (ii) It is not possible to recover large amount of pure insert DNA from individual clones; selective amplification of Y AC's DNA has recently allowed this problem to be overcome. Both these problems have been overcome in bacteriophage PI system, described earlier.