<P> Cosmids are plasmids that incorporate a segment of bacteriophage λ DNA that has the cohesive end site (cos) which contains elements required for packaging DNA into λ particles . It is normally used to clone large DNA fragments between 28 and 45 Kb . </P> <P> Insert size of up to 350 kb can be cloned in bacterial artificial chromosome (BAC). BACs are maintained in E. coli with a copy number of only 1 per cell . BACs are based on F plasmid, another artificial chromosome called the PAC is based on the P1 phage . </P> <P> Insert of up to 3,000 kb may be carried by yeast artificial chromosome . </P> <P> Human artificial chromosome may be potentially useful as a gene transfer vectors for gene delivery into human cells, and a tool for expression studies and determining human chromosome function . It can carry very large DNA fragment (there is no upper limit on size for practical purposes), therefore it does not have the problem of limited cloning capacity of other vectors, and it also avoids possible insertional mutagenesis caused by integration into host chromosomes by viral vector . </P>

How can a bacteriophage be used as a vector