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The comparison of ARS and ARS / CEN plasmids is tainted by the degree of uncertainty of the results . Plasmids are replicating units ( replicons ) of genetic material outside the chromosome . The data showing a considerably shorter lifespan for cells transformed with 2 plasmids compared to the rest of the cells suggests that the higher copy number of 2 plasmids as compared with ARS plasmids is more significant as a cause of aging than the greater bias of ARS plasmids for the mother during plasmid segregation . If a supercoiled plasmid is first linearized with another restriction enzyme or relaxed with topoisomerase , less enzyme may be needed for digestion . Where indicated ( Table I ) , the recombinant plasmids were then shot into a psbA deletion mutant . These plasmids are also expected to shorten lifespan . Plasmids are usually replicated by the cell , and some plasmids are even capable of being integrated into the host ’ s genome ( 1 ) . For example , bacteriophage P1 is capable of carrying inserts 80 kb , YACs ( Yeast Artificial Chromosomes ) carry hundreds of kb up to 1 Mb , the insert size in a plasmid is 3 kb . The ability of a restriction enzyme to find a single site by linear diffusion in the supercoiled plasmid is also presumed to be different than for any of the sites on a linear substrate . F ( fertility ) plasmids are involved in bacterial sex or conjugation , where they transfer between bacteria and sometimes take part of the bacterial genome with them , allowing recombination or replacement of parts of the host genome .