Bacteria often carry small circular DNA molecules called plasmids. These molecules sometimes carry genes that have important properties; for example, some plasmid genes make the host cell resistant to antibiotics, with important consequences for human health. Plasmids are widely used in genetics labs in the process of cloning genes. Like organelle DNA in eukary-otes, plasmid molecules are replicated independently of the cell chromosome, and one cell can contain many copies. Plasmid DNA molecules are replicated randomly and partitioned randomly to daughter cells, just like genotype set of genes organelle genes. As a result, a cell with two different genotypes of plasmids may produce daughter cells with only one or the other. This process, analogous to vegetative segregation, is called plasmid incompatibility.


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