Plasmid is a circular, double-stranded, and supercoiled DNA. Its molecular weight may range from 106to 108 which may encode from 40-50 genes. A plasmid may contain anywhere between 6 to hundreds of genes. It can exist autonomously within a cell and can self-replicate independently of the bacterial chromosome. Some types of plasmids are capable of integrating with the host chromosome. One such example is the episome. Two plasmid members belonging to the same group are incompatible and cannot coexist in a cell.