The E.coli genome is haploid and contains a single circular chromosome that is replicated and distributed to daughter cells via binary fission. Bacteria cells may also contain smaller DNA molecules, plasmids, that do not carry essential genes. Wildtype E.coli are prototrophic and thus are able to grow on minimal media. Geneticist are able to study rare genetic events in bacteria because of the ability to screen huge numbers of cells using techniques like replica plating.
Bacteria can transfer genes from one cell to another with relative ease.
In each case, the cell from which the DNA has originated is considered the donor cell and the cell that receives the DNA is the recipient. For conjugation to occur, the donor cell must contain a specialized plasmid called the F plasmid that contains genes that mediate DNA transfer. These genes will be transferred along with the F plasmid. Bacteriophage are the viruses that infect bacteria. Phage have been a valuable genetic model system because many generations and billions of individuals can be produced in a short time thus making it possible to isolate rare mutants.