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"Rolling replication" transfers the F (fertility) plasmid between bacteria. The F plasmid contains the gene for formation of pili, the hairlike projection through which the plasmid passes. Adapted from Curtis, 1989.

Bacterial chromosome

Bacterial chromosome

Cynthia A. Needham


Curtis, Helena, and Sue Barnes. Biology, 5th ed. New York: Worth, 1989.

Madigan, Michael T., John M. Martinko, and Jack Parker. Brock Biology of Microorganisms, 9th ed. Upper Saddle River, NJ: Prentice Hall, 2000.

Robinson, Richard, ed. Biology. New York: Macmillan Reference USA, 2001.

Snyder, Larry, and Wendy Champness. Molecular Genetics of Bacteria. Washington, DC: ASM Press, 1997.


Lederberg and his colleague E. L. Tatum set out to determine whether a sexual process might occur in bacteria. The bacterial species he used in the experiments was Escherichia coli. This was fortuitous, as it turned out, because E. coli often contains a special kind of conjugative plasmid that has the ability to insert itself into the cell's chromosome. Once this happens, the donor cell can transfer to a recipient not only plasmid genes but also large numbers of chromosomal genes.

Lederberg worked with two different nutritional mutants of E. coli. One strain required biotin and methionine to grow; the other strain required threonine and leucine. Lederberg mixed the two strains together and then attempted to grow them without supplying any of the four nutrients. His hypothesis was that any cell able to grow without the four nutrients would have all four genes intact, and would thus have received the functioning genes from the other strain and incorporated them into its chromosome. The incorporation of the genes in this manner is called genetic recombination.

As he predicted, Lederberg's experiment yielded cells that did not require any of the nutrients to grow. In a second set of experiments, Lederberg showed that cell-to-cell contact was necessary for genetic recombination to occur. Over several years, he and other scientists discovered the mechanics of the entire process that we now call conjugation.

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