<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2009) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (December 2009) (Learn how and when to remove this template message) </Td> </Tr> <P> A high - frequency recombination cell (Hfr cell) (also called an Hfr strain) is a bacterium with a conjugative plasmid (for example, the F - factor) integrated into its chromosomal DNA . The integration of the plasmid into the cell's chromosome is through homologous recombination . A conjugative plasmid capable of chromosome integration is also called an episome (a segment of DNA that can exist as a plasmid or become integrated into the chromosome). When conjugation occurs, Hfr cells are very efficient in delivering chromosomal genes of the cell into recipient F cells, which lack the episome . </P> <P> The Hfr strain was first characterized by Luca Cavalli - Sforza . William Hayes also isolated another Hfr strain independently . </P>

When does an f+ cell become and hfr cell