Bacterial Recombination

F' Conjugation

Just as F factors can occasionally integrate into the bacterial chromosome (producing an Hfr cell from an F+ cell), integrated F factors can occasionally excise themselves from the bacterial chromosome. If this excision occurs properly, the Hfr cell becomes an F+ again. The excision is sometimes sloppy, however, and the F factor takes a small segment of the bacterial chromosome with it. Some of the chromosomal DNA has therefore become associated with the episome. When this happens, the cell is called an F'.

Conjugation involving F' cells allows for the possibility of recombination, as shown below:

The F' cell has a full complement of chromosomal genes; however, some of those genes are now on the episome. F' cells are able to initiate conjugation with F- cells because of the presence of the F factor.

When the F factor begins to transfer its DNA to the recipient cell, it will transfer the small segment of chromosomal DNA as well.

Just as in the F+/F- mating, both cells wind up with a copy of the episome. The cell that was F- now has the F factor (along with the piece of chromosomal DNA) and is therefore now F'. This cell, however, also has a complete chromosome, so it will be diploid for the segment of chromosomal DNA on the episome. Such a partially diploid bacterial cell is called a merozygote. The chromosomal DNA on the episome can undergo recombination at high frequency with its homologous sequence on the chromosome.

Let's summarize the three examples of conjugation we've seen, and compare the outcomes:

F+ mating with F- produces 2 F+

Hfr mating with F- produces Hfr and F-

F' mating with F- produces F' and F' merozygote

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The F' cell has a full complement of chromosomal genes; however, some of those genes are now on the episome. F' cells are able to initiate conjugation with F- cells because of the presence of the F factor.
When the F factor begins to transfer its DNA to the recipient cell, it will transfer the small segment of chromosomal DNA as well.
Just as in the F+/F- mating, both cells wind up with a copy of the episome. The cell that was F- now has the F factor (along with the piece of chromosomal DNA) and is therefore now F'. This cell, however, also has a complete chromosome, so it will be diploid for the segment of chromosomal DNA on the episome. Such a partially diploid bacterial cell is called a merozygote. The chromosomal DNA on the episome can undergo recombination at high frequency with its homologous sequence on the chromosome.