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8.13 The F factor is transferred during conjugation between an F+ and F~ cell.

F+ cell

Bacterial ^ chromosome

Hfr cell

Crossing over takes place between F factor and chromosome.

The F factor is integrated into the chromosome.

Crossing over takes place between F factor and chromosome.

The F factor is integrated into the chromosome.

8.14 The F factor is integrated into the bacterial chromosome in an Hfr cell.

is replicated, producing a circular, double-stranded copy of the F plasmid ( FIGURE 8.13e). If the entire F factor is transferred to the recipient F~ cell, that cell becomes an F+ cell.

Hfr cells Conjugation transfers genetic material in the F plasmid from F+ to F~ cells but does not account for the transfer of chromosomal genes observed by Lederberg and Tatum. In Hfr (high-frequency) strains, the F factor is integrated into the bacterial chromosome ( FIGURE 8.14). Hfr cells behave as F+ cells, forming sex pili and undergoing conjugation with F~ cells.

In conjugation between Hfr and F~ cells ( FIGURE 8.15a), the integrated F factor is nicked, and the end of the nicked strand moves into the F~ cell ( FIGURE 8.15b), just as it does in conjugation between F+ and F~ cells. In the Hfr cells, the F factor is linked to the bacterial chromosome, so the chromosome follows it into the recipient cell. How much of the bacterial chromosome is transferred depends on the length of time that the two cells remain in conjugation.

Once inside the recipient cell, the donor DNA strand is replicated ( FIGURE 8.15c), and crossing over between it and the original chromosome of the F~ cell ( FIGURE 8.15d) may take place. This gene transfer between Hfr and F~ cells is how the recombinant prototrophic cells observed by Lederberg and Tatum were produced. When crossing over has taken place in the recipient cell, the donated chromosome is degraded, and the recombinant recipient chromosome remains ( FIGURE 8.15e) to be replicated and passed to later generations by binary fission.

In a mating of Hfr X F", the F~ cell almost never becomes F+ or Hfr, because the F factor is nicked in the middle during the initiation of strand transfer, placing part of F at the beginning and part at the end of the strand to be transferred. To become F+ or Hfr, the recipient cell must receive the entire F factor, requiring that the entire bacterial chromosome is transferred. This event happens rarely, because most conjugating cells break apart before the entire chromosome has been transferred.

The F plasmid in F+ cells integrates into the bacterial chromosome, causing an F+ cell to become Hfr, at a frequency of only about 1/10,000. This low frequency accounts for the low rate of recombination observed by Lederberg and Tatum in their F+ cells. The F factor is excised from the bacterial chromosome at a similarly low rate, causing a few Hfr cells to become F+.

F' cells When an F factor does excise from the bacterial chromosome, a small amount of the bacterial chromosome

Hfr cell F- cell

Hfr cell F- cell

Hfr Donor Conjugation Bacteria
In the Hfr cell, the F factor is integrated into the bacterial chromosome.

I 8.15 Bacterial genes may be transferred from an Hfr cell to an F~ cell in conjugation.

Hfr cell F- cell

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