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Modern Genetic Analysis Chapter 9. The Genetics of Bacteria and Phages
Solved Problem

3. | Compare the mechanism of transfer and inheritance of donor lac+ genes in crosses using Hfr, F+, and F-lac+ strains. How would an F cell that cannot undergo normal homologous recombination (rec) behave in crosses with each of these three strains? Would the cell be able to inherit the lac+ genes? See answer

Solution

Each of these three strains donates genes by conjugation. In regard to the Hfr and F+ strains, the lac+ genes on the host chromosome are donated. In regard to the Hfr strain, the F factor is integrated into the chromosome in every cell, so a chromosomal marker can be donated efficiently, particularly if the marker is near the integration site of F and is donated early. Therefore, for lac, we expect high frequencies of transfer. The F+ cell population contains only a small percentage of Hfr cells in which F is integrated into the chromosome. These rare cells are the ones responsible for the gene transfer displayed by cultures of F+ cells. Hence, for the lac gene, we expect a low frequency of transfer. In regard to Hfr- and F+-mediated gene transfer, inheritance requires the incorporation of a transferred fragment into the F chromosome by recombination (recall that two crossovers are needed). Therefore, any F strain that is rec (recombination defective) cannot integrate the donor fragment by recombination and hence cannot inherit donor chromosomal markers even though they are transferred from the donor cells. Because these donor fragments do not possess the ability to replicate within the F cell, they are rapidly diluted out in cell division.

In contrast, F cells transfer chromosomal genes on the F plasmid, a process that does not require chromosome transfer. We have seen that, in F-lac cells, lac+ genes are transferred on the plasmid at a high efficiency. In the F cell, no recombination is required, because the F-lac+ plasmid can replicate and be maintained in the dividing F cell population. Therefore, the lac+ genes are inherited even in a rec strain.

See question

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