Question

Why are the recombinants produced from an Hfr \(\times \mathrm{F}^{-}\) cross rarely, if ever, \(\mathrm{F}^{+}\) ?

Short answer

Recombinants produced from an Hfr and F- cross are rarely F+ due to the requirement of complete transfer and incorporation of the entire donor's F plasmid into the recipient cell, which is a time-consuming and rare event. Additionally, the excision of the F plasmid from the donor chromosome and the circularization of the excised DNA fragment in the recipient cell are also rare occurrences, further contributing to the scarcity of F+ recombinants in such crosses.

Step-by-step solution

Defining Hfr, F-, and F+ strains

Hfr strains are bacterial strains that contain an integrated F plasmid (Fertility plasmid) into their chromosomal DNA. F+ strains contain the F plasmid as an autonomously replicating circle, separately from their chromosomal DNA, while F- strains lack the F plasmid altogether. The F plasmid is responsible for the fertility or, more specifically, the ability to form sex pili and transfer genetic material to recipient cells during conjugation.

Understanding bacterial conjugation in Hfr x F- cross

During the bacterial conjugation between an Hfr donor and F- recipient, the F plasmid in the Hfr strain, which has been integrated into its chromosome, forms a bridge (sex pilus) with the recipient F- strain. This allows for the transfer of genetic material from the Hfr to the F- strain through a process called recombination.

Describing the transfer of genetic material

The transfer of genetic material from the Hfr donor to the F- recipient starts at the origin of transfer (oriT) on the F plasmid and proceeds in a linear fashion through the adjacent chromosomal DNA. As the donor DNA enters the recipient cell, it replaces the recipient's homologous DNA segments through homologous recombination. This transfer results in the formation of recombinant genetic material in the recipient cell.

Explaining the rarity of F+ recombinants

The rarity of F+ recombinants in an Hfr x F- cross can be explained by the following reasons: 1. The transfer of the entire F plasmid, including the genes necessary for the formation of sex pili and mating ability, is required for the recipient cell to become F+. 2. The F plasmid is integrated within the Hfr donor’s chromosomal DNA, meaning that during conjugation, the entire donor chromosome must be transferred to the recipient beforehand, which is a time-consuming process. 3. In most cases, bacterial conjugation is interrupted before the entire chromosome has been transferred, resulting in partial chromosomal transfer. Due to the nature of the F plasmid position within the chromosome, the essential F genes are usually not transferred. 4. Even when the donor chromosome is completely transferred, the formation of an autonomous F plasmid requires excision from the donor chromosome and the circularization of the excised DNA fragment in the recipient cell, which is also a rare event.

Conclusion

In summary, the recombinants produced from an Hfr x F- cross are rarely, if ever, F+ due to the requirement of complete transfer and incorporation of the entire donor F plasmid into the recipient cell, which is a time-consuming and rare event. Additionally, the excision of the F plasmid from the donor chromosome and the circularization of the excised DNA fragment in the recipient cell are also rare occurrences, further contributing to the scarcity of F+ recombinants in such crosses.