Question

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

Short answer

Answer: In an Hfr × F- cross, F+ recombinants would require the complete transfer of the F factor along with the entire Hfr cell's chromosomal DNA into the F- cell. However, since conjugation typically breaks off before the last genes are transferred, the F- cell rarely, if ever, receives the complete F factor. Thus, the recipient F- cell does not become F+, and it remains unable to initiate conjugation with other bacterial cells.

Step-by-step solution

Define Hfr and F- cells

Hfr (high frequency of recombination) cells are bacterial cells with an integrated F (fertility) factor into their chromosome. The F factor is a circular DNA molecule that contains around 25 genes, including those responsible for the formation of the sex pilus and plasmid transfer during bacterial conjugation. F- cells are bacterial cells that do not have the F factor, making them unable to initiate conjugation.

Describe bacterial conjugation

Bacterial conjugation is the process by which genetic information is transferred from one bacterial cell (the donor) to another (the recipient). In the case of an Hfr × F- cross, the donor cell is an Hfr cell, and the recipient is an F- cell.

Explain the role of the F factor in conjugation

The F factor allows a bacterial cell to produce a sex pilus, which establishes a connection between the donor (Hfr) and recipient (F-) cells. The F factor also forms the relaxosome complex that initiates the transfer of its DNA from the donor to the recipient cell.

Describe chromosome transfer during Hfr × F- crosses

During an Hfr × F- cross, the integrated F factor DNA in the Hfr cell breaks and opens up to initiate the transfer of genetic material. As the DNA transfer proceeds in a linear manner, genes closest to the origin of transfer (the starting point) enter the F- cell first, while those farthest away from the initial breaking point enter last. In most cases, the entire chromosome, including the part with the F factor, is not transferred, as conjugation usually breaks off before the last genes are transferred.

Explain why F+ recombinants are rare in Hfr × F- crosses

In an Hfr × F- cross, an F+ recombinant would require the complete transfer of the F factor along with the entire Hfr cell's chromosomal DNA into the F- cell. However, since conjugation typically breaks off before the last genes are transferred, the F- cell rarely, if ever, receives the complete F factor. Thus, the recipient F- cell does not become F+, and it remains unable to initiate conjugation with other bacterial cells.

Key concepts

Hfr Cells

The term 'Hfr cells' stands for high frequency of recombination cells. These cells play a critical role in the transfer of genetic material during bacterial conjugation. Unlike their counterpart—the 'F- cells'—which lack this capability, Hfr cells contain an integrated F (fertility) factor within their chromosome, not as a separate plasmid. This integration is the differentiator that allows Hfr cells to initiate conjugation and facilitates the genetic exchange between bacteria. The integrated F factor contains genes that are crucial for the assembly of the sex pilus and the transfer process of genetic material from the donor to the recipient.

During conjugation, the sex pilus connects Hfr to F- cells, beginning the gene transfer. The transfer starts with the breaking of the F factor sequence within the chromosome, and the genetic material flows into the F- cell in a linear fashion. Since the DNA transfer can often be interrupted before completion, the full set of genes—including the entire F factor—typically does not get passed on to the F- cell. This interruption is a key reason why Hfr cells are so named; they often result in recombination without turning the recipient into an Hfr or F+ cell.

F- Cells

In contrast to Hfr cells, 'F- cells' lack the F factor entirely, positioned as recipients in the bacterial conjugation process. They do not possess the genetic equipment necessary to begin the mating bridge with other bacteria. The F- designation indicates the cell's inability to initiate and carry out the transfer of genetic material on its own. These cells are essentially passive participants in the conjugation process, wholly reliant on donor cells like Hfr or F+ cells to receive genetic information.

Importantly, the interaction between an Hfr cell and an F- cell during conjugation can lead to genetic recombination in the F- cell's genome. However, without the complete transfer of the F factor, the F- cell will not convert into an F+ cell and so cannot propagate the conjugation process further. Consequently, F- cells are frequently used in genetic studies because they provide a stable, non-initiating backdrop for observing gene transfer and recombination events.

F factor

The 'F factor', also known as fertility factor or F plasmid, is a key element in the exchange of genetic material among bacteria. It is a specific DNA sequence that can exist as a separate, circular DNA molecule in F+ cells or can be integrated into the bacterial chromosome in Hfr cells. The F factor is endowed with genes imperative for the construction of the sex pilus and the conduction of conjugation. This includes the tra (transfer) genes, which are necessary for the mating pair formation and the physical transfer of genetic material from one cell to another.

Within the scope of the F factor, the concept of an episome is relevant—a genetic element that can replicate independently within a host cell or can integrate into the host's genome. The F factor's versatility in existing as both an episome and as part of the chromosome enables diverse modes of genetic recombination and contributes to the genetic variability observed in bacterial populations.

Genetic Recombination

Genetic recombination in bacteria is a foundational mechanism for genetic diversity and evolution. It occurs during bacterial conjugation when genetic material is exchanged between two bacterial cells, leading to a new combination of genes in the recipient cell. This process can involve segments of DNA being incorporated into the recipient's chromosome through a variety of methods, including homologous recombination, where regions of similarity are exchanged between two DNA molecules.

In the context of Hfr and F- cells, recombination happens when an Hfr cell transfers part of its genome to an F- cell. Because the F- cell does not typically receive the entire genome of the Hfr cell before conjugation is disrupted, the recombinant F- cells very rarely become F+ cells. This rarity safeguards the genetic integrity of the F- cell population, preventing them from universally gaining the ability to initiate conjugation, which could disrupt the natural balance of genetic exchange in the bacterial community.