Question

With respect to \(\mathrm{F}^{+}\) and \(\mathrm{F}^{-}\) bacterial matings, answer the following questions: (a) How was it established that physical contact between cells was necessary? (b) How was it established that chromosome transfer was unidirectional? (c) What is the genetic basis for a bacterium's being \(\mathrm{F}^{+}\) ?

Short answer

What is the genetic basis for a bacterium being F+? Answer: Key evidence demonstrating the necessity of physical contact between bacterial cells during mating involved experiments where F+ and F- cells were mixed, showing genetic transfer when in direct contact but not when separated by a porous membrane. The unidirectional nature of chromosome transfer was established by observing that only F+ cells transferred genetic material to F- cells, with no change in the F+ cells' genetic material. The genetic basis for a bacterium being F+ is the presence of the fertility factor (F plasmid), which contains genes necessary for conjugation and enables the cell to transfer genetic material during mating.

Step-by-step solution

(a) Establishing the necessity of physical contact between cells)

To establish that physical contact was necessary for bacterial mating, scientists performed experiments in which they mixed F+ and F- bacterial cells. They observed that the transfer of genetic material between the cells occurred, and F- strains of the bacteria acquired new genetic characteristics. In control experiments, they separated the bacterial cells using ultrafine porous membranes filtering molecules while preventing direct physical contact between the cells. These experiments showed that no genetic transfer occurred, indicating that physical contact between cells was necessary for bacterial mating. This provided evidence that the genetic exchange process (conjugation) required cell-to-cell contact.

(b) Establishing the unidirectional nature of chromosome transfer)

The unidirectional nature of chromosome transfer in bacterial mating was established by observing that only F+ cells (donor cells) transfer their genetic material to F- cells (recipient cells) during conjugation. Researchers observed that conjugation between F+ and F- bacterial cells led to a change in the genetic characteristics of F- cells only, making them F+. In contrast, the genetic material of the F+ cells did not change after conjugation, indicating that the genetic exchange occurred in only one direction - from F+ to F- cells. This demonstrated that chromosome transfer in bacterial mating is unidirectional.

(c) The genetic basis for a bacterium being F+)

The genetic basis for a bacterium being F+ comes from the presence of the fertility factor (F plasmid) inside the cell. The F plasmid is a small circular piece of DNA that distinguishes F+ from F- bacterial cells. F+ cells possess the F plasmid, which contains genes for the formation of sex pili and other functions necessary for conjugation. These sex pili help establish contact with F- cells and initiate the process of transferring a copy of the F plasmid from the donor (F+) to the recipient (F-) cell. Once the F plasmid is transferred, the recipient cell also becomes F+ and gains the ability to further transfer genetic material through conjugation. Thus, the genetic basis for a bacterium being F+ is the presence of the F plasmid in the cell.

Key concepts

F plasmid

The F plasmid, or fertility factor, plays a crucial role in the process of bacterial conjugation. It is a circular piece of DNA that exists independently of the bacterial chromosome. Bacteria that contain the F plasmid are designated as F+, and those without it are F-.

The presence of the F plasmid allows for the synthesis of sex pili, which are essential structures used by the bacteria to connect with F- cells. The transfer of the F plasmid is facilitated by these pili, enabling the F- cell to become F+, a process that extends the ability to engage in conjugation to the recipient. In essence, the F plasmid determines a bacterium's mating type and its potential to act as a genetic donor within its community.

Unidirectional chromosome transfer

Bacterial conjugation is characterized by its unidirectional nature of chromosome transfer, meaning that genetic material moves from the donor cell (F+) to the recipient cell (F-) without reciprocal exchange.

This process can be akin to one-way traffic, where the F+ cell initiates the transfer after making contact with an F- cell. The genetic material being transferred includes a copy of the F plasmid and sometimes additional genes from the donor cell's chromosome. This unilateral transfer has vital implications for genetic diversity and the spread of advantageous traits among bacterial populations, as new gene combinations emerge exclusively in the recipient cells.

Genetic material exchange

In the context of bacterial conjugation, the exchange of genetic material between two bacterial cells involves several steps. Initially, an F+ cell uses its sex pili to attach to an F- cell. Once a stable mating pair is formed, the F plasmid begins the replication of its DNA.

A copy of the plasmid is then transferred to the F- cell, converting it into an F+ cell with the newly acquired genetic information. This transfer is not only essential for the spread of the F plasmid but can also include parts of the donor's genomic DNA, leading to a rich mosaic of genetic traits across bacterial communities.

Sex pili

Sex pili are slender, tube-like structures protruding from the surface of F+ bacterial cells. They are the key mechanisms that facilitate the initial contact and subsequent attachment necessary for the genetic exchange during conjugation.

These appendages recognize and bind to specific sites on F- bacterial cells, effectively drawing the two cells closer to enable the transfer of the F plasmid. The formation of sex pili is directly governed by genes contained within the F plasmid, highlighting its central role in orchestrating the complex interaction during bacterial conjugation.