Question

With respect to \(\mathrm{F}^{+}\) and \(\mathrm{F}^{-}\) bacterial matings, (a) How was it established that physical contact was necessary? (b) How was it established that chromosome transfer was unidirectional? (c) What is the genetic basis of a bacterium being \(\mathrm{F}^{+}\) ?

Short answer

Question: Explain the process of bacterial matings involving F+ and F- bacteria, touching on the importance of physical contact, the directionality of chromosome transfer, and the genetic basis of a bacterium being F+. Answer: Bacterial matings between F+ and F- bacteria, known as bacterial conjugation, involve the transfer of genetic material from the donor F+ bacterium to the recipient F- bacterium through a sex pilus (tube-like structure). Physical contact is essential for this process, as demonstrated by the Lederberg and Tatum's experiment and the U-tube experiment. F+ and F- bacterial matings exhibit unidirectional chromosome transfer, as shown by the Interrupted Mating experiment. The genetic basis of a bacterium being F+ is the presence of the F (fertility) factor, a circular DNA molecule responsible for the synthesis of sex pili and controlling the conjugation process.

Step-by-step solution

Answer (a) - Establishing Physical Contact Necessity

F+ and F- bacterial matings refer to the process in which genetic information is exchanged between F+ (donor) and F- (recipient) bacteria through a sex pilus (tube-like structure). This process is known as bacterial conjugation. To answer part (a), we will discuss the experiment conducted by Lederberg and Tatum in 1946 which established that physical contact is necessary for bacterial matings. The experiment involved two strains of E. coli with different auxotrophic mutations. Here's how it was conducted: 1. Separately growing both strains: Strain A with mutations on genes X and Y, and Strain B with mutations on genes W and Z. Each could not grow on a minimal medium as they needed specific nutrients due to their mutations. 2. Mixing both strains and allowing them to grow on a minimal medium plate. The bacterial colonies which grew were called prototrophs as they were able to synthesize the necessary nutrients. 3. Reproducing the prototrophs and observing that they contained genetic material from both strains, indicating the exchange of genetic material. In another set of experiments (U-tube experiment), the strains were placed in separate compartments with a porous membrane blocking direct contact between them, while still allowing the exchange of nutrients. In this case, no conjugation took place because the bacteria could not physically come into contact. The U-tube experiment established that the F+ and F- bacterial matings require direct physical contact between the donor and recipient bacteria.

Answer (b) - Establishing Unidirectional Chromosome Transfer

To answer part (b), we need to understand how it was experimentally established that chromosome transfer is unidirectional in bacterial conjugation. F+ bacterium transfers the F (fertility) factor and some of its DNA to the F- bacterium, converting the latter into an F+ bacterium. Wollman and Jacob conducted an experiment, known as the "Interrupted Mating" experiment, which demonstrated the unidirectional transfer. They performed the following steps: 1. The F+ and F- strains were mixed and allowed to conjugate for specified time intervals. 2. At each interval, the conjugation was interrupted by agitation, separating the mating pairs. 3. The bacteria were then tested for the presence of genetic markers from both F+ and F- strains. The results indicated that the genetic markers from the F+ strain were transferred to the F- bacteria in a specific order, as a function of time. This demonstrated that the transfer of genetic material was unidirectional, from F+ to F- bacteria, and not reciprocal.

Answer (c) - Genetic Basis of a Bacterium being F+

The genetic basis of a bacterium being F+ lies in the presence of a plasmid called the F (fertility) factor. F factor is a circular, extrachromosomal DNA molecule that controls its own replication and encodes essential genes required for bacterial conjugation. The F factor carries genes responsible for the synthesis of sex pili which make contact with F- bacteria, in addition to other regulatory genes involved in the conjugation process. An F+ bacterium possesses this F factor and can donate genetic material to an F- recipient during bacterial conjugation, while an F- bacterium lacks the F factor and serves as the recipient for genetic material.

Key concepts

Physical Contact Necessity

Bacterial conjugation relies on a process where genetic information is exchanged between two bacterial cells. For this transfer to occur, physical contact is essential. This necessity was first demonstrated by the famous experiment conducted by Lederberg and Tatum in 1946. They used two strains of E. coli, each having different genetic mutations preventing them from growing on a minimal medium when grown separately. However, when the strains were mixed together, some bacterial colonies, known as prototrophs, grew, indicating that genetic exchange had occurred.

Another pivotal experiment, known as the U-tube experiment, further confirmed this requirement. In this experiment, the bacteria were placed in separate compartments of a U-shaped tube, with a filter in between. This filter allowed nutrients to pass through but blocked direct physical interaction between the bacteria. Without direct contact, no genetic exchange took place, solidifying the conclusion that direct physical contact is necessary for bacterial conjugation.

Unidirectional Chromosome Transfer

Bacterial conjugation is a fascinating process because it involves the transfer of genetic material in a one-way direction—from donor to recipient. This unidirectional transfer of chromosomes was illuminated by the celebrated "Interrupted Mating" experiment conducted by Wollman and Jacob. During this experiment, F+ (fertility factor positive) and F- (fertility factor negative) strains were allowed to mate for varying time intervals.

At specific intervals, mating was halted by shaking the mixture to separate the mating bacteria. By then checking for genetic markers, scientists found that genes were transferred in a specific sequence from F+ to F-. No reciprocal exchange of genetic material occurred. These results vividly established that DNA is transferred in a sequence-dependent manner from an F+ bacterium to an F- bacterium, confirming the unidirectional nature of this genetic exchange.

F Factor in Bacteria

The presence of the F factor, or fertility factor, in certain bacteria accounts for their ability to donate genetic material during conjugation. This F factor is a particular type of plasmid, which refers to a circular piece of extrachromosomal DNA capable of self-replication.

The F factor is equipped with genes that are instrumental for the synthesis of structures such as the sex pilus—a tube-like structure used to establish contact with F- recipient cells. This pilus forms a bridge, allowing the genetic material to transfer. Moreover, the F factor contains genes that regulate the conjugation process, ensuring smooth transfer of genetic material.

• An F+ bacterium contains the F factor and acts as a donor in the process.
• An F- bacterium lacks the F factor and acts as a recipient.

Thus, the presence of the F factor is the genetic cornerstone enabling conjugation in F+ bacteria, making it possible for them to change F- bacteria into F+ by transferring the F factor.