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 \(\mathrm{F}^{+}\) ?

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.

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.

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.