Transduction

Generalized transducing viruses are the most useful in mapping bacterial chromosomal genes. Since the amount of DNA that is packaged by the virus is determined by the size of the head of the virus, each viral particle holds the same amount of DNA. The initial cutting of the host chromosome is a random event, giving all genes approximately the same probability of being packaged and transferred. Each piece of DNA that is packaged will be the same length, meaning that the closer together two genes are, the higher the probability that the two genes will be present on the same fragment of packaged DNA. In other words, the closer together the genetic markers are, the higher the frequency of cotransduction. Therefore the distance between closely linked chromosomal genes can be calculated by measuring the frequency that two genes or genetic markers are cotransduced.

When the distance between two genes is greater than the size of the viral genome, it is physically impossible for the two genes to be packaged in the same viral capsid. Thus, these genes are said to be "unlinked" with regard to viral mapping. Since most transducing viral capsids can hold only from about fifteen to fifty genes, transductional mapping of bacterial chromosomal genes is most effective for genes that are relatively close to one another.

Historically, viruses, including transducing viruses, have played an important role in defining the basic principles of molecular biology. Perhaps the most important contribution to the study of transduction was that made by Alfred Hershey and Martha Chase. During the 1940s and 1950s there was still a great deal of controversy over whether DNA or protein was the genetically inheritable material. Hershey and Chase recognized that the simplicity of the virus, consisting of DNA wrapped in a protein coat, was the ideal model to directly address the question of the basis for inheritance.

They began their experiments by growing viruses on host bacteria in media containing radioactive forms of sulfur and phosphorus. The radioactive sulfur labeled the protein components of the virus, while the radioactive phosphorus labeled the DNA portions. This allowed them to independently track the protein and DNA. After separating the radioactively labeled viruses from their host cells, they used the viruses to infect host bacteria that were not radioactively labeled. After infection, they separated the bacterial cells from the growth media. Radioactive phosphorus (viral DNA) was found inside the host cells, while the radioactive sulfur (viral proteins) was found outside the cell. This indicated that only the DNA of the virus enters the host cell, while the protein was left on the outside.

Further details of their experiment make the case for DNA even more strongly. When viruses are grown on bacteria in a thin layer on the surface of agar plates, each virus will create a clear spot called a plaque, indicating infection. Hershey and Chase had two different mutants of their virus that resulted in plaques that looked different from the normal viral infection.

When the infected bacteria were replated, normal plaques were seen, indicating that the two different new mutants had both infected the same host cell and that recombination between the virus DNA occurred within, making a virus that had repaired both mutations. Consequently, since only DNA had entered the host cells and genetic change had occurred in the viruses, DNA had to be the inheritable material. Proteins could not be the source of inheritance because the viral proteins never entered the host cells.