Introduction: The delivery of DNA into the host is required for generation of genetically modified organism. DNA delivery to host is a 3 stage process, DNA sticking to the host cell, internalization and release into the host cell. As a result, it depends on 2 parameters-

Surface chemistry of host cell- Host cell surface charges either will attract or repell DNA as a result of opposite or similar charges, respectively. Presence of cell wall (in the case of bacteria, fungus and plant) causes additional physical barrier the entry of DNA.

Charges on DNA- Negative charge on DNA modulates interaction with the host cell especially cell surface.

Modulation of these two properties is achieved in different methods to deliver DNA into the host cell and it is the topic of the discussion of today's lecture.

Transformation- it is the natural process, through which bacterial population transfer the genetic material to acquire phenotypic features. The event of transformation was first time demonstrated by Frederick Griffith in 1928. The schematic presentation of the experiment is given in Figure 20.1. Griffith has used two different Streptococcus pneumonia strains, virulent (S, causes disease and death of mice) and avirulent (R, incapable of causing disease or death of mice). In a simple experiment he injected 4 different combination of bacterial mixture, (1) live S, (2) heat killed S, (3) live R, (4) mixture of live R and heat killed S in to the mice. The observation indicates that live S has killed the mice where as mice were healthy with heat killed S or live R. Surprisingly, mice injected with mixture of live R with heat killed S were found dead, and bacteria isolated from these dead mice were virulent. Based on these observations, Griffith hypothesized the existence of a transforming agent (Protein, DNA) being transferred from heat killed virulent strain to the avirulent strain and proposed the concept of transformation. Later, Oswald has proved that the transforming factor is DNA rather than protein.