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For transferring the foreign DNA into the host cell, there are various methods implemented. There are two broad categories of gene transfer occurring naturally or artificially with human interference.Horizontal Gene transfer: It can be defined as the transfer of material from one cell to the other cell other than parent to daughter cell.Vertical gene transfer: In this, the gene transfer is between the parent cell to the daughter cell.HGT[1][1][1][1] involves transformation, transfection and conjugation.Transformation: We are quite familiar with Griffith’s experiment and his transforming principle. It can be defined as the uptake if the naked DNA and its integration into the bacterial system. Transformation is naturally observed in plants, animals and Bacteria. Transformation can be done artificially implementing specific methods such as electroporation, ballistic method, chemical methods etc.Transfection[2][2][2][2]: It is similar to transformation. For eukaryotic cells, transformation word is used to relate to cancer and thus, transfection word is adopted. Transfection process includes the virus as the medium for the transfer of the genetic material into the eukaryotic system.Conjugation: It is a naturally occurring process in bacteria and other prokaryotes which facilitates sexual reproduction. Conjugation occurs by the formation of the conjugation bridge which allows the donor cell (male) to transfer the genetic material to the recipient cell (female). The conjugation bridge is formed by the pili (called sex pili) of the two bacteria. Note: bacteriophages like M13 uses these pili to transfer their genetic material directly into the bacterial cell.Methods of transformation used by genetic engineers and biotechnologist for the transforming the cells are broadly classified into two categories: Physical and chemical methods.1) Chemical methods:CaCl2 Treatment: CaCl2 would destabilise the cell membrane. It would bind to the DNA and cover the negative charge of the phosphate. Hence, the DNA can easily enter the cell through the lipid bilayer. CaCl2 cause the DNA to precipitate on the outer phase of the lipid membrane. Subjecting Heat shock (42 °C) to the sample increases the yield of the transformants.PEG Treatment: For plant cells, we use PEG instead of CaCl2. PEG is 4000 Da molecules and it destabilises the membrane and forms a complex with the DNA. The DNA is then easily transported into the cell.Other chemical methods involve the use of DEAE dextrans, Liposomes, nanocarriers etc.2) Physical methods:Electroporation: It involves the use of electric pulse to generate temporary pores in the membrane. The small plasmids and DNA can easily be incorporated by the use of chemical methods, but for the large DNA molecules, we need to use physical methods. However, for electroporation, the concentration should be high. In case of plant cells, the cell is surrounded by the layers of polysaccharides and other biomolecules such as hemicellulose, Cellulose, pectin etc. Thus, we treat the cell with enzymes such as hemicellulose and cellulose to remove the cell wall and convert the cell to a protoplast. Thus, pores in the membrane can be formed easily and DNA can be transferred into the cell through electroporation.Image[3][3][3][3]Biolistic method[4]: It is also called particle bombardment method, particle gun method, or simply gene gun. The instrument consists of microcarriers which are made up of heavy metals such as Au, Ag, W etc. The gun also consists of rupture disc, a plate which carries microcarriers (loaded with DNA), stopping plate, and at last, cell holding plate. When the gun is fired, the gas is pressurized into the gun leading to breaking of the rupture disk. The plate with the microcarrier (with the DNA) moves towards the stopping plate. The microcarrier then would move and enter the cell. The DNA is thus directly transferred into the cell. However, there are some limitations to the use of gene gun. There is a possibility of insertion of multiple particles and its accumulation which can cause overexpression of the gene or gene silencing.Image[5][5][5][5]Microinjections: They makes the use of very fine pipette to inject the DNA molecules directly into the nucleus of the cell to be transformed. It is used for plant and animal cells.Image[6][6][6][6]Note: I would appreciate your corrections and suggestions if the data is irrelevant.Reference and Further ReadingsBrown, T.A., 2016. Gene cloning and DNA analysis: an introduction. John Wiley & Sons.Wolff, J.A., Malone, R.W., Williams, P., Chong, W., Acsadi, G., Jani, A. and Felgner, P.L., 1990. Direct gene transfer into mouse muscle in vivo. Science, 247(4949 Pt 1), pp.1465-1468.Lorenz, M.G. and Wackernagel, W., 1994. Bacterial gene transfer by natural genetic transformation in the environment. Microbiological reviews, 58(3), pp.563-602.Lurquin, P.F., 1997. Gene transfer by electroporation. Molecular biotechnology, 7(1), pp.5-35.Förster, W. and Neumann, E., 1989. Gene transfer by electroporation. In Electroporation and electrofusion in cell biology (pp. 299-318). Springer US.Sanford, J.C., Klein, T.M., Wolf, E.D. and Allen, N., 1987. Delivery of substances into cells and tissues using a particle bombardment process. Particulate Science and Technology, 5(1), pp.27-37.Yang, N.S. and Sun, W.H., 1995. Gene gun and other non-viral approaches for cancer gene therapy. Nature medicine, 1(5), pp.481-483.Chourrout, D., Guyomard, R. and Houdebine, L.M., 1986. High efficiency gene transfer in rainbow trout (Salmo gairdneri Rich.) by microinjection into egg cytoplasm. Aquaculture, 51(2), pp.143-150.Footnotes[1] Horizontal gene transfer - Wikipedia[1] Horizontal gene transfer - Wikipedia[1] Horizontal gene transfer - Wikipedia[1] Horizontal gene transfer - Wikipedia[2] Transfection - Wikipedia[2] Transfection - Wikipedia[2] Transfection - Wikipedia[2] Transfection - Wikipedia[3] Image on bioelectrochemical-soc.org[3] Image on bioelectrochemical-soc.org[3] Image on bioelectrochemical-soc.org[3] Image on bioelectrochemical-soc.org[4] Gene gun - Wikipedia[5] https://www.researchgate.net/profile/Jose_Vidal4/publication/8400232/figure/fig1/AS:280088856809477@1443789795724/Fig-1-Components-of-the-Biolistic-R-PDS-1000He-particle-delivery-system-Draw.png[5] https://www.researchgate.net/profile/Jose_Vidal4/publication/8400232/figure/fig1/AS:280088856809477@1443789795724/Fig-1-Components-of-the-Biolistic-R-PDS-1000He-particle-delivery-system-Draw.png[5] https://www.researchgate.net/profile/Jose_Vidal4/publication/8400232/figure/fig1/AS:280088856809477@1443789795724/Fig-1-Components-of-the-Biolistic-R-PDS-1000He-particle-delivery-system-Draw.png[5] https://www.researchgate.net/profile/Jose_Vidal4/publication/8400232/figure/fig1/AS:280088856809477@1443789795724/Fig-1-Components-of-the-Biolistic-R-PDS-1000He-particle-delivery-system-Draw.png[6] Image on blogspot.com[6] Image on blogspot.com[6] Image on blogspot.com[6] Image on blogspot.com