Transgenics

One of the major obstacles in the creation of transgenic organisms is the problem of physically transferring DNA from one organism or cell into another. It was observed early on that bacteria resistant to antibiotics transferred the resistance characteristic to other nearby bacterial cells that were not previously resistant. It was eventually discovered that the resistant bacterial cells were actually exchanging plasmid DNA carrying resistance genes. The plasmids traveled between resistant and susceptible cells. In this way, susceptible bacterial cells were transformed into resistant cells.

The permanent modification of a genome by the external application of DNA from a cell of a different genotype is called transformation (in bacteria) or transfection (in plant or animal cells). Transformed cells can pass on the new characteristics to new cells when they reproduce because copies of the foreign transgenes are replicated during cell division. Transformation can be either naturally occurring or the result of transgenic technology. Scientists mimic the natural uptake of plasmids by bacterial cells for use in creating transgenic cells. Chemical, physical, and biological methods are used to introduce DNA into the cells.

Cells can be pre-treated with chemicals in order to more willingly take-up genetically engineered plasmids. Also DNA can be mixed with chemicals such as liposomes to introduce transgenes into cells. Liposomes are microscopic spheres filled with DNA that fuse to cells. When liposomes merge with host cells, they deliver the transgenes to the new cell. Liposomes are composed of lipids very similar to the lipids that make up cell membranes, which gives them the ability to fuse with cells.

Physical methods for DNA transfer include electroporation (bacterial and animal cells), microinjection of DNA and gene gun. Electroporation is a process where cells are induced by an electric current to take up pieces of foreign DNA. DNA can also be introduced into cells by microinjection using microscopic needles. Plant tissues are difficult to penetrate due to the presence of a cell wall so a gene gun shooting pellets covered with DNA is used to transfer DNA to plants.

Biological methods used in gene transfer include viruses, fungi, and bacteria that have been genetically modified. Viruses that infect bacterial cells are used to inject the foreign pieces of DNA.