Genetic Modification

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Genetic Modification of Bacteria

Three processes are known by which the genetic composition of bacteria can be altered: transformation, conjugation and transduction.

• Transformation is a process by which some bacteria are naturally capable of taking up DNA to acquire new genetic traits. This phenomenon was discovered by Fred Griffith in 1928, although the fact that it was specifically DNA molecules that carried the genetic information was not proven until 1944. Bacteria that are competent to undergo transformation are frequently used in molecular biology.
• In conjugation, DNA is transferred from one bacteria to another via a temporary connecting strand of DNA called a pilus (a process analogous to but biologically distinct from mating). Conjugation is not widely used for the artificial genetic modification of bacteria.
• Transduction refers to the introduction of new DNA into a bacterial cell by a bacteriophage (a virus that infects bacteria).

Genetic Modification of plants

The principle technique for the genetic modification of plants is based on a natural ability of the bacteria Agrobacterium tumefaciens. This bacteria infects plants and causes a tumor-like growth termed a crown gall. Agrobacterium causing crown galls contains a plasmid (a circular piece of DNA) that transfers from the bacteria into the infected plant and integrates into the plant's genome. The transferred genes cause the plant to form the gall, which houses the bacteria and produces nutrients that support the bacteria's growth. A number of scientists contributed to this discovery throughout the late 1960s and the 1970s, with key discoveries by Jeff Schell, Marc Van Montagu, Georges Morel and Jacques Tempé. By 1983 biotechnology had reached the point where it was possible to insert additional genes of interest into Agrobacterium and thus transfer those genes into plants.

Genetic Modification of animals

Like bacteria and plants, animals can be genetically modified by viral infection. However, the genetic modification occurs only in those cells that become infected, and in most cases these cells are eventually eliminated by the immune system. In some cases it is possible to use the gene-transferring ability of viruses for gene therapy, i.e. to correct diseases caused by defective genes by supplying a normal copy of the genes. Permanent genetic modification of whole animals can be accomplished in mice. The process begins by first genetically modifiying a mouse embryonic stem cell. This is normally done by physically introducing into the cell a plasmid that can integrate into the genome by homologous recombination. This altered cell is implanted into a blastocyst (an early embryo), which is then implanted into the uterus of a female mouse. A pup born from this blastocyst will be a chimera containing some cells derived from the unmodified cells of the blastocyst and some derived from the modified stem cell. By selecting mice whose germ cells (sperm or egg producing cells) developed from the modified cell and interbreeding them, pups that contain the genetic modification in all of their cells will be born.