TRANSGENESIS

The word ‘transgenic’ refers to an animal or plant that has been transplanted with an exogenous gene. Unlike ICSI and other manipulation techniques on human oocytes and embryos, embryo manipulation in animals, particularly in mice, involves the introduction of a foreign gene to produce either overexpression or disruption of expression of the targeted gene. Transgenic animals are generated to study gene regulation in a tissue-specific (spatial) and developmental stage-specific (temporal) manner. In livestock production, transgenic technology can be applied to develop improved strains and increased disease resistance (Wheeler and Walters, 2001). Transgenic animals can also be utilized as models for various human diseases, wherein the probable causative gene mutation or deletion can be analysed fully throughout development (Campbell, 2002; Petters and Sommer, 2000) and possible therapies can be trialled to alleviate the human disease conditions. Applications of the transgenic mouse include fate mapping of cells during development and acting as markers for unknown regions in the chromosome (Jaenish, 1988). A commonly used technique developed by Gordon et al. (1980) is direct DNA microinjection into the pronuclei of fertilized mouse zygotes. This results in a stable chromosomal integration of the foreign DNA, usually in a head-to-tail array. Since the foreign DNA is present in every cell, including the germ cells, generally integration occurs at the one-cell stage before DNA replication, although later-stage integration can also occur, creating a mosaic mouse. The number of copies integrated varies, and the transgenes are usually integrated in a single chromosomal locus.