Cloning and Transformation

Abstract

Competent cells are bacterial cells that can accept extra-chromosomal DNA or plasmids (naked DNA) from the environment. The generation of competent cells may occur by two methods: natural competence and artificial competence. Natural competence is the genetic ability of a bacterium to receive environmental DNA under natural or in vitro conditions. Bacteria can also be made competent artificially by chemical treatment and heat shock to make them transiently permeable to DNA. Natural competence dates back to 1928, when Frederick Griffith discovered that prepared heat-killed cells of a pathogenic bacterium could transform the nonpathogenic cells into pathogenic type. Natural competence has been reported in many bacterial strains, i.e. Bacillus subtilis, Streptococcus pneumonia, Neisseria gonorrhoeae and Haemophilus influenza. The natural competence phenomenon is highly regulated in bacteria and varies across genera. In some genera, certain portions of the population are competent at a time, and in others, the whole population gains competence at the same time. When the foreign DNA enters inside the cells, it may be degraded by the cellular nucleases or may recombine with the cellular chromosome. However, natural competence and transformation is efficient for linear molecules such as chromosomal DNA but not for circular plasmid molecules.