ABSTRACT
In 1928, F. Grifœths discovered an ingredient in heat-killed cells of the pathogenic bacterium Streptococcus pneumoniae that could transform a live, nonpathogenic mutant strain of the bacterium into a pathogenic strain. In 1944, O. T. Avery and his colleagues identiœed the transforming principle in the cell-free extracts as DNA. The introduction of a piece of homologous or foreign DNA molecule into an organism such that it is stably integrated into the host genome and stably alters its character is one of the most powerful tools in biology. It allows genes to be cloned through their ability to complement mutant phenotypes and to study their functions by selective knockout, the replacement of an endogenous gene with an engineered derivative.
