DNA-mediated transformation, referred to commonly as DNA transformation, involves the transfer of "naked'' DNA in the environment to recipient cells. If the cell walls of bacteria rupture, as frequently occurs in the stationary, death, and prolonged decline phases of bacterial growth, the long, circular
molecules of chromosomal DNA that are tightly jammed into the bacteria break up into several hundred pieces as they explode through the broken cell walls. The pieces of DNA average about 20 genes each. Some of these DNA macromolecules pass through the cell walls and cytoplasmic membranes of the recipient cells and are then integrated into their chromosomes, replacing the homologous genes. ■ bacterial growth curve, p. 102
DNA-mediated transformation can occur naturally in a wide variety of Gram-positive and Gram-negative bacteria (see Perspective 8.1). In addition, the yeast Saccharomyces cerevisi-ae, a eukaryote, can be transformed.
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