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Chapter 19 : Reading: Gene Transfer, Escherichia coli, and Disease
Transformation occurs when cells take up free DNA molecules from the environment and express encoded information. This phenomenon is of great importance to experimental molecular biology, because it provides a means of inserting new genes into cells. Bacterial cells in a state that allows them to be transformed are said to be competent. Internal proteins then compare the base sequence of the new DNA to the genome of the organism. Natural transformation is believed to be an important mechanism of genetic exchange for a number of species important to humans, notably, Streptococcus pneumoniae, a causative agent of pneumonia. Cells that are not naturally competent can often be artificially induced to take up DNA. In most cases, new linear DNA fragments are destroyed before they even have a chance to recombine into the Escherichia coli genome. Plasmids seem to be extra pieces of DNA; they do not contain any genes that are essential to the life of the organism. The ampicillin resistance gene encodes an enzyme called beta-lactamase that breaks down the ampicillin molecule, allowing cells to multiply in ampicillin-containing media. The kanamycin resistance enzymes stay in the periplasmic space (the area between the inner and outer membranes of E. coli) and modify the drug there so that it cannot cross the inner membrane.