Color Your Classroom: Engaging Students with Bacteria and Bio-Art
Background Information DNA CAN BE TRANSFERRED BETWEEN BACTERIA In nature, DNA is transferred between bacteria using two main methods— transformation and conjugation. In transformation, a bacterium takes up exogenous DNA from the surrounding environment (Figure 1). In contrast, conjugation relies upon direct contact between two bacterial cells. A piece of DNA is copied in one cell (the donor) and then is transferred into the other (recipient) cell. In both cases, the bacteria have acquired new genetic information that is both stable and heritable. Frederick Griffith first discovered transformation in 1928 when he observed that living cultures of a normally non-pathogenic strain of Streptococ- cus pneumonia were able to kill mice, but only after being mixed with a heat-killed pathogenic
Bacterial Cell
Plasmid
Transformed Cell
Figure 1: Bacterial Transformation
strain. Because the non-pathogenic strain had been “transformed” into a pathogenic strain, he named this transfer of virulence “transformation”. In 1944, Oswald Avery and his colleagues purified DNA, RNA and protein from a virulent strain of S. pneumonia to determine which was responsible for transformation. Each component was mixed each with a non-pathogenic strain of bacteria. Only those recipient cells exposed to DNA became pathogenic. These transformation experiments not only revealed how this virulence is transferred but also led to the recognition of DNA as the genetic material. The exact mode of transformation can differ between bacteria species. For example, Hae- mophilus influenzae uses membrane-bound vesicles to capture double-stranded DNA from the environment. In contrast, S. pneumoniae expresses competency factors that allow the cells to take in single-stranded DNA molecules. In the laboratory, scientists can induce cells—even those that are not naturally competent—to take up DNA and become transformed. To accomplish this, DNA is added to the cells in the presence of specific chemicals (like calcium, rubidium, or magnesium chloride), and the suspension is “heat shocked”—moved quickly between widely dif- ferent temperatures. It is believed that a combination of chemical ions and the rapid change in temperature alters the permeability of the cell wall and membrane, allowing the DNA molecules to enter the cell. Today, many molecular biologists use transformation of Escherichia coli in their experiments, even though it is not normally capable of transforming in nature. GENETIC ENGINEERING USING RECOMBINANT DNA TECHNOLOGY Many bacteria possess extra, non-essential genes on small circular pieces of double-stranded DNA in addition to their chromosomal DNA. These pieces of DNA, called plasmids, allow bacteria to exchange beneficial genes. For example, the gene that codes for ß-lactamase, an enzyme that provides antibiotic resistance, can be carried between bacteria on plasmids. Transformed cells secrete ß-lactamase into the surrounding medium, where it degrades the antibiotic ampicillin, which inhibits cell growth by interfering with cell wall synthesis. Thus, bacteria expressing this gene can grow in the presence of ampicillin. Furthermore, small “satellite” colonies of untrans- formed cells may also grow around transformed colonies because they are indirectly protected by ß-lactamase activity. Recombinant DNA technology has allowed scientists to link genes from different sources to bacterial plasmids (Figure 2). These specialized plasmids, called vectors, contain the following features:
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