Growing Cells for Genetics Experiments
A culture should be genetically pure before one attempts to isolate a new mutant or to study the properties of an existing mutant. Being geneti-cally pure means that all the cells of a culture are genetically identical. The easiest way to ensure the requisite purity is to grow cultures from a single cell. Then, all the cells will be descendants of the original cell, and the culture will be pure unless the spontaneous mutation rate is excessive.
The species of bacteria most widely used in molecular biology experi-ments is Escherichia coli. It can be simply purified by streaking a culture on a petri plate containing “rich” nutrient medium consisting of many nutrients such as glucose, amino acids, purines, pyrimidines, and vita-mins. These plates permit growth of virtually all Escherichia coli nutri-tional mutants, and wild-type, as well as many other types of cells. Streaking is performed by sterilizing a platinum needle, poking it into a colony or culture of cells and lightly dragging it across an agar surface so that at least a few of the deposited cells are sufficiently isolated that they will grow into isolated, and therefore pure, colonies. Essentially, the only modification in the above procedure necessary for other cell-types is to use appropriate media. Often cells from higher organisms display a density-dependent growth and will not divide unless the cell density is above a critical value. Then the isolation of a culture from a single cell requires the use of tiny volumes. One method is to use microdrops suspended from glass cover slips. Evaporation from the drops is prevented by placing the cover slips upside down over small chambers filled with the growth medium. The analog of beginning bacterial genetics experiments from a single cell is beginning genetics experiments on multicellular organisms with isogenic parents. Highly inbred laboratory strains serve such a purpose.
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