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Pictures Considered #3. How Do You Know There Is a Nucleoid?

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What is more commonplace than saying that prokaryotic cells possess a nucleoid? It is implicit in the term prokaryote itself. Still, it was not shown definitively until the 1940s that bacteria and archaea have such differentiated structures made up of condensed DNA. It was the careful work of “bacterial cytologists” (as they were then called) and especially that of the eminent Carl Robinow that furnished convincing evidence for the nucleoid’s existence. Bacterial cells, then and now, were commonly prepared for microscopic examination by fixing with heat and staining with basic dyes. Such dyes stain more intense than acid ones because bacteria are replete with acidic groups from the RNA in the ribosomes. So abundant are these RNAs that these dyes stain the cells uniformly, and the nucleoids are not apparent. To reveal the nucleoids, cells were treated with ribonuclease or with dilute acids that selectively degrade the RNA and then stained. The DNA-containing nucleoids retain the dye and are now easily seen. When thin sections of bacterial cells could be examined under the electron microscope, analogous structures were seen, thus providing additional evidence that bacterial nucleoids exist. And yet, a small lingering doubt remained.Both of these methods looked at chemically treated (“fixed”), dead bacteria. Could this lethal treatment create artifacts regarding the observed “nucleoids”?

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