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Drawing of feulgen stained cells Notes
The Feulgen procedure is undoubtedly the most widely used and most quantitative of all the cytochemical methods. Schiff's aldehyde reagent is the stain used. The great value of the Feulgen procedure is the manner in which cells are pretreated so that DNA furnishes the only available aldehyde to react with Schiff's reagent.
Extraction of cells with 1 N HCl at 60° C for 12 minutes provides optimum hydrolysis of purines from DNA, exposing the C1--aldehydes of deoxyribose.
The feulgen reaction is a semi--quantitative technique. If the only aldehydes remaining in the cell are those produced from the hydrolysis of DNA, then the technique is quantitative for DNA. It is possible to use an instrument known as a microdensitometer or microscpectrophotometer to actually measure the intensity of the pink feulgen reaction for a given organelle. Using this procedure, it was early determined that interphase cells were composed of two populations, those with 2C level of DNA and those with 4C. The nuclei looked identical, but one contained twice as much DNA within it.
This gave rise to the division of the interphase period of the cell cycle to a G1, an S, and a G2 based on the synthesis of that extra DNA.
For now, see if you can visually discern differences in the intensity of the feulgen stain reaction within the nuclei.
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