The effect of HCl hydrolysis on the dye content (Feulgen reaction) of normal cells and mouse ascites tumor cells was examined by means of cytophotometric measurements. After 11 min of hydrolysis, 16-day-old tumor cells showed a hypotetraploid DNA line with doubling peaks. The DNA values were in the ratios of 1:2:4:8 during all the tested hydrolysis times (3 to 21 min). The size of the nucleus and the DNA concentration did not influence the hydrolysis and the dye content. However, the time of the hydrolysis considerably influenced the dye content of normal and tumor cells. The course of the curves obtained by plotting dye absorption against hydrolysis time showed an inflection of the curve at 9 min' hydrolysis time in tumor cells, whereas the inflection occurred at 8 min in mitotic cells. These inflections were statistically significant. The DNA stem-line1 for tumor cells shifted during different hydrolysis times when compared to normal cells. The possibility is discussed of two types of DNA which differed in their acid sensitivity and which yielded atypical hydrolysis curves.

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