A method for the quantitative determination of DNA in the 50 to 500 µµg. range is presented. Cells or cell nuclei are isolated individually from fixed tissue by means of micromanipulation. The tissue units in question are extracted in an oil chamber with deoxyribonuclease solution. The extracts are evaporated to dryness and redissolved to lens-shaped drops, the DNA contents of which are determined by a photographic-photometric procedure in ultraviolet light. Determinations on calf thymocytes and rat spermatids show a relatively good agreement with biochemical data. The present method tends, however, to give some. what higher values than those reported earlier. The coefficient of variation for analytical values from test material is about ± 10 per cent. The method has been applied to cells from the axolotl, adults as well as tadpoles. Germ cells (spermatids and spermatocytes) do not show any evidence of a biological variation in DNA content. Cells from proliferating tissues give an increased spread of the DNA values. It could be shown, for epithelial cells, that there are at least two factors determining the DNA content of these cells. One is the fact that the cells are investigated at different phases of the mitotic cycle; the other is the fact that the DNA synthesis cycle occupies different ranges for different cells.

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