Macronuclear DNA content in Paramecium caudatum was found to be almost unchanged with a mean of about 400C during the earlier two-thirds of the life span in terms of the number of fissions and then it dropped rapidly to about one-fifth of the initial content. The age when rapid DNA decline occurred corresponded to that when the characteristics of senescence appeared. This decreasing pattern of macronuclear DNA content contrasted with earlier observations in P. tetraurelia, P. bursaria and Tetrahymena thermophila. The data suggested that in P. caudatum the distribution pattern of macronuclear DNA to daughter cells changed from an almost equal distribution in younger cells to an unequal distribution in older cells, while the relative volume of the macronucleus to the whole cell remained almost constant throughout the life cycle.

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