The Journal of Cell Biology, Vol 76, 116-126, Copyright © 1978 by The Rockefeller University Press
Regulation of macronuclear DNA content in Paramecium tetraurelia
JD Berger and HJ Schmidt
The macronucleus of Paramecium divides amitotically, and daughter
macronuclei with different DNA contents are frequently produced. If no
regulatory mechanism were present, the variance of macronuclear DNA content
would increase continuously. Analysis of variance within cell lines shows
that macronuclear DNA content is regulated so that a constant variance is
maintained from one cell generation to the next. Variation in macronuclear
DNA content is removed from the cell population by the regulatory mechanism
at the same rate at which it is introduced through inequality of
macronuclear division. Half of the variation in macronuclear DNA content
introduced into the population at a particular fission by inequality of
division is compensated for during the subsequent period of DNA synthesis.
Half of the remaining variation is removed during each subsequent cell
cycle. The amount of variation removed in one cell cycle is proportional to
the postfission variation. The cell's power to regulate DNA content is
substantially greater than that required to compensate for the small
differences that arise during division of wild-type cells. For example, a
constant variance was still maintained when the mean difference between
sister cells was increased to ten times its normal level in a mutant
strain. The observations are consistent with a replication model that
assumes that each cell synthesizes an approximately constant amount of DNA
which is independent of the initial DNA content of the macronucleus. It is
suggested that the amount of DNA synthesized may be largely determined by
the mass of the cell.
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