The question of whether distribution of chromatids to daughter cells in mitosis is a random or nonrandom process was investigated by study of the distribution of labeled chromatin in anaphase pairs at M1 and M2 after a pulse of tritiated thymidine. Diploid and tetraploid rat and diploid human fibroblast-like cells in serial monlayer culture were synchronized by two different methods to "purify" M1 and M2 anaphases: metaphase shake, and FUdR block to DNA synthesis followed by exogenous thymidine. Exposed grains of NTB-2 emulsion were counted over M1 and M2 anaphase pairs. An analysis (by pair) of diploid M2 anaphase grain counts showed two discrete populations of daughters with less and with more radioactivity. A similar analysis of diploid M1 and tetrapolid M2 anaphases showed a single grain-count distribution. These findings may support a nonrandom model of chromatid segregation for diploid mammalian cells but do not rule out random segregation until sound mathematical models are formulated for expected random grain distributions in M2 anaphases of cells with differing numbers of chromosomes.

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