DISTRIBUTION OF LABELED CHROMATIN : I. M₁ and M₂ Anaphases of Diploid and Tetraploid Cultured Mammalian Cells

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.