Is intracellular pH a clock for mitosis?

Experiments have shown that the intracellular pH of many cells rises to a maximum at the onset of mitosis, subsequently decreasing 0.3 to 0.5 pH units by the end of mitosis. This result, and observations that tubulin net charge depends strongly on pH, may be critical for microtubule (MT) dynamics during mitosis. In vivo studies demonstrate that MT dynamics is sensitive to pH, with MT growth favored by higher pH values. Therefore it seems likely that the shift from the dominance of microtubule growth during prophase, and to a lesser extent during prometaphase, to a parity between MT polymerization and depolymerization during metaphase chromosome oscillations is a consequence of gradually decreasing intracellular pH during mitosis. Thus the timing and sequencing of prophase, prometaphase, and metaphase chromosome motions may be understood as an increase in the MT disassembly to assembly probability ratio resulting from a continuously declining intracellular pH.