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Abstract
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.
Highlights
In the cytoplasmic medium within biological cells, it is generally thought that electrostatic fields are subject to strong attenuation by screening with oppositely charged ions, decreasing exponentially to much smaller values over a distance of several Debye lengths
The Debye length within cells is typically given as 1 nm [1], and since eukaryotic cells have much larger dimensions one is tempted to conclude that electrostatics is not a major factor in explaining mitotic chromosome movements
Changes in microtubule dynamics are integral to changes in chromosome motions during mitotic stages
Summary
In the cytoplasmic medium (cytosol) within biological cells, it is generally thought that electrostatic fields are subject to strong attenuation by screening with oppositely charged ions (counterion screening), decreasing exponentially to much smaller values over a distance of several Debye lengths. Intracellular pH as a clock for mitosis In addition to addressing force generation for post-attachment chromosome motions [3,15,24], a continuum electrostatics approach to mitotic motions can account for the timing and sequencing of the detailed changes in these motions These can be attributed to changes in microtubule dynamics based on a progressively increasing microtubule disassembly to assembly ratio for kinetochore microtubules that is caused by a steadily decreasing pHi during mitosis. As pHi increases beyond interphase the presence of nucleating centers, along with the favoring of microtubule polymerization in a higher pHi environment, suggests that the pool of tubulin from interphase microtubule disassembly will polymerize around prophase centrosomes As discussed, this self-assembly would be aided by reduced counterion screening due to layered water and the reduced dielectric constant between charged protein surfaces. As discussed elsewhere [3,15], any additional lowering of pHi after metaphase may work in conjunction with increased [Ca2+] [29,30] as major determinants of anaphase-A and anaphase-B motions
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