L. John Gagliardi (856-225-6159)
Patrick Michael West
Rutgers University
Camden, NJ 08102
Contact info between April 27-29, 2001:
The Renaissance Hotel, Washington, 202-898-9000
Popular Version of Paper
B11.04
Saturday, April 28, 2001, 11 AM
APS April 2001 Meeting, Washington, DC
When cells first appeared, they had not yet had time to evolve complex mechanisms. Nevertheless, they needed to divide. Thus a new model, to be presented at this meeting, is grounded in the reasonable assumption that most of the motions of cell division were forced to rely on basic physical properties of the molecules involved. These properties, of course, were present from the very beginning.
One of the most fundamental properties of a molecule is its electric charge. Biological molecules are large enough to have different charges in different areas. So it is with the molecules that are the building blocks of microtubules, the familiar spindle fibers of cell division, along which the chromosomes appear to move as the duplicates separate and move to opposite sides (poles) of the cell. Do the spindle fibers actually pull the chromosomes through the cytoplasm, the fluid region inside the cell membrane, or outer covering of the cell? How is the force needed to move the chromosomes generated? These have proved difficult questions to answer, but physics offers some insights.
The building blocks from which spindle fibers form, termed subunits, have a negative charge at one end and a positive charge at the other end. The negative end of one subunit tends to attract the positive end of another subunit, and so on, assisting in the assembly of a long structure -- the completed spindle fiber -- that is somewhat unstable in the cytoplasm.
During cell division, the negative end of the spindle fiber becomes embedded in the chromosome's kinetochore, the structure essential for the movements of the chromosome during cell division. The spindle fiber then tends to break, leaving a positively-charged stub protruding. The other end of the broken fiber will have a negative charge, which naturally attracts the positive stub in the kinetochore. Because of the fiber's inherent instability, the terminal subunit of the broken fiber will soon fall off, exposing the negative end of the next subunit. As each subunit in turn drops off the spindle fiber, the location of the negative charge moves nearer and nearer to the pole of the cell, dragging one of the chromosome duplicates with it.
Currently, the leading models for chromosome motion involve molecular motors, envisioned as protein molecules that change shape, then change back, as conditions in the cell go through their cycle of changes. For the portion of cell division addressed in this paper (called anaphase A motions), however, the rate at which molecular motors would move the chromosomes does not match the rates at which the chromosomes actually move. The observed rates are ten times slower than would be predicted for molecular motors, a serious drawback that as yet has not been explained within these models.
The paper to be presented will feature computer simulations of the anaphase motions with accurate forces and rates incorporated.
Although the motion of chromosomes to the poles will be the focus of this presentation, the new model, complete with explanations and calculations, can account for all the data presently known about the motions of cell division. If the model continues to be supported by new evidence, it will constitute a comprehensive and radically different understanding of the motions that occur during cell division.
Determining the correct model for motions of chromosomes during cell division is crucial to understanding cancer, which is simply uncontrolled cell division. This model will also impact the study of aging, since during aging many types of cells divide more slowly or not at all. Consequently, skin and hair are renewed more slowly, so they become thinner.