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Figure 7. Electron microscopic image of the yeast nucleus revealing the electron dense spindle pole bodies imbedded in the nuclear membrane with microtubules emanating from them. Reprinted from ref 8 with permission.

I thought we might learn more about the mutants by looking at them at higher resolution in the electron microscope. When I approached Breck Byers about this, he immediately responded that there was only one structure worth looking at and that was the spindle and its poles. He and Loretta Goetsch examined the spindle and were able to resolve additional events of spindle pole duplication, separation of the two poles, and elongation of the spindle. This analysis resolved three mutants, all required for the initiation of DNA synthesis into successive steps involving duplication of the spindle pole body (CDC28), separation of the poles (CDC4) and the initiation of DNA synthesis (CDC7).