The spindle apparatus is only mentioned within Figure 5-2, yet it is essential for cell division to occur. It is simply an organization of microtubules. A cell that is undergoing mitosis will develop the spindle apparatus from the poles toward the equator. Here is a picture of the spindle apparatus in a dividing cell, at about the time of metaphase. The green line around the entire image is the cell membrane. The black lines are the microtubules of the spindle apparatus, and the pink sets of lines at each pole are the centrioles. Centrioles are described in chapter 4 of your book, but we skipped them before. In the chapter 5 of your book, it just shows centrioles within the poles, but doesn't talk about them.
A centriole is nothing really complicated. It's just a short segment of microtubules in an arrangement pretty much like you find them within cilia and flagella. The centrioles sit in pairs, with the 2 centrioles of each pair lying perpendicular to each other. Cells typically only have one pair of centrioles, but when a cell is getting ready for mitosis, it replicates its centrioles as well as other items so that it ends up with 2 pairs of centrioles. Centrioles are not essential for mitosis-- since plant cells don't even have them.
Note that the spindle is able to fill the entire cell because the nucleus has already vanished. Earlier, before the nucleus is gone (like during prophase), the future poles are just starting to grow their microtubules. They are first lying next to each other, but then start to migrate toward opposite sides of the cell and grow their microtubules. When they make it to opposite sides, that defines the poles. By the end of telophase, the spindle has disassembled again so that the cell can get back to its regular business.
All microtubules have an intrinsic direction to them... you should thus be able to imagine that each half of the spindle is set up so that chromosomes can only move from the middle toward the poles... kind of like going down a one-way street.
Some chemicals, like "taxol," mess up microtubules in one way or another. Taxol stabilizes them so that they cannot disassemble. Think about what would happen to a cell that divides in the presence of taxol:
Chemicals like taxol are used in chemotherapy. Do you see how they kill cancer cells?
© 2006 STCC Foundation Press, content by Dawn A. Tamarkin, Ph.D.
Last changed: January 21, 2007