We covered this above.
Microtubule Instability Increases Greatly at M Phase
Compared to interphase, the assembly and disassembly of microtubules is much faster
during M phase. This is termed dynamic instability. Rather than a few long microtubules,
the cell contains many short microtubules emanating from the centrosomes. The
phosphorylation of motor proteins and MAPs is largely responsible for this change. In
addition, catastrophins remove microtubule caps, which permits disassembly of a
Mitotic Chromosomes Promote Bipolar Spindle Assembly
Kinetochores Attach Chromosomes to the Mitotic Spindle
Kinetochore microtubule attachment is a dynamic, random process. Due to dynamic
instability, many microtubules assemble and disassemble during prophase. If a microtubule
happens to collide with a kinetochore, the microtubule binds to the kinetochore. Such a
microtubule doesn’t totally retract back to the centrosome. Instead, it remains attached to the
kinetochore throughout mitosis, although its length can change. Initially, the capture is
usually a lateral interaction, but eventually the plus end of the microtubule becomes affixed
to the kinetochore (see Figures 17-36, 17-37, 17-38, 17-39).
As the capturing process is taking place, the kinetochore microtubules lengthen and shorten.
Microscopically, the sister chromatids are seen to be tugged back and forth between the
poles. Eventually, the tug of war ceases, and the sister chromatids form a straight line at the
metaphase plate (Figure 17-43).
Microtubules Are Highly Dynamic in the Metaphase Spindle
Experimentally, if a labeled tubulin protein is incorporated into a kinetochore or overlapping
microtubule, it will eventually be lost at the minus end (see the progression in Figure 17-41).
(Note: this is a special property of these microtubules and typically doesn’t happen during
interphase). The dynamic instability that these microtubules exhibit involves growth and
disassembly at the plus end, and primarily disassembly at the minus end.
Bi-Orientation is Achieved by Trial and error
Several factors promote biorientation: (1) proper cohesion between the sister chromatids is
essential for allowing biorientation to occur; (2) microtubules must be highly dynamic in
order to search for and capture a kinetochore; (3) kinetochores are positioned in a back-to-
back fashion, thus favoring interaction with microtubules from opposite spindle poles; (4)
kinetochores also stabilize the +end of microtubules, making them less dynamics once
attached to the kinetochores; (5) Lastly, there is a “Trial and Error” mechanism, sometimes
called “Error Correction”. All this means is that biorientated chromosomes are mechanically
stable because the pulling forces (or tension) on each one of the pair of kinetochores are in
balance. The cell can detect when a pair of kinetochores are not under balanced tension, for
example if microtubules from the same pole are attached to the same kinetochore. In this case