Key Concepts of Section 18.6

• Mitosis—the accurate separation of duplicated chromosomes—involves a molecular machine comprising dynamic microtubules and microtubule-associated motors.

• The mitotic spindle has three classes of microtubules, all emanating from the spindle poles: kinetochore microtubules, which attach to chromosomes; polar microtubules, which extend from each spindle pole and overlap in the middle of the spindle; and astral microtubules, which extend to the cell cortex (see Figure 18-38).

• In the first stage of mitosis, prophase, the nuclear chromosomes condense and the spindle poles move to either side of the nucleus (see Figure 18-37).

• At prometaphase, the nuclear envelope breaks down and microtubules emanating from the spindle poles capture sister chromatid pairs at their kinetochores. The two kinetochores (one on each chromatid) become attached to opposite spindle poles (bi-oriented), which allows the chromosome to congress to the middle of the spindle.

• The chromosomal passenger complex (CPC) associated with the inner kinetochore keeps microtubule attachments weak by the activity of its kinase component Aurora B, which phosphorylates critical kinetochore proteins. When a chromosome is bi-oriented, tension is generated, and the Aurora B substrates are pulled away from the kinase (see Figure 18-42). Without phosphorylation of kinetochore proteins by Aurora B, the chromosome-kinetochore attachment becomes stable.

• At metaphase, chromosomes are aligned on the metaphase plate. The spindle assembly checkpoint pathway monitors unattached kinetochores and delays anaphase until all chromosomes are attached.

• At anaphase, duplicated chromosomes are separated and moved toward the spindle poles by shortening of the kinetochore microtubules at both the kinetochore and spindle pole (anaphase A). The spindle poles also move apart, pushed by bipolar kinesin-5 moving toward the (+) ends of the polar microtubules (anaphase B). Spindle separation is also facilitated by cortically located dynein pulling on astral microtubules (see Figure 18-43).

• Since the mitotic spindle has the ability to self-assemble in the absence of centrosomes, additional mechanisms contribute to the assembly of the mitotic spindle.

• The actin-myosin–based contractile ring, the position of which is determined by the position of the spindle, contracts to pinch the cell in two during cytokinesis.

• In plants, cell division involves the delivery of membranes by microtubules to assemble the phragmoplast, which becomes the plasma membrane of the two daughter cells.