ANAPHASE4 At anaphase, the sister chromatids synchronously separate to form two daughter chromosomes, and each is pulled slowly toward the spindle pole it faces. The kinetochore microtubules get shorter, and the spindle poles also move apart; both processes contribute to chromosome segregation. TELOPHASE5 During telophase, the two sets of daughter chromo-somes arrive at the poles of the spindle and decondense. A new nuclear envelope reassembles around each set, completing the formation of two nuclei and marking the end of mitosis. The division of the cytoplasm begins with contraction of the contractile ring. CYTOKINESIS6 During cytokinesis, the cytoplasm is divided in two by a contractile ring of actin and myosin flaments, which pinches the cell in two to create two daughters, each with one nucleus. shortening kinetochore microtubule spindle pole moving outward set of daughter chromosomes at spindle pole contractile ring starting to contract centrosomeinterpolar microtubules nuclear envelope reassembling around individual chromosomes daughter chromosomes completed nuclear envelope surrounds decondensing chromosomes contractile ring creating cleavage furrow re-formation of interphase array of microtubules nucleated by the centrosome (Micrographs courtesy of Julie Canman and Ted Salmon.) 981
The condensation and resolution of sister chromatids depend, at least in part, on a five-subunit protein complex called condensin. Condensin structure is related to that of the cohesin complex that holds sister chromatids together (see Figure 17–19). It contains two SMC subunits like those of cohesin, plus three non-SMC subunits (Figure 17–22). Condensin may form a ringlike structure that somehow uses the energy provided by ATP hydrolysis to promote the compaction and resolution of sister chromatids. Condensin is able to change the coiling of DNA molecules in a test tube, and this coiling activity is thought to be important for chromosome condensation during mitosis. Interestingly, phosphorylation of condensin subunits by M-Cdk stimulates this coiling activity, providing one mechanism by which M-Cdk may promote chromosome restructuring in early mitosis.
The mitotic spindle Is a microtubule-Based machine
The central event of mitosis—chromosome segregation—depends in all eukaryotes on a complex and beautiful machine called the mitotic spindle (see Panel 17–1). The spindle is a bipolar array of microtubules, which pulls sister chromatids apart in anaphase, thereby segregating the two sets of chromosomes to opposite ends of the cell, where they are packaged into daughter nuclei (Movie 17.6). M-Cdk triggers the assembly of the spindle early in mitosis, in parallel with the chromosome restructuring just described. Before we consider how the spindle assembles and how its microtubules attach to sister chromatids, we briefly review the basic features of spindle structure.
The core of the mitotic spindle is a bipolar array of microtubules, the minus ends of which are focused at the two spindle poles, and the plus ends of which radiate outward from the poles (Figure 17–23). The plus ends of some microtubules—called the interpolar microtubules—overlap with the plus ends of micro-tubules from the other pole, resulting in an antiparallel array in the spindle mid-zone. The plus ends of other microtubules—the kinetochore microtubules—are attached to sister-chromatid pairs at large protein structures called kinetochores, which are located at the centromere of each sister chromatid. Finally, many spindles also contain astral microtubules that radiate outward from the poles and contact the cell cortex, helping to position the spindle in the cell.