Mpf

Inhibitory Activating site site

Inhibitory Activating site site

MPF turns off, phosphatase acts

Active myosin

MPF turns off, phosphatase acts

Cytokinesis cytokinesis. With completion of cytokinesis, two daughter cells are formed. In Chapter 19, we saw how the contractile ring of actin and myosin II constricts the cell during cytokinesis but does not constrict the mechanism that determines the plane of cleavage through the cell. It is clear that the contractile ring and hence the cleavage furrow always develop where the chromosomes line up during metaphase, but which component of the mitotic apparatus dictates where the contractile ring will assemble is not clear.

Findings from micromanipulation experiments have shown that different cells rely on either the spindle or the asters to determine the cleavage plane (Figure 20-41). For instance, the presence of two asters determines where cleavage occurs in fertilized sand dollar eggs, whereas the spindle determines the cleavage plane in animal cells.

One hypothesis is that astral or spindle microtubules send a signal to the region of the cortex midway between asters. This signal promotes the interaction of actin microfilaments and myosin II, resulting in the formation of the contractile ring followed by the development of the cleavage furrow. The signal is unidentified as yet, but an alluring candidate is CDK1. Not only does activation of this cyclin-dependent kinase trigger entry into mitosis, but the myosin light chain is one of its known substrates. Phosphorylation of the regulatory light chains in myosin II by CDK1 early in mitosis inhibits interaction of myosin with actin filaments, thus preventing premature formation of the contractile ring (Figure 20-42). The decrease in CDK1 activity in late anaphase relieves this inhibition, leading to assembly of the contractile ring and its contraction to form the cleavage furrow (see Figure 19-20). In Chapter 21, we examine how the activity of CDK1 and other cyclin-dependent kinases waxes and wanes in replicating cells.

Plant Cells Reorganize Their Microtubules and Build a New Cell Wall in Mitosis

As noted previously, interphase plant cells lack a single perinuclear microtubule-organizing center that organizes microtubules into the radiating interphase array typical of animal cells. Instead, numerous MTOCs line the cortex of plant cells and nucleate the assembly of transverse bands of microtubules below the cell wall (Figure 20-43, left). These cortical microtubules, which are cross-linked by plant-specific MAPs, aid in laying down extracellular cellulose microfibrils, the main component of the rigid cell wall (see Figure 6-33).

Although mitotic events in plant cells are generally similar to those in animal cells, formation of the spindle and cytokinesis have unique features in plants (see Figure 20-43). Plant cells bundle their cortical microtubules and reorganize them into a spindle at prophase without the aid of centro-somes. At metaphase, the mitotic apparatus appears much the same in plant and animal cells. Golgi-derived vesicles, which appear at metaphase, are transported into the mitotic apparatus along microtubules that radiate from each end of the spindle. At telophase, these vesicles line up near the center of the dividing cell and then fuse to form the phragmo-plast, a membrane structure that replaces the animal-cell contractile ring. The membranes of the vesicles forming the phragmoplast become the plasma membranes of the daughter cells. The contents of these vesicles, such as polysaccha-ride precursors of cellulose and pectin, form the early cell plate, which develops into the new cell wall between the daughter cells.

Interphase

Prophase

Metaphase

Telophase

Interphase

Prophase

Preprophase band

Preprophase band

Phragmoplast

Cell plate

Phragmoplast

▲ FIGURE 20-43 Mitosis in a higher plant cell.

Immunofluorescence micrographs (top) and corresponding diagrams (bottom) showing arrangement of microtubules in interphase and mitotic plant cells. A cortical array of microtubules girdles a cell during interphase. Webs of microtubules cap the growing ends of plant cells and remain intact during cell division. As a cell enters prophase, the microtubules are bundled around the nucleus and reorganized into a spindle that appears similar

Cell plate to that in metaphase animal cells. By late telophase, the nuclear membrane has re-formed around the daughter nuclei, and the Golgi-derived phragmoplast has assembled at the equatorial plate. Additional small vesicles derived from the Golgi complex accumulate at the equatorial plate and fuse with the phragmoplast to form the new cell plate. [Adapted from R. H. Goddard et al., 1994, Plant Physiol. 104:1; micrographs courtesy of Susan M. Wick.]

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