Membrane deposition is particularly important for cytokinesis in higher-plant cells. These cells are enclosed by a semirigid cell wall. Rather than a contractile ring dividing the cytoplasm from the outside in, the cytoplasm of the plant cell is partitioned from the inside out by the construction of a new cell wall, called the cell plate, between the two daughter nuclei (Figure 17–48). The assembly of the cell plate begins in late anaphase and is guided by a structure called the phragmoplast, which contains microtubules derived from the mitotic spindle. Motor proteins transport small vesicles along these microtubules from the Golgi apparatus to the cell center. These vesicles, filled with polysaccharide and glycoproteins required for the synthesis of the new cell wall, fuse to form a disc-like, membrane-enclosed structure called the early cell plate. The plate expands outward by further
Figure 17–48 Cytokinesis in a plant cell in telophase. In this light micrograph, the early cell plate (between the two arrowheads) has formed in a plane perpendicular to the plane of the page. The microtubules of the spindle are stained with gold-labeled antibodies against tubulin, and the dna in the two sets of daughter chromosomes is stained with a fluorescent dye. note that there are no astral microtubules, because there are no centrosomes in higher-plant cells. (Courtesy of andrew Bajer.)
Figure 17–47 Localization of cytokinesis regulators at the central spindle of the human cell. (a) at center is a cultured human cell at the beginning of cytokinesis, showing the locations of the gTpase Rhoa (red) and a protein called Cyk4 (green), which is one of several regulatory proteins that form complexes at the overlapping plus ends of interpolar microtubules. These proteins are thought to generate signals that help control Rhoa activity at the cell cortex. (B) When the same three-dimensional image is viewed in the plane of the contractile ring, as shown here, Rhoa (red) is seen as a ring beneath the cell surface, while the central spindle protein Cyk4 (green) is associated with microtubule bundles scattered throughout the equatorial plane of the cell. (Courtesy of alisa piekny and michael glotzer.) vesicle fusion until it reaches the plasma membrane and the original cell wall and divides the cell in two. Later, cellulose microfibrils are laid down within the matrix of the cell plate to complete the construction of the new cell wall (Figure 17–49).
membrane-Enclosed Organelles must Be distributed to daughter Cells during Cytokinesis
The process of mitosis ensures that each daughter cell receives a full complement of chromosomes. When a eukaryotic cell divides, however, each daughter cell must also inherit all of the other essential cell components, including the membrane-enclosed organelles. As discussed in Chapter 12, organelles such as mitochondria and chloroplasts cannot be assembled de novo from their individual components; they can arise only by the growth and division of the preexisting organelles. Similarly, cells cannot make a new endoplasmic reticulum (ER) unless some part of it is already present.
How, then, do the various membrane-enclosed organelles segregate when a cell divides? Organelles such as mitochondria and chloroplasts are usually present in large enough numbers to be safely inherited if, on average, their numbers roughly double once each cycle. The ER in interphase cells is continuous with the nuclear membrane and is organized by the microtubule cytoskeleton. Upon entry into M phase, the reorganization of the microtubules and breakdown of the nuclear envelope releases the ER. In most cells, the ER remains largely intact and is cut in two during cytokinesis. The Golgi apparatus is reorganized and fragmented during mitosis. Golgi fragments associate with the spindle poles and are thereby distributed to opposite ends of the spindle, ensuring that each daughter cell inherits the materials needed to reconstruct the Golgi in telophase.
some Cells Reposition Their spindle to divide asymmetrically