kinetochore .
During
metaphase , all the chromosomes are aligned in a plane called the metaphase plate, or the
equatorial plane, midway between the two poles of the cell. The sister chromatids are still tightly attached to
each other. At this time, the chromosomes are maximally condensed.
During
anaphase , the centromere separates the sister chromatids at the equatorial plane. Each
chromatid, which is now referred to as a chromosome, is quickly drawn in the direction of the centrosome
where its microtubule was attached. The non-kinetochore microtubules slide against one another at the
metaphase plate where they overlap, causing the cell to appear elongated.
During
telophase , all the events that set up the duplicated chromosomes for mitosis during the first
three phases are reversed. Chromosomes reach their antipodal positions and start to decondense (unravel). Each
daughter cell's cytoskeleton will be assembled from the monomers that are separated from the mitotic spindles
during this process. Chromosomes develop nuclear envelops around them.
Cytokinesis The second stage of the mitotic phase, known as cytokinesis, is when a cell divides into two daughter
cells by physically separating its cytoplasm. Although most eukaryotes go through the same stages of mitosis,
plant cells and other eukaryotes with cell walls go through cytokinesis in a very different way.
In cells such as
animal cells that lack cell walls, cytokinesis begins following the onset of anaphase.
At the former metaphase plate, a contractile ring made of actin filaments develops just inside the plasma
membrane. The actin filaments cause a fissure by pulling the cell's equator inward. The term "crack" for this
split is the "cleavage furrow." As the actin ring contracts, the furrow widens until it eventually cleaves the
membrane and cell in half (Figure 33).
In
plant cells , a cleavage furrow is not possible because of the rigid cell walls surrounding the plasma
membrane. The daughter cells must form a new cell wall. Enzymes, structural proteins, and glucose molecules
are gathered by the Golgi apparatus during interphase before it fragments into vesicles and spreads throughout
the dividing cell. These Golgi vesicles travel along microtubules during telophase to assemble at the metaphase
plate. A cell plate is the structure created when the vesicles fuse there, moving from the center toward the cell
walls. As more vesicles fuse, the cell plate enlarges until it merges with the cell wall at the periphery of the
22
(credit "diagrams": modification of work by Mariana Ruiz Villareal; credit "mitosis micrographs": modification of
work by Roy van Heesbeen; credit "cytokinesis micrograph": modification of work by the Wadsworth Center, NY State
Department of Health; donated to the Wikimedia foundation; scale-bar data from Matt Russell)
58
cell. Enzymes use the glucose that has accumulated between the membrane layers to build a new cell wall of
cellulose. The Golgi membranes become the plasma membrane on either side of the new cell wall (Figure 33).