10.6:
Mitosis and Cytokinesis
Comprised of five unique stages, mitosis is a form of division where a cells genetic material is partitioned between two daughter cells. First, during prophase in humans, nucleic chromatin condenses into x-shaped chromosomes composed of sister chromatid pairs attached at centromere junctions.
Concurrently, outside the nucleus, two l-shaped organelles called centrosomes migrate to opposite cell sides. As they do so, microtubule rods begin to grow from each, either towards the cell's interior or exterior forming a web-like spindle apparatus.
Next, the nuclear envelope dissolves during prometaphase, exposing the chromosomes to the cells other contents. Protein structures also appear on both sides of the centromeres, one for every chromatin.
Once these kinetochores form, extending interior microtubules fasten to them, with each sister chromatid being tethered to a different cell pole.
Mitosis then progresses to metaphase where the spindle apparatus rearranges the chromosomes so that they are similarly oriented in a fixed row along the cell's equator.
With anaphase, kinetochore afixed microtubules shorten and sister chromatids, now individually referred to as chromosomes, are dragged apart. These and other microtubule dynamics also elongate the cell.
Finally, the chromosomes land at opposite cell sides during telophase, where upon the spindle apparatus disbands. The genetic material loosens, and two nuclear envelopes, one around each chromosome set, arise.
During telophase, in a distinct process called cytokinesis, the cell is cytoplasmically divided. Thus, the end result of mitosis is a cell pair genetically identical to their precursor.
10.6:
Mitosis and Cytokinesis
In eukaryotes, the cell division cycle is divided into distinct, coordinated cellular processes that include cell growth, DNA replication/chromosome duplication, chromosome distribution to daughter cells, and finally, cell division. The cell cycle is tightly regulated by its regulatory systems as well as extracellular signals that affect cell proliferation.
The processes of the cell cycle occur over approximately 24 hours (in typical human cells) and in two major distinguishable stages. The first stage is DNA replication, during the S phase of interphase. The second stage is the mitotic (M) phase, which involves the separation of the duplicated chromosomes into two new nuclei (mitosis) and cytoplasmic division (cytokinesis). The two phases are separated by intervals (G1 and G2 gaps), during which the cell prepares for replication and division.
The Process of Mitosis
Mitosis can be divided into five distinct stages—prophase, prometaphase, metaphase, anaphase, and telophase. Cytokinesis, which begins during anaphase or telophase (depending on the cell), is part of the M phase, but not part of mitosis.
Prophase
As the cell enters mitosis, its replicated chromosomes begin to condense and become visible as threadlike structures with the aid of proteins known as condensins. The mitotic spindle apparatus begins to form between the centrosomes—which were duplicated during S phase—and migrate to opposite poles of the cell. The spindle is made up of filamentous structures called microtubules that are comprised of tubulin protein monomers. Spindle microtubules start extending towards the condensed chromosomes. The nucleolus, a component of the nucleus that produces ribosomes, vanishes, indicating the impending breakdown of the nucleus.
Prometaphase
During prometaphase, the microtubule filaments from the spindle apparatus continue to grow, and the chromosomes finish condensing. The nuclear envelope completely breaks down, releasing the chromosomes. Some of the microtubules attach to the released chromosomes, binding at a protein structure called the kinetochore that is present on the centromere of each pair of sister chromatids. Spindle microtubules from opposite poles attach at the kinetochores and capture the condensed sister chromatid pairs. Spindle microtubules that do not attach to chromosomes—polar and astral microtubules—help push the spindles apart and anchor the spindle poles to the cell membrane.
Metaphase
The spindle microtubules align each pair of the fully condensed sister chromatids along the equator of the cell—at the metaphase plate. The cell is now ready to divide.
Anaphase
The microtubules from opposite spindle poles, which are attached to the kinetochore structure, shorten and separate the sister chromatids at the centromere. The cohesion proteins that hold the chromatids together now break down. The shortening kinetochore microtubules cause each chromatid of the pair—now called chromosomes—to migrate to an opposite pole.
Telophase
Once the chromosomes reach opposite poles of the cell, they decondense and uncoil to form chromatin. The spindle microtubule filaments depolymerize into their tubulin monomers, which are then utilized as cytoskeletal elements in daughter cells. Nuclear envelopes reassemble around each set of chromosomes.
Cytokinesis
During cytokinesis in animal cells, actin filaments form a contractile ring in the plasma membrane to create a cleavage furrow, which eventually pinches the cell into two. In plant cells, vesicles from the Golgi apparatus carrying glucose, enzymes and structural proteins join to form a new cell plate at the location of the former metaphase plate. The growing cell plate fuses with the plasma membranes on each side, eventually forming a new cell wall that divides the cell into two.
Mitosis is now complete, generating two daughter cells that are identical to the parent cell. In most human cells, mitosis accounts for about one hour of the approximately 24-hour cell cycle.
Suggested Reading
Guertin, David A., Susanne Trautmann, and Dannel McCollum. “Cytokinesis in Eukaryotes.” Microbiol. Mol. Biol. Rev. 66, no. 2 (June 1, 2002): 155–78. [Source]