18.1: 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.
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.
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.
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.
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.
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.
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.