By Levi Clancy for Student Reader on
Mitosis is the division of a parent cell into two daughter cells that are identical to itself. A cell does this to increase population size and to increase genetic diversity.
The parent cell duplicates each of its chromosomes before mitosis; these duplicate chromosomes are called sister chromatids, and are attached at their centromere. In animals and plants, the nuclear envelope degrades during mitosis. Microtubules emanate from opposite ends of the cell to latch onto the sister chromatids, then shorten and pull the sister chromatids apart into sister chromosomes. The cell elongates and the nuclear envelope reforms.
By now, cytokinesis is already underway, and by the time it finishes the parent cell has been split in half into two daughter cells that are identical to it. This is similar to binary fission in prokaryotes, but is different in that prokaryotes lack a nucleus and have a single chromosome without a centromere.
Chromosomes duplicate and condense. Sister chromatids are bound by cohesin. Centrosomes form and emanate microtubules.
Chromosomes are fully condensed. Microtubules from different centrosomes repel each other, pushing centrosomes to opposite sides. The endoplasmic reticulum reabsorbs the nuclear envelope.
The nuclear envelope finishes its reabsorption into the endoplasmic reticulum. Microtubules bind and bi-orient chromosomes, then pull them toward the equator between the two centrosomes.
Chromosomes aligned at the equator, forming an imaginary bisecting plane called the metaphase plate
APC/C activated, degrading cohesin binding sister chromatids. Chromosomes separate to the poles of the cell.
The nuclear envelope reassembles, the cell is pinched at the metaphase plate, forming a contractile ring.
The interphase microtubule array reforms. The contractile ring forms the cleavage furrow.
|Prophase||The genome condenses. Visible chromosomes form. Each chromosome has 2 sister chromatids bound at the centromere by cohesin.||Spindle fibers (aka spindle poles) emanate from the centromere. The two centrosomes form G2 sprout microtubules by polymerizing free-floating tubulin. The microtubules repel each other, pushing the centrosomes to opposite ends of the cell. This microtubular network is the start of the mitotic spindle. The nuclear double-membrane begins to reabsorb into the endoplasmic reticulum; when this finishes in prometaphase, the cell no longer has a nucleus.|
|Prometaphase||Each sister chromatid forms a kinetochore at its centromere as a place for microtubules to latch onto the chromosome (thus, there are two kinetochores per centromere). With the nuclear envelope gone, microtubules invade the nuclear space and (once the centromere's spindles are sufficiently long) clasp onto the kinetochore. The kinetochore has an ATP-dependent motor that is activated by the microtubules. Kinetochore motors push each chromosome along the microtubules toward the metaphase plate. The metaphase plate is an imaginary plane that is perpendicular to the plane between the two centrosomes. Microtubules not bound to a kinetochore find and bind other "free" microtubules from the opposite centrosome; this forms the mitotic spindle.|
|Metaphase||The chromosomes have aligned such that their centromeres convene on metaphase plate. This precise alignment is due to opposing kinetochores functioning like equally strong people in a tug of war. Some species' chromosomes do not align, but instead move randomly back and forth between the poles before roughly lining up at the metaphase plate. The mitotic spindle checkpoint refers to an arresting signal emitted by any kinetchores still unattached to microtubules. This is important because proper chromosomes separation in anaphase requires attachment of every kinetochore to many microtubules. Anaphase commences only once every kinetochore has attached to a cluster of microtubules, with every chromosome thus lined up along the metaphase plate.|
Sister chromatids separate, resulting in two distinct populations of genetic material (one at each centrosome) that are identical. This occurs in two steps sometimes labeled early anaphase and late anaphase. Also, cytokinesis begins in anaphase and ends in telophase.
|Telophase||The nonkinetochore microtubules continue to lengthen, elongating the cell even more. Sister chromosomes attach to opposite ends of the cell. Fragments of the parental nuclear envelope attach around each set of separated sister chromatids. Each set of nuclei, now surrounded by a new nuclear envelope, unfold back into chromatin. The cleavage furrow forms.|
|Cytokinesis||Often, (mistakenly) thought to be the same process as telophase cytokinesis, if slated to occur, is usually well under way by this time. In animal cells, a cleavage furrow develops where the metaphase plate used to be, pinching off the separated nuclei. In plant cells, vesicles derived from the Golgi apparatus move along microtubules to the middle of the cell, coalescing into a cell plate that develops into a cell wall, separating the two nuclei. Each daughter cell has a complete copy of the genome of its parent cell. Mitosis is complete.|
|Growth 1||Each chromosome is copied during S phase, forming two identical sister chromatids that then separate into the future daughter cells during anaphase. Thus, each generation has exactly as much DNA as its predecessor.||Upon completion of cytokinesis, the two daughter cells are identical to their predecessor and enter G1, an interphase. Most cells spend the majority of the cell cycle in G1; some animal cells never undergo mitosis.|
This is called open mitosis, and it occurs in most multicellular organisms. Some protists, such as algae, undergo a variation called closed mitosis where the microtubules are able to penetrate an intact nuclear envelope.