From a basic biology standpoint, the successful end of any individual eukaryotic cell's life is the division of that cell into two daughter cells, each of them carrying a complete copy of the parent cell's DNA, or deoxyribonucleic acid (i.e., its genetic material). This division of the cell is called cytokinesis, and is immediately preceded by mitosis, the multi-step process that separates the cell's DNA into two daughter nuclei.
Mitosis and cytokinesis together represent the fourth and final stage of the eukaryotic cell cycle, called the M phase. The M phase is preceded by the three stages that together make up interphase, the part of the cell cycle in which no nuclear or cellular division processes are taking place.
The mechanics of cytokinesis are not yet completely understood, but a great deal is known about the critical timing of its events and other aspects of the final step in the cycle of any one cell.
- The four stages of cytokinesis are initiation, contraction, membrane insertion and completion.
The Eukaryotic Cell Cycle
Living things can be divided into prokaryotes and eukaryotes. Prokaryotes are single-celled organisms that carry only a small amount of DNA and have no internal membrane-bound structures in their cells, including nuclei. They reproduce by simply dividing in half after replicating their DNA and growing larger overall, a process called binary fission. Little of consequence occurs before the next division. Because these organisms have only one cell, binary fission is equivalent to reproduction.
Eukaryotes (plants, animals and fungi) do have nuclei and a number of other organelles, making reproduction of the cell a more complex process. At the moment one of these cells comes into being, it enters the G1 (first gap) stage of interphase. This is followed by S (synthesis), G2 (second gap) and finally M (mitosis). The cell grows generally larger in G1, replicates its chromosomes in S, checks its work in G2 and divides its contents into equal halves in M. Interphase is far longer than the M phase.
In the event you are ever asked "In what phase are daughter cells in as a result of mitosis?" you can answer "the M phase," because interphase does not begin until cytokinesis, which begins while mitosis is underway and usually ends shortly after mitosis does, is complete.
The Stages of Mitosis
Mitosis can be divided into either four or five stages, with the second stage in the five-stage scheme (prometaphase) being a later addition to the scheme. For the sake of completeness, all five stages are described here.
Prophase: Mitosis gets underway when the chromosomes, which were duplicated in the S phase, become more condensed, making it easier to see them as individual forms under a microscope. At the same time, a structure called the centriole is replicated and the two daughter centrioles migrate to opposite poles, or ends, of the cell, where they begin to generate the mitotic spindle, mostly from microtubule proteins.
Prometaphase: In this step, the chromosome sets, consisting of identical sister chromatids joined at a structure called the centromere, begin their pilgrimage toward the midline of the cell. Meanwhile, the centrioles continue to assemble the mitotic spindle, which serves as a set of tiny ropes or chains.
Metaphase: At this stage, all of the chromosomes (46 in humans) are lined up in a neat line on the metaphase plate, a plane passing through the "equator" of the cell and perpendicular to the spindle apparatus. This line passes through the centromeres, meaning that one sister chromatid from each set lies on one side of the plate while its twin lies on the opposite side.
Anaphase: In this phase, the spindle fibers physically pull the chromatids apart and toward opposite poles of the cell. Cytokinesis actually begins at this stage with the appearance of a cleavage furrow. At the end of anaphase, a complete set of 46 chromatids (single chromosomes) sits in a clump at each pole.
Telophase: With the genetic material now duplicated and separated, the cell goes about giving each chromosome set its own nuclear envelope. In addition, the chromosomes de-condense. In essence, telophase is prophase run in reverse. Early cytokinesis proceeds during telophase.
At the end of mitosis, cytokinesis is the only process remaining in the cell cycle. Although many sources list mitosis and cytokinesis as being consecutive events, this is misleading. While it's true that cytokinesis typically finishes not long after mitosis does, the two processes overlap considerably in time and, to some extent, space.
The cleavage furrow that signifies the onset of cytokinesis appears, as noted, during anaphase. If you picture what is happening during this stage of mitosis, you can understand why this is the earliest point at which it is safe for the cell as a whole to initiate the process of its own division. If your mental image has the two sets of chromatids moving to the left and the right within a nucleus, imagine the cell membrane starting to "pinch in" from above, setting in motion a cleavage that ultimately squeezes the middle of the cell from both the top and the bottom.
If this cell cleavage were to occur before anaphase was underway, it could produce an asymmetrical distribution of chromatids within the nuclear region. The result would almost certainly be lethal to the cell, which requires a full complement of the organism's DNA to function properly.
The Contractile Ring
The predominant functional feature of cytokinesis is the contractile ring, a structure that consists of various proteins, mainly actin and myosin, and sits just under the cell membrane. Picture an enormous hoop running just under the Earth's equator (the imaginary line passing around the planet's middle), and you get an idea of the overall set-up.
- The contractile ring is a feature of animal cells and a handful of single-celled eukaryotes only. In plant cells, which are more cubical in shape, the cleavage plane forms without the appearance of a furrow.
The plane of the contractile ring is determined by the orientation of the mitotic spindle fibers. When you look at a diagram of a cell, virtually every time you are looking at a two-dimension representation. But if you envision the cell as a sphere instead of a globe, and conjure an image of chromosomes hanging out on both "edges," you can probably intuit that the ideal plane of cleavage would have to run perpendicular to the general direction of the spindle fibers, which reach between the two cell poles.
As the ring becomes smaller, drawing the membrane inward along with it, new cell membrane material emerges from vesicles on either side of the cleavage plane. As the cell is gradually split, the new pieces of membrane plug the gaps that would otherwise appear on the sides of both daughter cells and allow cytoplasmic contents to spill out.
Cells occasionally divide in an asymmetrical way. They do not divide their chromatids asymmetrically, since, as noted, this would have decidedly unpleasant results for the cell. However, reasons occasionally arise for dividing the cytoplasm and its contents into unequal portions.
The cell ordinarily employs this cytokinesis strategy when the daughter cells have different ultimate functions and destinations. The asymmetry can manifest in an uneven distribution of organelles, an uneven mass of cytoplasm, or some combination of these features.