The cell cycle is one of the many things that distinguishes eukaryotic cells from their simpler counterparts, prokaryotic cells. The cell cycle describes one complete journey from the point a cell is "born" (at the end of cytokinesis of its "parent" cell) to the point it divides in half in carrying out its own cytokinesis (creating two genetically identical "daughter" cells).
In accordance with this progression, the cell cycle consists of interphase and the M (mitotic) phase. The former consists in turn of G1 (first gap), S (synthesis) and G2 (second gap) phases, while the latter includes mitosis and cytokinesis.
Mitosis is the only one of these that includes further formal divisions, and includes prophase, metaphase, anaphase and telophase.
Interphase under a microscope would not appear nearly as dramatic as, say, anaphase of mitosis, when chromosomes are condensed (and therefore more visible) and active (in this case being pulled apart, as you'll soon explore).
A basic interphase definition is "everything in a cell's life not involving division." Instead, cells grow larger overall and duplicate many of their own contents. The duplication, or replication, of a cell's genetic material is reserved for its own stage of interphase.
In the G1 stage, right after a cell is "born," not much appears to be happening at a microscopic glance, but the cell in this stage is readying itself for action. Energy stores and the building blocks of DNA accumulate inside the cell.
In S phase, the genetic material of the cell, the DNA within the nucleus, is replicated. This means that all 46 single chromosomes are copied. These remain physically linked in the form of sister chromatids.
The G2 stage has organelles within the cell such as mitochondria and the endoplasmic reticulum are being replicated, and the cell as a whole grows larger. In this stage, the cell also checks its own work, looking for replication errors and other manufacturing miscues and also preparing the "ingredients" of mitosis.
M Phase Summary
The M Phase starts with the beginning of mitosis and ends with the conclusion of cytokinesis. These processes, however, overlap to a slight extent; that it, mitosis is still underway as cytokinesis is getting a tentative start nearby in the cell.
Mitosis can be thought of as the division of the nucleus and all of its contents into two genetically identical daughter nuclei, with the most important part of "its contents" being the DNA that assures the "genetically identical" part. Cytokinesis is the division of the cell as a whole that occurs to place the daughter nuclei from mitosis into new cells outright.
Steps of Mitosis
Prophase: In this step, the replicated chromosomes, in the form of joined sister chromatids, become condensed. The mitotic spindle apparatus forms as the centrioles move to their positions in the poles and the nuclear membrane dissolves.
Metaphase: The chromosomes begin to migrate to the plane of cell division in the cell, called the metaphase plate. Remember that chromosomes are duplicated in interphase; metaphase keeps one copy on each side of the metaphase plate.
Anaphase: The sister chromosomes are pulled apart at their centromeres by the spindle fibers and move to opposite poles of the cell. Cytokinesis, meanwhile, is just getting started at the level of the cell membrane.
Telophase: This is essentially prophase run backward, as nuclear membranes forms around the daughter chromosome sets to form two daughter nuclei.
The process of cytokinesis gets underway in anaphase of mitosis, when the cytoplasm begins to constrict inward, creating a "pinching" appearance. In plant cells, this does not occur because of the presence of a cell wall; instead, the entire cell uses the metaphase plate from mitosis as a plane of cleavage for the cell as a whole.
Cytokinesis ends with the formation of complete cell membranes around the two daughter cells, and each daughter cell has now entered interphase of a brand-new cell cycle.
About the Author
Kevin Beck holds a bachelor's degree in physics with minors in math and chemistry from the University of Vermont. Formerly with ScienceBlogs.com and the editor of "Run Strong," he has written for Runner's World, Men's Fitness, Competitor, and a variety of other publications. More about Kevin and links to his professional work can be found at www.kemibe.com.
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