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Mitosis

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Mitosis is a major form of cell division. In multicellular cells it is the somatic cells that undergo mitosis. Mitosis is the even distribution of the cell's genome. When a cell undergoes mitosis it creates two identical cells, called daughter cells. The other major form of cell division is meiosis which is the division between germ cells which perform sexual reproduction. Mitosis, however, divides via asexual reproduction. [1]

Mitosis Phases

stages of mitosis from the African globe lily (Scadoxus katherinae).

Mitosis is a highly complex cellular process. It can be divided into five major Phases. Although this is the basic process of mitotic cell division there are many other chemical reactions that happen instantaneously that are as of yet not understood. [2]

Interphase

Interphase is the point in a cell's life when everything is normal. All normal functions are taking place and nothing out of the ordinary occurs. There are three stages of Interphase however. There is G1, S, and G2. In this case "G" stands for gap, and "S" stands for synthesis. G1 is the stage in which the cell can stay for hours, days, weeks or even years. The cell is triggered to enter the S phase and the G2 phase by certain chemical reactions. For this to occur there needs to be a promoting factor that enables the cell to change stages. Controlling factors are also necessary to change stages. Scientists discovered that a Cdk was stalling yeast to pass into the S stage. They then realized that Cdk was also necessary for Mitosis to occur. However, they do not work by themselves. They must link themselves to a protein called cyclin. When these two proteins bind together, they are able to trigger the transition from S to G1. The cyclin-Cdk acts as a protein kinase in this process. There are different Cyclin configurations that bind to different Cdk's at different points in the life of a cell that change the cell stages from M to G1, S, G2 and back to M.

After this occurs, Interphase enters into the S stage. This stage is the beginning of the synthesis or replication of the DNA in the cell. This is when all of the chromosomes are coiled up and tightened into the shapes we can see under a microscope. After the DNA has been replicated, it again enters another time of waiting called G2. When DNA winds itself up, there are some key proteins that help tighten its coil. One such protein is cohesin. The double helical strand of DNA also wraps itself around histones to compact the size of the chromosome. After, the chromosome condenses and the cell's life cycle leaves the stage G2 through another reaction between Cdk1 and Cyclin B. The M (Mitosis) stage is then divided into five separate stages.(Purves 167-69)

Prophase

This fireworks explosion of color is a dividing newt lung cell seen during metaphase under a light microscope and colored using fluorescent dyes: chromosomes in blue, intermediate filaments in red, and spindle fibers (bundled microtubules assembled for cell division) in green.

Once the cell has entered mitosis, the first stage it goes through is Prophase. During prophase, the chromosomes continue to condense. Once the organism's DNA completely condenses, it creates sister chromatids. A chromatid is one of two identical pieces of DNA which make up a chromosome. Chromatids are two chromosomes bound together with a protein called cohesin. During prophase, the centrosomes start to develop polar microtubules that later pull the chromosome pairs apart. Also, the Kinetochores form in the centromere region. These structures connect each to one to an individual chromatid. These structures will become very important later in chromosome movement. (Purves 171-74)

Prometaphase

Prometaphase is very similar to prophase. The distinction between the two phases is the breakdown of the nuclear envelope during the beginning of prometaphase. This is to prepare for the cell to evenly distribute chromosomes to the two poles of the cell set up by the centrosome. The material that made up the nuclear envelope does not disintegrate however but stays there waiting to be used again when the cell is divided and there are two cells one parent and one identical daughter cell. (Purves 171-74)[3]

Metaphase

Metaphase is marked by all of the chromosomes aligning along the equatorial plate. This plate is created when all of the centrosomes align correctly. When they align like this one chromatid is on one side and the other on the opposite side of the plate. This shows that one side is clearly connected to one pole and the other side connected to the other pole. In this phase a enzyme separase comes in and initiates the breakdown of cohesin which is the connecting point in the chromatids. (Purves 171-74)[4][5]

Anaphase

As a cell undergoes mitosis, spindles pull apart the original chromosomes and their copies. Kinetochores fasten the chromosomal centromeres to the spindles.

This phase is started when the chromatids aligned during metaphase start to separate. The separation of the chromatid is not entirely understood as of yet. However the kinetochores work as motors bring the chromatid to its designated pole. This reaction is powered by the phosphorylation of ATP to ADP releasing a Phosphate releasing energy. During this phase the poles all push further apart doubling the length of the cell. (Purves 171-74)[6]

Telophase

During Telophase all of the chromosomes reach the poles. The chromatids completely detach and start to decondense. During this phase the nuclear envelope is recreated from the old nuclear envelop. The cell starts to restart its normal functions during this stage. This stage marks the start of the cell entering back into Interphase, specifically G1. In some cells it can enter into what is called G0, which is a completely dormant state where the cell does not form and major functions. (Purves 171-74)[7]

Cytokinesis

Cytokinesis takes place just after telophase. In some cases it starts to begin during telophase, while the chromatids are still moving to the poles and the nuclear envelope is being created. Cytokinesis is like pulling a thread loop tight around Jello. It splits in two. In cytokinesis the cell pinches down in the middle and splits into two, leaving the two nucleases separate from each other creating two new cells. Identical to each other in genetic makeup. (Purves 171-74)

Gallery

Related References

  • Mitosis Wikipedia
  • Purves, William K. et al. Life: The Science of Biology. Gordensville, VA. 2004.
  • Chromatid wikipedia