Mitosis.

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Make sure you have read about chromosomes and the cell cycle first!

 

Animal cell drawing: interphase

Introduction: Mitosis ( a type of division of the nucleus) and cytokinesis are the methods by a eukaryotic cell divides to produce two identical daughter cells. In single celled eukaryotes as well as some multicellular organisms, mitosis can be for asexual reproduction. This is most commonly seen in plants, for example the so-called "spider" or "airplane" plant that many people have in their homes. But mitosis is mainly the means by which multicellular organisms grow and replace damaged or worn out cells.

To understand how mitosis works we must first understand something about the overall process of cell development. Just like you, eukaryotic cells go through various developmental periods and for the cell these are called the cell cycle.

 

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Interphase is that part of the cell cycle when the cell is doing its 'normal job. Typically Interphase is viewed as having three big phases. During the S phase, new DNA is synthesized by the cell resulting in each chromosome with two molecules of DNA. These are attached together at a structure called the centromere. Each chromosome is called a duplicated or replicated chromosome because it consists of two identical DNA molecules. Remember that interphase is not part of mitosis!

Each DNA molecule and its associated proteins is now called a chromatid.

Next, the cell goes into the so called G2 phase during which the cell begins to get ready for mitosis by putting together much of the machinery needed to move the chromosomes around.

Interphase is the part of the cell cycle that is in between separate mitotic divisions.

The G1 phase is when the cell is doing is its normal every day job. Each chromosome contains only a single molecule of DNA and the DNA is all stretched out so that under the light microscope an individual chromosome is not visible. Typically there will be one or more nucleoli which are the sites of ribosomal RNA synthesis. Each chromosome is called an unduplicated or unreplicated chromosome.

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Mitosis is the process by which the DNA in the nucleus is divided up into two genetically nuclei. Sometimes mitosis taken to mean cell division. However this is not true. For example the are many one celled parasites such as the malaria parasite (Plasmodium) which undergo repeated nuclear divisions without dividing the cytoplasm. Thus, at certain stages of the life cycle the cell has many nuclei. The cell will then undergo a modified form of cytokinesis(division of the cytoplasm). A similar example from animals involves the eggs of insects and certain other invertebrates where the original nucleus may divide numerous times before division of the cytoplasm takes place.

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Prophase, the first step in mitosis involves the chromosomes becoming visible. And the nuclear membrane disintegrating. In animals and other organisms that have them, paired centrioles(centrosomes) move to opposite ends of the cell. If you look at a stained cell during prophase typically you will see what looks like a mass of spaghetti. During very early prophase a big tip that the cell is no longer in interphase is that the nucleoli have disappeared and the nuclear material appears grainy.

As the centrosomes move apart the so called mitotic spindle forms. This is part of the cytoskeleton. In animals the mitotic spindle consists of the asters which radiate in a star like pattern away from each centrosome, and the spindle microtubules which go toward the equator of the cell. Some of these fibers attach to the chromosomes at the centromere during late prophase and begin to pull the chromosomes toward the equator of the cell.

Late prophase is sometimes given a special name Prometaphase because of the two distinct events that happen then, namely, the disintegration of the nuclear membrane and, the attachment of the chromosomes to the spindle.

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Metaphase is easily recognizable because during metaphase the chromosomes are lined up at the equator of the cell.

Remember that the chromosomes are still duplicated chromosomes during metaphase.

 

 

 

 

 

 

 

 

 

 

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Anaphase begins when the chromatids of each duplicated chromosome separate and become unduplicated chromosomes in their own right. The new unduplicated chromosomes travel toward the centrosomes, or in plants the end of the spindles, on the side of the cell closest to them. When the chromosomes reach the end of the spindles the telophase begins.

 

 

 

 

 

 

 

 

 

 

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Telophase is the last phase of mitosis. Basically the membranes for the two new nuclei. from, the spindles are broken down by the cell and the unduplicated chromosomes begin to unravel and stretch out. At the end of telophase the nuclei have taken on the appearance of interphase nuclei. and the cell cycle is ready to begin again in each of the two cells, assuming cytokinesis has occurred. Since it's hard to see the actual start of telophase, scientists often arbitrarily say that telophase has begun with the beginning of cytokinesis. At least that's safe in that if cytokinesis has begun the cells are typically already in telophase.

 

 

 

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Cytokinesis.(Cytoplasmic Division).

The basic point of cytokinesis is to divide the cytoplasm of the original cell such that each of the nuclei. from mitosis gets roughly equal amounts of cytoplasm and the organelles each new cell is going to need. For instance, you may know that some of the organelles, (e.g. plastids and the

Cytokinesis is different for plant and animal cells. In plant cells a so called cell plate forms roughly equidistant between the ends of the old cell. This cell plate consists of new cell membranes for each new cell. Between these membranes the cell walls for each cell are laid down. Also the new daughter cells then typically get longer. When actively growing plants parts such root tips or new twigs are examined under the microscope one can often see a zone of active cell division right near the tip of the root or twig and then a zone of elongation further back.

In animal cells, cytokinesis involves the formation of a contractile ring made up of cytoskeleton elements. This ring is located right underneath the cell membrane of the original cell and is attached to it. When this ring contracts, a groove called the cleavage furrow forms. This groove eventually deepens and the cell is split into two daughter cells.

Remember that each daughter cell produced by mitosis contains an identical set of unduplicated chromosomes and that now the cell cycle can begin again. It is important also to remember that certain cells may not divide often under normal conditions. For example, the nerve cells in your body typically do not divide after birth. This is one reason why nervous system injuries are so difficult to deal with.

 

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pgd revised 8/15/02