Mitosis and Meiosis

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Mitosis and Meiosis

Cell division may happen by either mitosis or meiosis, depending on what type of cell is invovled.  Mitosis is a process by which a cell divides to form two daughter cells.  They each have the same exact number and kind of chromosomes as the parent cell.  Meiosis occurs in the primary sex cells leading to the formation of viable egg and sperm cells.  They reduce the number of chromosomes to half in each gamete so that when they are getting furtilized, the species chromosome number is kept even.  
Mitosis happens in the reproduction of unicellular organisms and in the addition of cells to a tissue or organ in a multicellular organism.
There are four stages of mitosis.  The first stage is the prophase.  In this stage the chromosomes become shorter and thicker and they each consist of two chromatids.  The nuclear membrane begins to disintegrate.  The centrioles in animal cells, fungi, algae and some other ferns double and a pair move toward each pole and the chromosomes begin to move toward the equator of the cell.
The second stage is called the metaphase.  In this stage the centrioles move to the poles.  The chromosomes line up at the equator of the spindle and the spindle fibers are attached to the centromeres connecting them to the poles of the spindle.
The third stage is called the anaphase.  By this time, the centromeres split apart.  The chromatid pairs of each chromosome separate from each other and they move quickly in opposite directions, one toward each pole.
The last stage is called the telophase.  The chromosomes that separated apart reach the poles.  The pole is the place where the new nucleus of each daughter cell will be located.  The spindle fibers extending from the poles to the centromeres disappear and those fibers that lie in the plane between the rows of chromosomes remain for a longer time.  A nuclear membrane reforms around each bundle of chromosomes at the poles.  At the center of animal cells, the cytoplasm turns inward, pinching the old cell into two new cells.
Meiosis happens in the primary sex cells, which leads to the formation of viable egg and sperm cells.  The purpose of meiosis is to cut the number of chromosomes in half.
There are two stages of meiosis, meiosis I and II.  Meiosis I has five stages called prophase I, metaphse I, anaphase I, telophase I, and interkinesis I.  Meiosis II has only four stages.  They are prophase II, metaphase II, anaphase II, and telophase II.
The stages of meiosis I result in the reduction of the number of chromosomes.
In prophase I the chromosomes become shorter and thicker and the nucleolus disappears.  The chromosomes pair with their homologues forming a group of four chromatids.  The tetrads wrap around each other and may exchange like parts.  The centrioles move and the spindle fibers appear and the nuclear membrane disappears.
The tetrads move as a group to the equator in metaphase I.  The centromeres of each of the homologous pairs of chromosomes become attached to spindle fibers extending from opposite poles.
Each pair of double-stranded chromosomes, in anaphaseI, is pulled away from its homologue toward opposite poles.
The chromosomes are double-stranded in telophase I.  In some organisms the nuclear membrane reappears.
The last stage of meiosis I is interkinesis.  The chromosomes in this stage disappear and there are two haploid nuclei.  Interkinesis lasts for a very short time.
Meiosis II separates the chromatids, finishing in four haploid cells.
The first stage of meiosis II is prophase II.  In this stage the chromosomes reappear, as do the spindle fibers.  The centrioles migrate to opposite poles.
The spindle fibers form in metaphase II.  The double-stranded chromosomes migrate to the equator.  Their centromeres become attached to the spindle fibers.  The centromeres then separate as they did in mitosis.
The chromosomes pull apart to opposite poles in anaphase II.
In telophase II four haploid nunclei are formed.  Each nucleus has one member of each pair of chromosomes that began the original meiosis.  The nuclear membrane reforms and cytokinesis comes to a finish.
There are two major ways of reproduction, asexual and sexual.
Asexual reproduction only involves one parent.  They split their cell in half.  Sexual reproduction involves two parents and they each have one cell (either an egg of sperm)