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Mitosis

Mitosis is the term for cell division that produces two daughter cells identical to the parent cell.  In humans, each of these cells will have 46 chromosomes.  The five stages of mitosis are:

Interphase

Interphase is the stage where the cell carries on it's normal processes.  DNA replicates during this stage.

Prophase

During prophase all the fun begins.  First the chromatin begin to coil up and prophase condense.  Once that happens they are referred to as chromatids.  When two chromatid join, the pair is called a chromosome.  The chromosome is held together by the centromere.  Chromosomes first become visible during prophase.  The nuclear membrane, as well as the nucleoli, disappears, and the centrioles move to opposite ends of the cell while the mitotic spindle forms between them.           

Metaphase

During metaphase, the chromosomes all meet up at the middle of the cell.  The metaphase centromeres of the chromosome align with the spindle fibers.

Anaphase

The chromosome splits at the centromere, and each chromatid pulls apart and Image moves toward opposite poles of the cell as the spindle fibers shorten.

Telophase

The most obvious landmark of telophase is the formation of the cleavage furrow which will divide the cytoplasm, and hence the cell, in two.  The chromosomes once again become chromatin (long, unwound threads of DNA), and the nuclear membrane reforms.   The final result of mitosis has been the formation of two identical daughter cells, each containg 46 chromosomes.

 

Meiosis

Meiosis is the type of cell division that produces sex cells.  Spermatogonia and oogonia are primitive sex cells and have 46 chromosomes (the same number as other cells in our bodies), but in order for them to become mature gametes (sperm and eggs) they must reduce this number to 23.  To do this, meiosis has more stages:

Interphase I

The same events take place in this stage as those that take place in the interphase stage of mitosis.  DNA replicates.

Prophase I

Just like in mitosis, the chromatin begin to coil up and condense, the nuclear membrane disappears and the centrioles begin their migration while the spindle fiber forms between them.  The difference between this stage and what happens in mitosis is that crossing over occurs.  This is where pieces of chromosomes exchange with pieces of other chromosomes.

Metaphase I

During metaphase, the chromosomes all meet up at the middle of the cell.  The centromeres of the chromosome align with the spindle fibers.

Anaphase I

During anaphase during mitosis, the centromere split and each chromatid moved to opposite poles.  This does not happen here.  The centromere does not split, and instead whole chromosomes undergo this migration.  

Telophase I

Same as mitosis in that the cleavage furrow forms and divides the cell into two daughter cells.  Each of these cells contains.

Interphase II

Ha!  Fooled you....there is no interphase II.  The DNA has already been replicated.

Prophase II

Same as during prophase during mitosis.  Each of the cells has 23 chromosomes (remember that the chromosome is actually 2 chromatid to keep from getting confused).

Metaphase II

Chromosomes meet at the middle.

Anaphase II

This time the centromere does split and one chromatid of the chromosome goes to one pole and the other to the opposite pole.  

Telophase II

Same as in mitosis except that each cell has 23 chromosomes.

 

 

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