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Mitosis and Meiosis Review

 

All human cells undergo division.  They do this in order to replace dead or dying cells and to maintain the quantity of cells.  The division of each cell is called mitosis. The division of sex cells is called meiosis.  The division of the cytoplasm is called cytokinesis.

There are specific stages that the cell must undergo before it completely divides.  The stages of MITOSIS are:

1)       Interphase:  This phase consists of 3 separate phases

  • G1 phase: the cell grows
  • S phase: the DNA in the cell replicates. Each human cell contains 23 pairs of chromosomes, or 46 chromosomes. Therefore, at the end of replication, there are 92 chromosomes.
  • G2 phase: specific molecules and structures duplicate (such as centrioles) and the cell becomes prepared for division.

2)       Prophase:  During this phase, the chromosomes in the nucleus become more noticeable and darker.  Each chromosome consists of two identical portions called a chromatid, which are attached at the centre by a structure called the centromere. 

 

The two centrioles that were duplicated during interphase now move to opposite ends of the cell.  The nuclear envelope and the nucleolus break up and are no longer visible.  Microtubules are made from proteins, and join together in a spindle shape.  They are now called spindle fibres. The spindle fibres form between the two centrioles as they move away from each other.

 

3)       Metaphase: During this, all the sets of chromosomes (remember, there are now 46 pairs!) line up halfway between the centrioles.  The spindle fibres attach to each set of chromosomes' centromere

4)       Anaphase: The centromeres become pulled apart and the chromatids separate and move in opposite directions, half going to one end of the cell, half going to the other.  The spindle fibres shorten and pull their chromosomes towards the centrioles.

 

5)       Telophase: The chromosomes now reach the end of each side of the cell.  As they approach the centrioles, they begin to unwind and become thread-like.  A nuclear envelope surrounds each set of chromosomes and nucleoli appear.  The microtubules dissemble.

6)       Cytokinesis:  The cell membrane begins to move inwards in the middle of the cell and a ring of microfilaments attach to the inner surface of the cell membrane, dividing the cytoplasm.  Now we have two separate cells, each with 46 chromosomes.  Each chromosome in one cell is identical to a chromosome in another cell. 

The stages of MEIOSIS are similar to mitosis, but there are slight differences.  Meiosis involves only the sex cells, which are the female ovarian cell that forms eggs (oocytes), and the male spermatocyte that forms sperm cells.  Remember, since every person has only 46 chromosomes, we can't receive 46 from each parent.  Instead, we can only receive 23 from each parent to make a total of 46! This is why people have half of their genetic information from their mother, and half from their father.

The process of Meiosis is the same as Mitosis except there are 2 divisions:

1)       Interphase : The number of chromosomes, which is 46 (or 23 pairs), duplicates and the cell prepares for division.

2)       Prophase 1:  Each chromosome begins to condense and pair together with its homologue, or matching chromosome.  Afterwards, they swap segments in a process called crossover.

3)       Metaphase 1:  Proteins attached to microtubules move the spindle poles apart and the chromosomes line up in position in the middle of the cell.

4)       Anaphase 1:  Microtubules extending from the spindle poles begin to lengthen and pull each chromosome away from its homologous partner.  The separated chromosomes begin moving to the poles, with each homolog moving to opposite ends.

5)       Telophase 1: The cytoplasm divides and now we have two cells, each with 46 chromosomes.

6)       Prophase 2:  Microtubules attach to the chromosomes and proteins drive the movement of these chromosomes towards the equator of the cell.

7)       Metaphase 2:  In each daughter cell, all the chromosomes are now lined up at the spindle equator in the middle of the cell.

8)       Anaphase 2:  The sister chromosomes break and each sister moves to opposite ends of the cell, towards each pole.

9)       Telophase 2: Now each cell divides in half.  Since we had two cells dividing, we now have 4 cells.  Each of these 4 cells contains only 23 chromosomes since there was no Interphase 2.

 

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