Meiosis Close Up


As in the case of mitosis, the chromosomes replicate during an interphase resting period when they are an undifferentiated mass.  At the beginning of the first stage of meiosis, prophase I, the chromosomes contract and become visible.  These doubled chromosomes then come together in homologous pairs.   Breakage of the ends of chromatids usually occurs at this time, followed by crossing-over, which results in chromosomes with new combinations of genes.

drawings of interphase and prophase 1 stages of meiosis--undifferentiated chromosome mass becomes visible chromosomes; homologous chromosomes come together and crossing-over occurs


Interphase to prophase I

By the end of prophase I in o÷genesis , the immature ova cells that are being produced are referred to as primary o÷cytes.  Before birth, human females produce approximately seven million primary o÷cytes that are stored in their ovaries for future use.

During metaphase I, all of the doubled homologous chromosome pairs line up along the midline of the cell between the two centrioles.

drawing of metaphase 1 stage of meiosis--doubled chromosomes pairs line up along the midline of the cell between two centrioles


Metaphase I


1st Division (Reduction Division)

During anaphase I, the homologous chromosome pairs separate and are pulled to opposite poles of the cell by spindle fibers attached to the centrioles.

drawing of anaphase 1 stage of meiosis--homologous chromosome pairs separate and are pulled to opposite poles of the cell by spindle fibers attached to the centrioles


Anaphase I

This first cell division process is completed during telophase I.  Separate nuclear membranes form.  In o÷genesis, the cell membrane then begins to pinch inward to enclose the two new cells with an unequal division of the cytoplasm. The larger one, containing about 95% of the cytoplasm, is a secondary o÷cyte.  It potentially will go on to become an ovum.  The smaller cell is a polar body which is not reproductively functional and which will ultimately be reabsorbed.   In spermatogenesis, telophase I results in two cells with equal volume.

drawings of telophase 1 stage of meiosis in females and males--separate nuclear membranes form around what will be distinct new cells


    Telophase I

The cell division during this phase (telophase I) is a reduction division.  That is to say, the resulting cells have only one of each pair of doubled homologous chromosomes.  In the case of humans, this new haploid number is 23.  The original diploid number was 46.


2nd Division

The second division process occurs in metaphase II.  It begins with the chromosomes once again lining up on the equatorial plane of the cells, equidistant between the two centrioles.  The still doubled chromosomes then split at their centromeres and the single chromosome strands migrate to opposite sides of the cell.

drawings of metaphase 2 stage of meiosis in females and males--chromosomes once again line up on the equatorial plane of the cells, equidistant between the two centrioles; the still doubled chromosomes then split at their centromeres and the single chromosome strands migrate to opposite sides of the cell

   
Metaphase II

The result of this last division in o÷genesis is the production of one ovum and two more non-reproductively functional polar bodies.  In spermatogenesis, four sperm cells are produced.

drawings of the culmination of meiosis--in females one ovum and two more polar bodies are produced; in males four sperm cells are produced


Culmination of meiosis

 

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Copyright ę 1997-2012 by Dennis O'Neil. All rights reserved.
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