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.

Interphase to prophase I

By the end of prophase I in oögenesis , the immature ova cells that are being produced are referred to as primary oöcytes.  Before birth, human females produce approximately seven million primary oöcytes 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.

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.

Anaphase I

This first cell division process is completed during telophase I.  Separate nuclear membranes form.  In oögenesis, 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 oöcyte.  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.

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.

Metaphase II

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

Culmination of meiosis

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