During which of the following phases of meiosis do homologous chromosomes separate?

Understanding:

•  Homologous chromosomes separate in meiosis I

•  Sister chromatids separate in meiosis II

    
Meiosis consists of two divisions, both of which follow the same stages as mitosis (prophase, metaphase, anaphase, telophase)

• Meiosis is preceded by interphase, in which DNA is replicated to produce chromosomes consisting of two sister chromatids
• A second growth phase called interkinesis may occur between meiosis I and II, however no DNA replication occurs in this stage

Meiosis I

The first meiotic division is a reduction division (diploid → haploid) in which homologous chromosomes are separated 

• P-I: Chromosomes condense, nuclear membrane dissolves, homologous chromosomes form bivalents, crossing over occurs
• M-I: Spindle fibres from opposing centrosomes connect to bivalents (at centromeres) and align them along the middle of the cell
• A-I: Spindle fibres contract and split the bivalent, homologous chromosomes move to opposite poles of the cell
• T-I: Chromosomes decondense, nuclear membrane may reform, cell divides (cytokinesis) to form two haploid daughter cells

Meiosis II

The second division separates sister chromatids (these chromatids may not be identical due to crossing over in prophase I) 

• P-II: Chromosomes condense, nuclear membrane dissolves, centrosomes move to opposite poles (perpendicular to before)
• M-II: Spindle fibres from opposing centrosomes attach to chromosomes (at centromere) and align them along the cell equator
• A-II: Spindle fibres contract and separate the sister chromatids, chromatids (now called chromosomes) move to opposite poles
• T-II: Chromosomes decondense, nuclear membrane reforms, cells divide (cytokinesis) to form four haploid daughter cells 

The final outcome of meiosis is the production of four haploid daughter cells

• These cells may all be genetically distinct if crossing over occurs in prophase I (causes recombination of sister chromatids)

Meiosis Overview

Animation of the Stages of Meiosis

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 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.

Metaphase 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.

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 o�genesis 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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