Meiosis

I. Introduction
   
II. The Meiotic Cell Cycle
   
     A. Functions
   
          1. One function of meiosis is to produce four
               new cells which have half the number of
               chromosomes of the original cell; when
               two of these cells (gametes) come
               together during fertilization the original
               diploid number is restored.
   
          2. Another function of meiosis is to make sure
               the genetic information in the cells
               produced is different from that in the
               original cell (i. e., genetic recombination
               occurs).
   
     B. Process
   
          1. The process consists of two modified
               mitotic divisions.
   
NOTE: In the example for meiosis, the diploid number is assumed to be 2 so
that what happens to individual chromosomes will be easier to follow.
   
III. The First Meiotic Division
   
     A. Interphase (I)
   
          1. Similar to that in mitosis.
   
          2. Germinal Cells (diploid)
   
   
   
     B. Prophase I
   
          1. Similar to that in mitosis, but genetic
               recombination takes place.
   
          2. Genetic Recombination
   
               a. Synapsis
   
                    i. Homologous chromosomes line
                         up next to each other.
   
                    ii. A synaptonemal complex forms.
   
   
   
               b. Crossing Over
   
                    i. Chiasmata [sing. = Chiasm] form when
                         homologous chromatids cross over
                         each other; they break, and exchange
                         genetic material.
   
   
   
     C.  Metaphase I
   
          1. Entire homologous chromosomes line up
               on either side of the metaphase plate
               (not just the chromatids as occurs during
               mitosis).
   
   
                    
    D. Anaphase I
   
          1. Homologous chromosomes move apart
               (but splitting of the centromere does not
               occur).
   
   
   
     E. Telophase I (and Cytokinesis)
   
          1. Two daughter cells, each with half the
               genetic material of the original cell, are
               produced.
   
   
   
IV. The Second Meiotic Division
   
     A. Interphase (II)
   
          1. An interphase may or may not be present,
               depending on the organism; if an interphase
               is present, it is normal except that DNA is
               not duplicated.
   
     B.  Prophase II
   
          1. If present (if there was an interphase),
               it is similar to other prophases.
   
     C.  Metaphase II
   
          1. Chromosomes line up along the
               metaphase plate as in mitosis.
   
   
   
     D.  Anaphase II
   
          1. Chromatids separate as in mitosis.
   
   
   
     E.  Telophase II (and Cytokinesis)
   
          1. At the end of the process four daughter
               cells are produced, each of which is not
               genetically identical to the others.
   
   
   
V. Review
   
VI. The Significance of Meiosis
   
     A. Each of the four cells produced is haploid.
   
     B. The four sets of chromosomes produced are
          not identical because genetic recombination 
          took place.
   
     C. When fertilization takes place the resulting cell
          (zygote) regains the diploid number and is a
          genetic mixture.
   
VII. Spermatogenesis and Oogenesis
   
     A. Spermatogenesis
   
   
   
     B. Oogenesis
   
   
   
VIII. Summary

Page obtained from link at URL: http://www.ric.edu/faculty/jmontvilo/109.htm Prepared by Jerome A. Montvilo, Ph.D. for the use of his students. Copyright © by Jerome A. Montvilo. All rights reserved. Please send questions, comments, or suggestions to jmontvilo@ric.edu. Last updated 15 January, 2008