BSCI 124 Lecture Notes

I. Meiosis

• A. Definition - a process in the sexual cycle by which the chromosome number is reduced by 1/2 (2n -> n)
  B. Other terms to define
  • 1. Diploid - having two complete sets of chromosomes (2n)
    2. Haploid - having a single set of chromosomes (n)
    3. Homologous chromosomes - members of a chromosome pair alike - one came from male, one from female - genes in same location
    4. Gametes - haploid reproductive cells that fuse to form a fertilized egg - sperm and eggs, sex cells
    5. Zygote - a fertilized egg

II. How meiosis fits into the life cycle

• A. Animal life cycle (plants more complex)
  • 1. Only haploid stage is gamete
    2. Gametes form from meiosis

  B. Plant life cycle will be discussed later

III. Stages of meiosis- animated

• Photomicrographs of each stage
  A. Interphase - duplication of DNA
  B. Prophase I - similar to mitosis except for synapsis
  • 1. Synapsis - pairing of homologous chromosomes
    2. Chromatin shortens & thickens to become chromosomes
    3. Nuclear envelope disappears
    4. Nucleolus disappears

  C. Metaphase I

  • 1. Homologous chromosomes line up in pairs across the equator
    2. Spindle fibers attach to the centromeres of the chromosomes

  D. Anaphase I - separation of homologous chromosomes
  E.Telophase I

  • 1. Chromosomes arrive at spindle poles
    2. Nuclear envelope reforms in some cells
    3. Each cell has one each homologous chromosome, cytokinesis occurs to split cells
    
  • No Interphase

  F. Prophase II

  • 1. Chromosomes shorten and thicken
    2. Nuclear envelope disappears
    3. No duplication occurs

  G. Metaphase II

  • 1. Chromosomes line up along the equator
    2. Spindle fibers attach to the centromeres

  H. Anaphase II

  • 1. Centromeres divide
    2. Sister chromatids separate, become chromosomes

  I. Telophase II

  • 1. Nuclear envelope forms around each set of chromosomes
    2. Nucleolus appears in each nucleus
    3. Chromosomes lengthen and become indistinct
    • Cytokinesis occurs forming 4 cells

IV. Mitosis vs Meiosis A. Table

               Mitosis          versus           Meiosis



purpose    produces somatic cells          produces reproductive cells

            (body, growth)

process    cell duplication                reduction division

            (diploid -> diploid)            (diploid -> haploid)

number of  1 cell division                 2 cell divisions

divisions

product    1 -> 2 daughter cells           1 -> 4 cells (gametes)

             daughter cells identical        daughter cells different

            (to each other & mother cell)

B. Detailed comparison of mitosis and meiosis - see also Quick Time^® movies of meiosis and mitosis - requires high-speed access to download

Other sites of interest
Animated review of Meiosis
Excellent review of meiosis in plants
Good review
Review
Basics of mitosis: A good review

Meiosis tutorial- The Biology Project
Definitions (detailed and complete)

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Last revised: 17 Feb 1999 - Bottino