1. MEIOSIS

2. Chromosome Number Diploid (2N) Homologous Haploid (N)
Cell has both sets of homologous chromosomes
Ex: human somatic (body) cells have 46 chromosomes
Homologous
Chromosomes that have a corresponding chromosome from the opposite-sex parent
Haploid (N)
Cell has only a single set of chromosomes
Ex: human gametes (sex cells) have 23 chromosomes

4. Defined as: Has two stages: Meiosis
Reduction division where the number of chromosomes per cell is halved by the separation of homologous chromosomes in a diploid (2N) cell
Has two stages:
meiosis I  results in 2 haploid (n) cells
meiosis II results in 4 haploid (n) cells

5. Phases of Meiosis Interphase occurs just prior to meiosis I
NOT part of meiosis
Cell grows in size
DNA is duplicated
Chromatin condenses into chromosomes (made up of two chromatids)

6. Meiosis I – Prophase I Prophase I Tetrad
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad
Tetrad
Contains 4 chromatids

7. Meiosis I – Crossing-over
Exchange of portions of sister chromatids during Prophase I
Results in exchange of genetic material between homologous chromosomes
Essential for genetic diversity

8. Meiosis I – Metaphase I Metaphase I
Spindle fibers attach to chromosomes
Tetrads line up in the middle of the cell
Photo obtained:

9. Prophase I – Anaphase I Anaphase I
Fibers pull apart tetrads separating the homologous chromosomes
Photo obtained:

10. Meiosis I – Telephase & Cytokinesis
Chromosomes gather at opposite poles
Nuclear membranes reform
Cytokinesis
two cells separate as cytoplasm pinches in
Photo obtained:

11. Meiosis II Prophase II Metaphase II
notice there is not an Interphase II (!!)
Nuclear membranes break down
Metaphase II
Chromosomes line up like in mitosis
Photo obtained:
Photo obtained:

12. Meiosis II Continued Anaphase II Telephase II Cytokinesis
Sister chromatids separtate & move toward poles
Telephase II
Chromosomes gather at opposite poles of cell
Cytokinesis
Cell splits resulting in 4 haploid (N) daughter cells
Photo obtained:
Photo obtained:

13. Gamete Formation In Males In Females Animals – sperm
Plants – pollen grains contain haploid
cells
In Females
Animals and plants – egg

14. Review – Meiosis I

15. Review – Meiosis II

16. Animation
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