1. MEIOSIS chapter 11 section 4
SC SCIENCE STANDARD B-4.5
Students will demonstrate understanding of characteristics of phases of Meiosis I and II.

2. Meiosis Vocabulary
Homologous: term used to refer to chromosomes that each have corresponding chromosome from opposite-sex parent
Example: Your # 12 chromosome from your mom is homologous to your # 12 chromosome you received from your dad

3. Diploid: refers to a cell that contains both sets of homologous chromosomes
Referred to as 2n
2n in humans = 46

4. Haploid: refers to cells that have one set of chromosomes
Referred to as n
In humans n = 23

5. Meiosis Phases
Meiosis is a process of reduction division in which the # of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.
Only occurs in sex cells
Involves 2 distinct divisions:
Meiosis I
Meiosis II

6. Meiosis I
Prior to:
each chromosome has replicated

7. Meiosis I: Prophase I
Homologous chromosomes are paired forming a tetrad

8. Prophase I
Chromatids (#4) in tetrad exchange portions with their homologous partner = crossing-over
Results in new combinations of alleles

9. Meiosis I continued Metaphase I Anaphase I
Homologous pairs line up on metaphase plate
Anaphase I
Homologous chromosomes separate and move to opposite poles

10. Meiosis I continued Telophase I Cytokinesis I
Same as telophase in mitosis
Cytokinesis I
2 daughter cells
1set of duplicated chromosomes (n)
Chromosomes are different from parent cell

11. Meiosis II 2 cells made in Meiosis I immediately enter Meiosis II
Except there is no replication of chromosomes prior to Prophase II

12. Meiosis II Prophase II Each chromosome has 2 sister chromatids
No crossing over
Spindles start to form
Nuclear envelope & nucleolus fade away
(a lot like prophase in mitosis)

13. Meiosis II continued Metaphase II
Chromosomes line up on metaphase plate
Spindles attach to centromeres
(just like metaphase in mitosis)

14. Anaphase II Centromeres split separating sister chromatids
Individual chromosomes move toward poles of cell

15. Telophase II & Cytokinesis II
Nuclear envelopes and nucleoli reform
Spindle disappears
Cleavage furrow forms
4 daughter cells formed each with haploid # (n) of chromosomes

16. Meiosis

17. Comparing : Mitosis Meiosis
Parent cell produces 2 genetically identical daughter cells
Parent cell produces 4 genetically different daughter cells

18. Gamete Formation Males: Females:
Even cell divisions at end of Meiosis I & II
Females:
uneven cell divisions in Meiosis I & II (most of cytoplasm goes to 1 cell)

19. Gametes Males: Females:

20. Meiosis Animations