1. Meiosis
SC.912.L.16.16

2. Meiosis
A process of reduction division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell.
Diploid – 2 sets of chromosomes
Haploid – 1 set of chromosomes
Homologous – chromosomes that each have a corresponding chromosome from the opposite sex parent

3. Meiosis Stages Meiosis usually involves 2 distinct stages Meiosis I
Meiosis II

4. Meiosis 1
Two important events in meiosis 1: crossing over in prophase, and the pairing of homologues in metaphase.
Crossing over. Homologues break at identical locations, then rejoin opposite partners. This creates new combinations of the alleles on each chromosome. Occurs randomly several times on every chromosome. Results in mixing of the genes you inherited from your parents.

5. Prophase I
Each chromosome pairs with its corresponding homologous chromosome to form a tetrad.
There are 4 chromosomes in a tetrad.
The pairing of homologous chromosomes is the key to understanding meiosis.
Crossing-over may occur here
Crossing-over is when chromosomes overlap and exchange portions of their chromatids.

6. Prophase I

7. Metaphase I
Spindle fibers attach to the chromosomes

8. Metaphase I

9. Anaphase I
The fibers pull the homologous chromosomes toward opposite ends of the cell.

10. Anaphase I

11. Telophase I & Cytokinesis
Nuclear membranes form.
The cell separates into 2 cells.

12. Telophase I

13. Result of Meiosis 1
Go from 1 cell to 2 cells. Each daughter cell contains 1 copy of each chromosome: they are haploid, with the chromosomes still having 2 chromatids each.
For humans: start with one cell containing 46 chromosomes (23 pairs) to 2 cells containing 23 chromosomes.
As a result of crossing over, each chromosome is the mixture of the original homologues.

14. Meiosis 2 Meiosis 2 is just like mitosis
No replication of DNA between meiosis 1 and meiosis 2.
Chromosomes line up individually on the equator of the spindle at metaphase.
At anaphase the centromeres divide, splitting the 2 chromatids.
The one-chromatid chromosomes are pulled to opposite poles.

15. Prophase II Meiosis I results in two haploid (N) cells.
Each cell has half the number of chromosomes as the original cell.

16. Prophase II

17. Metaphase II
The chromosomes line up similar to metaphase in mitosis.

18. Metaphase II

19. Anaphase II
Sister chromatids separate and move to opposite ends of the cell.

20. Anaphase II

21. Telophase II
Meiosis II results in 4 haploid cells.

22. Telophase II

23. Summary of Meiosis 2 cell divisions.
Start with 2 copies of each chromosome (homologues), each with 2 chromatids.
In meiosis 1, crossing over in prophase mixes alleles between the homologues.
In metaphase of meiosis 1, homologues pair up, and in anaphase the homologues are separated into 2 cells.
Meiosis 2 is just like mitosis. The centromeres divide in anaphase, giving rise to a total of 4 cells, each with 1 copy of each chromosome, and each chromosome with only 1 chromatid.

24. Gamete Formation In males, meiosis results in 4 sperm cells
In females, meiosis results in 1 egg cell and three polar bodies, which are not used in reproduction.

25. Mitosis vs Meiosis Mitosis Meiosis Results in 2 Diploid Cells (2N)
4 Haploid Cells (N)
Cells are
Genetically Identical
Genetically Different
Occurs in
Somatic (Body) Cells
Sex Cells