1. Meiosis  Cells are divided into two groups: Reproductive cells = gametes Reproductive cells = gametesAND

2. Meiosis Non-reproductive cells = somatic cells Non-reproductive cells = somatic cells Somatic cells include nerve cells, skin cells, muscle cells, skeletal cells, etc.Somatic cells include nerve cells, skin cells, muscle cells, skeletal cells, etc. A somatic cell contains a nucleus with TWO sets of chromosomes. One set of chromosomes comes from each parent (mother and father)A somatic cell contains a nucleus with TWO sets of chromosomes. One set of chromosomes comes from each parent (mother and father)

3. Meiosis  Each chromosome from one parent can be matched to a chromosome from the other parent.  The matching chromosomes contain the same genes. They are called homologous chromosomes.

4. Meiosis  A somatic cell contains two complete sets of homologous chromosomes.  This makes a somatic cell diploid. (Di = two)  The symbol for a diploid cell is 2N

5. Meiosis  In humans the total number of chromosomes in each somatic cell = 46 (2N = 46)  The total number of genes found in humans is not known, but it is estimated at 20,000-25000.  Each chromosome contains hundreds or thousands of genes

6. Meiosis  If an offspring received ALL of the genetic information from its parents it would end up with  And the next generation would have  And so on… 92+ 184 chromosomes= 46+ 92 chromosomes=

7. Meiosis  Instead an organism produces gametes through meiosis.  Each gamete is haploid = contains one set of chromosomes.  The symbol for a haploid cell is N.  In humans: N = 23

8. Meiosis  Cell division in which a diploid cell produces 4 haploid cells (sperm or eggs). Occurs only in reproductive organs. In flowers: stamen and ovary In flowers: stamen and ovary In animals: testes and ovary* In animals: testes and ovary*ovary

9. Meiosis  Before meiosis begins the cell is in Interphase:  At the end of interphase the chromatin doubles and forms sister chromatids.  Each pair of sister chromatids matches up with its homologous chromosome.

10. Meiosis 1  Prophase 1- The homologous chromosomes come together and form tetrads. (tetra =4) The homologous chromosomes come together and form tetrads. (tetra =4) Nucleus dissolves. Nucleus dissolves. Centrioles move to opposite ends of cell. Centrioles move to opposite ends of cell. Spindle fibers form. Spindle fibers form. Tetrad

11. Meiosis 1  Prophase 1- During this first stage of meiosis, crossing over occurs. During this first stage of meiosis, crossing over occurs. Crossing over = genetic material gets swapped between the homologous chromosomes*. Crossing over = genetic material gets swapped between the homologous chromosomes*. chromosomes*.

12. Meiosis 1  Metaphase 1 Tetrads line up in middle of cell. Tetrads line up in middle of cell. Spindle fibers attach to each chromosome Spindle fibers attach to each chromosome

13. Meiosis 1  Anaphase 1 Tetrads pulled apart to opposite ends of cell Tetrads pulled apart to opposite ends of cell Homologous chromosomes are separated. Homologous chromosomes are separated. Sister chromatids remain attached Sister chromatids remain attached

14. Meiosis 1  Telophase and Cytokinesis Nuclear membranes form Nuclear membranes form Sister chromatids remain attached Sister chromatids remain attached Cell is pinched into two new cells Cell is pinched into two new cells

15. Meiosis 1  End of Meiosis 1: Two new cells formed The cells are NOT identical to each otherThe cells are NOT identical to each other The cells are NOT identical to the parent cellThe cells are NOT identical to the parent cell Each cell is made of half the number of original chromosomes, but each chromosome is made of a pair of sister chromatids*.Each cell is made of half the number of original chromosomes, but each chromosome is made of a pair of sister chromatids*.chromatids*.

16. Meiosis 1- Each new cell is unique!

17. Meiosis 2  Prophase 2 – Occurs in the two new cells at the same time. Centrioles move to opposite ends of cell Centrioles move to opposite ends of cell Spindle fibers form Spindle fibers form

18. Meiosis 2  Metaphase Sister chromatids line up in the middle of each cell Sister chromatids line up in the middle of each cell

19. Meiosis 2  Anaphase 2 Sister chromatids SEPARATE! Sister chromatids SEPARATE! Pulled to opposite ends of the cell. Pulled to opposite ends of the cell.

20. Meiosis 2  Telophase 2 and Cytokinesis Two new cells form from each cell resulting in a total of 4 NEW haploid cells! Two new cells form from each cell resulting in a total of 4 NEW haploid cells! Each cell is different from the others due to crossing over. Each cell is different from the others due to crossing over.over

21. Meiosis  The description of meiosis above results in the production of 4 new cells in males (sperm/pollen).

22. Meiosis  Each egg (ova/ovules) of a female is much larger than a sperm.  During the production of a female gamete, most of the cytoplasm ends up in one cell.  The other three cells are called polar bodies—they are non-functional.

23.  Activity on Meiosis!