Meiosis

1. Meiosis

2. Review Mitosis • What is the purpose of Mitosis? • cell division • 2 daughter cells produced that are exact copies of the parent cell • What are the parts of the cell cycle? • Interphase • G1, S, G2 • M-Phase • Mitosis • Cytokinesis • What were the phases of Mitosis? • PMAT – Prophase, Metaphase, Anaphase, & Telophase

3. MeiosisYes, it is different from Mitosis!! • Mitosis occurs in any cell in the body that was NOT a reproductive cell • Body cells are called somatic cells • MEIOSIS is cell division by which gamete cells (reproductive cells---sperm and egg) are produced

4. Remember!!! • Chromosomes = tightly coiled DNA (sister chromatids) • Contain genes which determine hereditary traits

5. TOTAL NUMBERS • We have a total of 46 chromosomes in EVERY cell of our body • We get… • 23 from Mom • 23 from Dad *During the S phase of Interphase, Those chromosomes are duplicated Dad Mom So each chromosome duplicates to make an identical one; these structures are now called SISTER CHROMATIDS!!!!! So each pair of chromosomes will have 2 “X” (sister chromatids) shapes

6. SO…… • We call the pair of matching sister chromatids Homologous Chromosomes • Homologous chromosomes contain the same genes for the same traits, but not necessarily the same alleles

7. Genes are your traits • Eye color, hair color, height, foot length, etc… • Alleles are alternate forms your traits can take • Usually one is dominant (brown eyes) vs. recessive (blue eyes)

8. *Somatic cells and Gametes have different numbers of chromosomes • DIPLOID (2N) = Somatic cells (Body Cells) • describes a cell that has two copies of each chromosome: 1 from mom and 1 from dad • Contains 2 sets of alleles for each gene • Chromosome # is 46 • HAPLOID (N) = Gametes (Sex Cells) • describes a cell that has only one set of chromosomes • Contains 1 set of alleles for each gene • Each egg or sperm has 22 autosomes and 1 sex chromosome. • Autosomes – pairs 1-22 • Sex chromosome - pair 23

9. Chromosome Number Example: Humans Chromosome # (N) = 23 Diploid Cells = 2N = 2x23 = 46 Haploid Cells = N = 23

10. SexualReproduction • During sexual reproduction, an egg is fertilized by a sperm to create a zygote • The zygote should be diploid; that is, it should have the same number of chromosomes as all normal human body cells • Sperm + Egg = 46 • (23) + (23)

11. Sexual Reproduction • What if gametes (sex cells) went through Mitosis? • Mitosis produces 2 identical daughter cells so… • Diploid produces diploid • If sperm and egg were diploid they would create a polyploid (“many sets”) zygote when they came together • Obviously we do not have “many sets” of chromosomes do we? SO WHAT’S THE SOLUTION?

12. MEIOSIS! • Where a Diploid cell makes 4 genetically different Haploid daughter cells • Meiosis is 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.

13. Genetic Recombination • So, when a haploid sperm fertilizes a haploid egg, a diploid zygote is the result • Meiosis helps create Genetic recombination, where 2 different sets of chromosomes come together. This keeps you from being an exact copy of one of your parents! • It’s what creates genetic diversity in our world!!

14. Stages of Meiosis • Meiosis I • Meiosis II

15. Meiosis I • Prophase I • Metaphase I • Anaphase I • Telophase I • Cytokinesis

16. Interphase Same as in Mitosis • G1: cell growth, duplication of organelles • S: DNA synthesis/replication of DNA • G2: preparing for division

17. Prophase I • Nuclear membrane and nucleolus disappear • Centrioles begin to separate and spindles begins to form • The homologous chromosomes pair up creating a tetrad • a tetrad = 2 sets of sister chromatids = 4 chromatids • The chromosomes pair up precisely, gene for gene, down their entire length.

18. During Prophase I • Crossing-over can occur • Involves the exchange of genetic material between homologous chromosomes • Breaks arms of each chromosome and switches the material

19. Metaphase I • Spindle fibers attach to the tetrads • The tetrads line up along the metaphase plate • Each side of the equator has chromosomes from both parents

20. Anaphase I • The paired homologous chromosomes are pulled apart and move toward opposite ends of the cell • Sister chromatids remain attached

21. Telophase I and Cytokinesis • The nuclear membrane forms again in some species, the spindle fibers disassemble and the cell undergoes cytokinesis. • 2 daughter cells are produced • A “reduction division” has occurred because each daughter cell has half of the number of chromosomes the original parent cell had

22. Important Note: Meiosis II will occur in both of the daughter cells created in Meiosis I

23. Review • Meiosis I – Homologous Chromosomes separate • Begins w/ 1 diploid cell • Prophase I, Metaphase I, Anaphase I, Telophase I • Ends with 2 haploid cells

24. Interphase Prophase I Metaphase I Anaphase I Cells undergo a round of DNA replication, forming duplicate Chromosomes. Each chromosome pairs with its corresponding homologous chromosome to form a tetrad. Spindle fibers attach to the chromosomes. The fibers pull the homologous chromosomes toward the opposite ends of the cell. Meiosis I

25. Meiosis II • Prophase II • Metaphase II • Anaphase II • Telophase II • Cytokinesis

26. Prophase II • The DNA DOES NOT duplicate • The nuclear membrane disappears • Centrioles move to opposite sides of the cell • Spindle fibers start to assemble

27. Metaphase II • Spindle fibers attach to the chromosomes at the centromere • The 23 chromosomes line up along the metaphase plate • Each chromosome still has two sister chromatids

28. Anaphase II • Centromeres break, spindle fibers shorten, and the sister chromatids (now chromosomes) are pulled apart from each other and move to opposite ends of the cell

29. Telophase II and Cytokinesis • Spindle fibers break down • Nuclear envelopes and nucleolus reform • Cell membrane pinches in and cytoplasm is divided • 4 haploid daughter cells are produced • A “reduction division” has occurred because each daughter cell now has half of the number of chromosomes the original parent had

30. Results of Meiosis • The 4 haploid daughter cells… • Have only 1 set of genes • Are not considered to be in any phase because they will not grow and divide • Now referred to as gametes (sex cells)

31. Review • Meiosis II – Sister chromatids separate • NO DNA replication • Prophase II, Metaphase II, Anaphase II, Telophase II • Ends with 4 haploid cells

32. Prophase II Metaphase II Anaphase II Telophase II Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original. The chromosomes line up in a similar way to the metaphase stage of mitosis. The sister chromatids separate and move toward opposite ends of the cell. Meiosis II

33. Review Questions • What type of cells are produced from meiosis? • Gametes (sex cells), haploid • What happens to the chromosome number in the cells that are produced from meiosis? • Goes through reduction division • Cell chromosome number is cut in half - Start with 2N and end with 1N • 4 haploid gametes are produced, each genetically different from the parents • What do genes code for? • traits

34. Spermatogenesis • Meiosis in males to produce sperm • Thousands of reproductive cells undergo meiosis each day to produce large numbers of sperm

35. Oogenesis • Meiosis in females to produce ova (eggs) • Only occurs in one cell once a month • The cytokinesis at end of meiosis I and meiosis II is uneven and cytoplasm is divided unequally • One cell gets the majority of the cytoplasm and becomes the egg • The other cells are called polar bodies and do not participate in reproduction

36. Comparing Mitosis and Meiosis Comparison Animation http://www.pbs.org/wgbh/nova/body/how-cells-divide.html

38. Mitosis vs. Meiosis

39. Mitosis results in the production of two genetically identical diploid cells, whereas meiosis produces four genetically different haploid cells