MITOSIS MEIOSIS

1. MITOSIS & MEIOSIS

2. CELLS & CELL DIVISION Organisms are composed of cells Cells arise from preexisting cells Cells have various components Organelles Chromosomes Contain genetic material (DNA) Different species have different numbers of chromosomes Cell division segregates these components into daughter cells

3. CELL DIVISION Two types of eukaryotic cell division Mitosis Produces identical cells Meiosis Produces sperm & egg

4. CELL CYCLE Interphase No division Preparation for mitosis Chromosomes replicate Mitotic phase Mitosis Division of the nucleus Equally divides chromosomes Cytokinesis Divides cytoplasm (& organelles)

5. INTERPHASE Preparation for mitosis Chromosomes replicate 1 chromosome ? 2 sister chromatids

6. MITOSIS Chromosomes equally divided into two daughter cells Four stages Prophase Metaphase Anaphase Telophase

7. PROPHASE Chromosomes condense Become thick and distinct Nuclear membrane disassociates Mitotic spindle begins to form

8. METAPHASE Chromosomes line up on equatorial plate Mitotic spindle complete Attached to chromosomes at centromere (kinetochore) Also attached to centrioles (north & south poles) Sister chromatids attached to opposite poles

9. ANAPHASE Sister chromatids separate Now independent chromosomes Move to opposite poles Ultimately reach opposite poles

10. ANAPHASE Forces separating chromatids/chromosomes Separation of poles Shortening of microtubules Kinetochore “eats” its way toward pole

11. TELOPHASE Prophase in reverse Chromosomes decondense Nuclear membranes reform Mitotic spindle disappears Cytokinesis generally begins

12. CYTOKINESIS Division of the cytoplasm Generally begins during telophase Protein ring contracts Cleavage furrow forms Cells ultimately separated Organelles distributed to each daughter cell

13. MITOTIC CELL DIVISION Summary of mitotic cell division One cell ? two identical cells In humans 46 ? 46 + 46

14. MITOTIC CELL DIVISION Mitotic cell division produces daughter cells identical to the original cell Many organisms reproduce asexually via mitotic cell division Sexual reproduction requires a different type of cell division

15. SEXUAL REPRODUCTION Some cells can form gametes Germ cells ? sperm & egg Occurs in gonads Testes & ovaries Gametes must have reduced chromosome number 46 sperm + 46 egg ? 92 (too many!!!) 23 sperm + 23 egg ? 46 (normal number) Meiosis halves chromosome number

16. CELL DIVISION Two types of eukaryotic cell division Mitosis Produces identical cells Meiosis Produces sperm & egg

17. HUMAN GENOME Most human cells possess 46 chromosomes 23 pairs of chromosomes 2 copies of each chromosome Pairs of “homologous” chromosomes

18. HUMAN GENOME Most human cells possess 46 chromosomes 23 pairs of chromosomes 2 copies of each chromosome Pairs of “homologous” chromosomes “Diploid” Sperm & egg possess 23 chromosomes No pairs 1 copy of each chromosome Produced by meiosis “Haploid”

19. MEIOSIS Ultimately produces four haploid cells Involves two divisions Meiosis I Prophase I Metaphase I Anaphase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II

20. INTERPHASE Preparation for meiosis Chromosomes replicate 1 chromosome ? 2 sister chromatids

21. PROPHASE I Chromosomes condense Homologous chromosomes paired Nuclear membrane disassociates Spindle begins to form

22. METAPHASE I Chromosomes line up on equatorial plate Spindle complete Attached to chromosomes and poles Paired homologous chromosomes attached to opposite poles

23. ANAPHASE I Homologous chromosomes separate Sister chromatids remain attached Move to opposite poles Ultimately reach opposite poles

24. TELOPHASE I Prophase in reverse Chromosomes decondense Nuclear membranes reform Spindle disappears Cytokinesis Products of meiosis I are haploid No homologous pairs No DNA replication prior to meiosis II

25. MEIOSIS II Same as mitosis

26. MEIOSIS SUMMARY

27. MEIOTIC CELL DIVISION Summary of meiotic cell division One cell ? four non-identical cells In humans 46 ? 23 + 23 + 23 + 23

28. FERTILIZATION

29. WHY BOTHER WITH SEX? Asexual reproduction produces offspring genetically identical to the parent Sexual reproduction produces genetically unique offspring Increases genetic variation within a population How is this accomplished? Why is this genetic variation important?

30. GENETIC VARIATION Meiosis produces haploid cells Different from the diploid parent cell Different from each other This genetic difference is due to two factors Independent segregation of chromosomes Genetic recombination (crossing over)

31. INDEPENDENT SEGREGATION The segregation of one chromosome pair is independent of the segregation of other pairs

32. INDEPENDENT SEGREGATION What is the chance that any particular gamete will receive only red chromosomes? 2n = 4 2n = 6 2n = 8 2n = 46

33. GENETIC VARIATION The independent segregation of different pairs of homologous chromosomes alone allows one person the ability to produce more genetically unique gametes than the number of people on earth Crossing over further increases this genetic variation

34. CROSSING OVER While homologous chromosomes are paired in prophase I of meiosis, crossing over can occur Breaking and rejoining of chromosomes Reciprocal exchange of portions of homologous chromosomes Crossing over further increases genetic variation

35. MITOSIS & MEIOSIS

36. NONDISJUNCTION Errors sometime occur, resulting in the unequal segregation of chromosomes

37. NONDISJUNCTION The effects of nondisjunction and other chromosomal alterations will be discussed later, during our discussion of genetics