Mitosis and Meiosis

1. Mitosis and Meiosis Chapter 3 – Part 3

2. Mitosis Notes Starts Here

3. Why Do Cells Divide Instead of Growing? • The cell's DNA would no longer be able to serve the increasing needs of the growing cell. • The cell has more trouble moving enough nutrients and wastes across the cell membrane. • Larger cells have a smaller ratio of surface area to volume.

4. Cell Division • Cell Division - The process by which a cell divides into two new “daughter” cells. • Before cell division occurs, the cell replicates, or copies, all of its DNA. • Each daughter cell gets one complete set of genetic information.

5. Prokaryotic vs. Eukaryotic • Prokaryotic Cell Division: • Simple matter of separating the contents of the cell into two parts. • Eukaryotic Cell Division: Two Stages: • Mitosis (division of the cell nucleus) • Cytokinesis (division of the cytoplasm)

6. Asexual Reproduction • Reproduction by mitosis is classified as asexual, since the cells produced by mitosis are genetically identical to the parent cell. • Asexual Reproduction - Reproduction without the fusion of gametes; not necessary to have two parents • Examples Include: • Single-celled organisms • Source of new cells when a multicellular organism grows and develops 

7. Chromosomes • In eukaryotic cells, the genetic information that is passed on from one generation of cells to the next is carried by chromosomes. • The cells of every organism have a specific number of chromosomes • The cells of fruit flies have 8 chromosomes. • Human cells have 46 chromosomes. • Carrot cells have 18 chromosomes.

8. Chromosomes • Chromosomes are not visible in most cells except during cell division. • DNA and protein molecules that make up the chromosomes are spread throughout the nucleus. • At the beginning of cell division, however, the chromosomes condense into compact, visible structures.

9. Chromosomes • Well before cell division, each chromosome is replicated, or copied.

10. Chromosomes • Each chromosome consists of two identical “sister” chromatids. • When the cell divides, the “sister” chromatids separate from each other. • One chromatid goes to each of the two new cells.

11. Chromosomes • Each pair of chromatids is attached at an area called the centromere. • Usually located near the middle of the chromatids, although some lie near the ends.

12. The Cell Cycle • Cell Cycle - Series of events that cells go through as they grow and divide. • Cell grows and prepares for division • Divides to form two daughter cells • Begins the cycle again • Interphase - Period of the cell cycle between cell divisions (consists of S, G1, and G2 phase)

13. Four Phases of the Cell Cycle • M Phase - Mitosis and cytokinesis take place. • S Phase - Chromosome replication takes place. • G1 and G2 Phase - Periods of intense growth and activity that occurs between the M and S phases.

15. Four Phases of Mitosis • Prophase 3. Anaphase • Metaphase 4. Telophase (Four phases are followed by Cytokinesis)

16. Prophase • The first and longest phase of mitosis • The chromosomes become visible. • The centrioles, two tiny structures located in the cytoplasm near the nuclear envelope, separate and take up positions on opposite sides of the nucleus. Centrioles

17. Prophase • The condensed chromosomes become attached to fibers in the spindle (attach near the centromere of each chromatid). • Near the end of prophase: • The chromosomes coil more tightly • The nucleolus disappears • The nuclear envelope breaks down. Spindle - A fanlike microtubule structure that helps separate the chromosomes.

18. Metaphase • The second phase of mitosis • The chromosomes line up across the center of the cell. • Microtubules connect the centromere of each chromosome to the two poles of the spindle.

19. Anaphase • Third phase of mitosis. • The centromeres split, allowing the sister chromatids to separate and become individual chromosomes. • Chromosomes continue to move until they have separated into two groups near the poles. • Ends when the chromosomes stop moving.

20. Telophase • Fourth and final phase of mitosis. • The chromosomes begin to disperse into a tangle of dense material. • A nuclear envelope re-forms around each cluster of chromosomes. • The spindle begins to break apart. • A nucleolus becomes visible in each daughter nucleus.

21. Mitosis • To help you remember the order of the events of mitosis: PMAT • https://www.besttvbuys.com/pottypatch/ver3/index.asp?did=978&refcode=p2

22. Cytokinesis • Usually occurs at the same time as telophase. • Cytokinesis - The division of the cytoplasm itself.

23. Cytokinesis: Plant vs. Animal Cells • Animal Cells: • The cell membrane is drawn inward until the cytoplasm is pinched into two nearly equal parts. • Plant Cells: • A structure known as the cell plate forms midway between the divided nuclei and gradually develops into a separating membrane. • A cell wall then begins to appear in the cell plate.

25. Regulating the Cell Cycle • Not all cells move through the cell cycle at the same rate. • Cell growth and cell division are controlled. • Cyclins are proteins that regulate the timing of the cell cycle in eukaryotic cells.

26. Regulating the Cell Cycle • If cells are placed in a petri dish containing nutrient broth (provides food for the cells): • Cells will grow until they form a thin layer covering the bottom of the dish. • Then, the cells stop growing. When cells come into contact with other cells, they respond by not growing.

27. Regulating the Cell Cycle • If cells are removed from the center of the dish: • The cells bordering the open space will begin dividing until they have filled the empty space. • This occurs in an injury as well: • If a cut in the skin or a break in a bone occurs, cells at the edges of the injury are stimulated to divide rapidly. • When the healing process nears completion, the rate of cell division slows down.

28. Cancer • Cancer - Disorder in which some of the body's own cells lose the ability to control growth. • Cancer cell divide uncontrollably and form masses of cells (tumors) • Cancer cells may spread throughout the body: • Disrupts normal activities and causes serious medical problems or even death

29. Meiosis Notes Start Here

30. Homologous Chromosomes- Term used to refer to chromosomes that each have a corresponding chromosome from the opposite-sex parent.

31. Haploid vs. Diploid • Haploid - Term used to refer to a cell that contains only a single set of chromosomes and therefore only a single set of genes • “One Set” • Represented by N. • Diploid - Term used to refer to a cell that contains both sets of homologous chromosomes • “Two Sets” • Represented by 2N.

32. Gamete vs. Zygote • Gamete - A mature sexual reproductive cell that has a haploid numbers that unites with another cell to form a new organism. • Example: Sperm or egg cell • Zygote - The cell formed by the union of two gametes.

33. Diploid zygote Haploid sperm (gamete) Haploid egg (gamete) 1n 1n 2n Sexual Reproduction + =

34. …which grows into a DIPLOID fetus HAPLOID gametes join… …to form a DIPLOID zygote Remember, in SEXUAL reproduction…

35. Meiosis • Meiosis - Process by which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell • Haploid (N) gamete cells are produced from diploid (2N) cells. Begin with Diploid (2N) Cells Produce Haploid (N) Cells

36. Four gametes are made during cell division by meiosis. The gamete cells have half the number of chromosomes as the cell.

37. Meiosis • Meiosis usually involves two distinct divisions: • Meiosis 1 • Meiosis 2

38. Meiosis 1 • Prior to meiosis I, each chromosome is replicated. • Meiosis 1 is similar to mitosis EXCEPT: • Each chromosome pairs with its corresponding homologous chromosome to form a structure called a tetrad • Each tetrad exchanges portions of their chromatids in a process called crossing-over.

39. Meiosis 1 • Tetrad - Structure containing four chromatids that forms during meiosis • Crossing Over - Process in which homologous chromosomes exchange portions of their chromatids or alleles.

40. Crossing Over • Produces new combinations of alleles. • Meiosis 1 is similar to mitosis EXCEPT: • The two cells produced by meiosis I have sets of chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I.

41. Meiosis 2 • Two cells produced by meiosis I now enter a second meiotic division. • Unlike the first division, neither cell goes through a round of chromosome replication before entering meiosis II. • Those four daughter cells now contain the haploid number (N)—just 2 chromosomes each.

42. Gamete Formation • Males: The haploid gametes produced by meiosis are called sperm. • Females: Generally only one of the cells produced by meiosis is involved in reproduction. This female gamete is called an egg in animals. • In females, cell divisions at the end of meiosis I and meiosis II are uneven. • A single cell, which becomes an egg, receives most of the cytoplasm. • The other three cells are known as polar bodies and usually do not participate in reproduction.

44. Mitosis vs. Meiosis • Mitosis: Produces two genetically identical diploid cells. • Meiosis: Produces four genetically different haploid cells.

45. Mitosis vs. Meiosis • Mitosis • Allows an organism's body to grow and replace cells. • Used in asexual reproduction to produce a new organism. • New (daughter) cell is identical to the parent celland to each other. • Produces two diploid (2N) daughter cells. • Meiosis • Used in sexual reproduction to produce gametes. • New (daughter) cells are genetically different from the parent cells and from one another. • Produces four haploid (N) cells.

46. Somatic (Body) Cells • You can only inherit a trait from gamete cells (sperm or egg cell)! • If your mother has skin cancer, you will not inherit this mutation because the mutation is on her somatic (body) cells and these are not inherited.