1. Cell Division
Unit 4: Chapter 10, 11.4

2. Why do cells divide?
Growth
Cell Repair
Cell Replacement

3. Limits to growth Cells cannot exceed a certain size. 1. DNA overload
as cell size increases, more demands are placed on DNA
ex. WIFI network with too many users
2. Nutrient / Waste exchange
as cell size increases, the cell cannot move nutrients and waste in/out of the cell
Cells need high ratios of surface area to volume

4. Surface Area to Volume Ratio
As the length of a cell increases, its volume increases faster than the surface area.
The decrease in the cell’s ratio of surface area to volume makes it more difficult to move materials in and waste products out quickly enough to survive.

5. Surface Area to Volume Ratio

6. Cell Division

7. Cell Division
cell division - process by which a cell divides into two new daughter cells
cells divide to avoid DNA overload, and low surface area to volume ratio

8. Cell Division Stages of Cell Division
mitosis – division of the nucleus
cytokinesis – division of the cytoplasm

9. Cell Division - DNA
DNA - contains all of the genetic information in a cell.
chromosomes – thread-like structures containing DNA and protein
centromeres –protein that attaches sister chromatids

10. Cell Cycle

11. Cell Cycle During the cell cycle, the cell: grows
prepares for division
divides into two daughter cells

12. Cell Cycle Interphase G1 Phase S Phase G2 Phase M Phase Mitosis
Cytokinesis

13. Interphase 1. G1 Phase – first growth phase cell increases in size
synthesizes new proteins and organelles

14. Interphase 2. S Phase – synthesis chromosomes are replicated
DNA synthesis takes place
Once a cell enters the S phase, it usually completes the rest of the cell cycle.

15. Interphase 3. G2 Phase – second growth phase
Organelles and molecules required for cell division are produced
M phase begins at completion of G2

16. Cell Cycle

17. M Phase 1. Mitosis – division of nucleus (DNA) Prophase Anaphase
Metaphase
Telophase
2. Cytokinesis – division of cytoplasm

18. M-Phase: Mitosis / Cytokinesis

19. Mitosis 1. Prophase Chromatin condenses into chromosomes.
The nuclear envelope breaks down.
The centrioles separate and a spindle begins to form.
Spindle fibers made from microtubules form a web between centrioles.

20. Prophase

21. Prophase

22. Mitosis
2. Metaphase
The chromosomes line up across the center of the cell.
Microtubules connect the centromere of each chromosome to the poles of the spindle.

23. Metaphase

24. Metaphase

25. Mitosis
3. Anaphase
The sister chromatids separate into individual chromosomes.
The chromosomes continue to move until they have separated into two groups.

26. Anaphase

27. Anaphase

28. Mitosis
4. Telophase
Telophase is the fourth and final phase of mitosis.
Chromosomes gather at opposite ends of the cell and lose their distinct shape.
A new nuclear envelope forms around each cluster of chromosomes.

29. Telophase

30. Telophase

31. Cytokinesis Cytokinesis: Animals
During cytokinesis, the cytoplasm pinches in half.
Each daughter cell has an identical set of duplicate chromosomes

32. Cytokinesis

33. Cytokinesis: Animals

34. Cytokinesis: Plants Cytokinesis: Plants
The cell plate forms midway between the divided nuclei
The cell plate gradually develops into a separating membrane.
A cell wall then begins to appear in the cell plate.

35. Cytokinesis: Plants

36. Cytokinesis: Plants

37. Mitosis Overview

38. Cell Cycle

39. Regulating the Cell Cycle
Experiments show that normal cells will reproduce until they come into contact with other cells.
When cells come into contact with other cells, they respond by not growing.
This demonstrates that controls on cell growth and division can be turned on and off.

40. Regulating the Cell Cycle

41. Cyclins
cyclins – proteins that regulate the timing of the cell cycle in eukaryotic cells
internal regulators - proteins that respond to events inside the cell and allow the cell cycle to proceed only when certain processes have happened inside the cell (ex. cyclins)
external regulators - proteins that respond to events outside the cell and direct cells to speed up or slow down the cell cycle. (ex. hormones)

42. Regulating the Cell Cycle

43. Cancer
cancer – uncontrolled growth of cells that do not respond to signals which regulate the cell cycle
Cancer cells divide uncontrollably and form masses of cells called tumors that can damage the surrounding tissues.
Cancer cells may break loose from tumors and spread throughout the body, disrupting normal activities and causing serious medical problems or even death.

44. Cancer

45. Chromosomes
Unit 4: Chapter 11.4

46. Chromosomes
chromosome – condensed thread-like chromatin containing genes of an organism
allele – one particular form of a gene
homologous chromosomes – two sets of chromosomes in a cell
diploid (2N) - cell that has 2 sets of chromosomes (somatic cells)
haploid (N) - cell that has 1 set of chromosomes (gamete cells)
gamete – sex cell of sexually reproducing organism
Why do gametes only have half the genetic material of a normal cell?

47. Chromosome Number Chromosome Number: Humans Fruit Flies
2N = 46 (Diploid Cell) 2N = 8
N = 23 (Haploid Cell) N = 4
Humans have 23 pairs of chromosomes or 46 total chromosomes.
Fruit Flies have 4 pairs of chromosomes or 8 total chromosomes.

48. Chromosomes

49. Meiosis
Unit 4: Chapter 11.4

51. Meiosis Meiosis I Meiosis II
meiosis - process of reduction cell division in which the number of chromosomes per cell is cut in half through the separation of homologous chromosomes in a diploid cell to create haploid cells.
2 Stages
Meiosis I
Meiosis II
1 diploid cell produces 4 haploid cells.

52. Meiosis I

53. Meiosis II

54. Meiosis I
1. Interphase I
Cells undergo a round of DNA replication, forming duplicate chromosomes.

55. Interphase I

56. Meiosis I
2. Prophase I
tetrad - each chromosome pairs with its corresponding homologous chromosome
There are 4 chromatids in a tetrad.
crossing over – process where homologous chromosomes form tetrads in meiosis I, exchanging portions of their chromatids
Crossing-over produces new combinations of alleles.

57. Prophase I

58. Prophase I: crossing over

59. Meiosis I 3. Metaphase 1 Chromosomes line up in the middle of cell
Spindle fibers attach to centromeres

60. Metaphase I

61. Meiosis I
4. Anaphase I
Spindle fibers pull analogous chromosomes to opposite ends

62. Anaphase I

63. Meiosis I 5. Telophase and Cytokinesis I Nuclear membranes form.
The cell separates into two cells.
The two cells produced by meiosis I have chromosomes and alleles that are different from each other and from the diploid cell that entered meiosis I (genetic variation).

64. Telophase and Cytokinesis I

65. Meiosis II
The two cells produced by meiosis I now enter a second meiotic division.
Unlike meiosis I, Interphase II does not occur - neither cell goes through chromosome replication.
Each of the cell’s chromosomes has 2 chromatids.

66. Meiosis II
1. Prophase II
Meiosis I results in two haploid (N) daughter cells, each with half the number of chromosomes as the original cell.

67. Meiosis II 2. Metaphase II
The chromosomes line up in the center of cell.

68. Meiosis II
3. Anaphase II
The sister chromatids separate and move toward opposite ends of the cell.

69. Meiosis II 4. Telophase and Cytokinesis II
Meiosis II results in four haploid (N) daughter cells.

70. Gamete Formation

71. Mitosis vs. Meiosis
Mitosis results in the production of two genetically identical diploid cells (2N).
Meiosis produces four genetically different haploid cells (N).

72. Mitosis vs. Meiosis Mitosis
Cells produced by mitosis have the same number of chromosomes and alleles as the original cell.
Mitosis allows an organism to grow and replace cells.
Some organisms reproduce asexually by mitosis.

73. Mitosis vs. Meiosis Meiosis
Cells produced by meiosis have half the number of chromosomes as the parent cell.
These cells are genetically different from the diploid cell and from each other.
Meiosis is how sexually-reproducing organisms produce gametes.