1. General Biology Mr. Cobb
Mitosis & Meiosis
General Biology
Mr. Cobb

2. Cell Theory
According to the cell theory, all cells come from pre-existing cells.
But why do cells reproduce?

3. Cell Reproduction
Cells reproduce to replace lost or damaged cells with new healthy cells.
New cells gradually replace dead cells.
Cell reproduction is responsible for your growth.

4. Reproduction
Asexual reproduction occurs when one organism makes an exact duplicate of itself.
Sexual reproduction occurs when the genetic information from two organisms combine to make a new, genetically different, organism.

5. Genetic material (DNA or RNA) can exist in your cells in three forms:
Chromatin
Chromosomes
Chromatids

6. Chromatin
Genetic material is called chromatin when it is in a mass of very long fibers.

7. Chromosomes
As the cell prepares to divide, the genetic material condenses and winds around itself, forming chromosomes.
Chromosomes are visible under powerful microscopes

8. Chromosomes Chromosomes are pictured in a karyotype.
Humans have 23 pairs of chromosomes.
These pairs are called homologous pairs of chromosomes.

9. Diploid
Cells that have homologous pairs of chromosomes are called diploid. (2n)
Most organisms are diploid.

10. Chromosomes

11. Chromatids
Before the cell divides, the genetic material is copied. The two identical pieces of DNA join together to form “sister chromatids.”
These chromatids are joined at the centromere.

12. Chromosomes & Chromatids

13. Genetic Information

14. Cell Cycle
In order for cells to reproduce, they must first go through the cell cycle, which lasts about hours.
During the cell cycle, the DNA and organelles are copied, the cell increases in size, and then divides.

15. The cell cycle is composed of several steps:
Interphase
Mitosis
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis

16. Interphase
Interphase is made up of 3 phases:
G1 – growth and function.
During S-phase, the cell’s DNA is synthesized (copied). This phase lasts for 3 – 6 hours.
During G2 phase the cell prepares to divide (organelles and cytoplasm increase)

17. Cell Division
When the cell is actually dividing, it is called mitosis, followed by cytokinesis.

18. Mitosis
During mitosis, the nucleus and the duplicated DNA divide and are moved into two daughter cells.

19. Cytokinesis
During cytokinesis, the cytoplasm of the cell divides into two parts.
At the end of cytokinesis, the two daughter cells completely separate, forming two identical cells.

21. FLIP BOOK – Label flaps like this
CELL CYCLE (title page)
MITOSIS – Prophase
Metaphase
Anaphase
Telophase/Cytokinesis
Interphase

22. Mitosis
Cell Cycle

23. Cell Cycle The cell cycle is divided into three parts: Interphase
Mitosis
Cytokinesis

24. Interphase During interphase, the cell is very active.
Organelles and DNA are duplicated

25. Interphase
Individual chromosomes are not visible because they are loosely packed chromatin
ChromatIN is found only during INterphase

26. Mitosis Mitosis is divided into 4 phases: Prophase Metaphase Anaphase
Telophase

27. Prophase
During prophase, the chromatin fibers condense and become chromosomes
Sister chromatids join at the centromere.
Sister chromatIDs are IDentical

28. Prophase The nucleolus disappears. The nuclear membrane disappears
Spindle fibers form from spindle

29. Metaphase
All the chromosomes line up in the middle of the cell (called the “metaphase plate”).

30. Metaphase
Spindle fibers attach to the chromosomes at the centromere.

31. Anaphase
Spindle fibers condense, causing the sister chromatids to separate.
Each chromatid is now a “daughter chromosome.”

32. Telophase
Telophase begins when the chromosomes reach the poles of the spindle.
Processes that occurred during prophase are reversed.

33. Telophase The nuclear envelope and nucleolus reappear.
The spindle disappears
Chromosomes uncoil

34. Cytokinesis
During cytokinesis, the cytoplasm separates into the two daughter cells.
The two daughter cells separate from each other.

35. Cytokinesis
In animal cells a “cleavage furrow” forms when the two cells begin to separate.
In plants, a cell plate forms and separates the two cells.

36. Cell division
The result of the cell cycle is one cell becoming two identical cells.

38. Meiosis

39. Mitosis Mitosis occurs in “somatic” cells.
Somatic cells are body cells (ex: muscle cells, brain cells, liver cells, skin cells, etc.)
But what about sex cells?

40. Meiosis Sex cells divide by a different process.
This process, called Meiosis, allows 4 unidentical cells to be formed from one cell.

41. Meiosis
In females, meiosis occurs in egg cells before birth, and again during puberty.
In males, meiosis occurs in sperm cells throughout their lives.

42. Definitions Somatic Cells - Body cells Sex cells - Sperm & Egg
Homologous chromosomes - chromosomes that are similar in size, shape, and genetic content. They are NOT identical, just similar.

43. Homologous Chromosomes
Homologous chromosomes are similar, but not identical.

44. Definitions
Diploid - Have both pairs of homologous chromosomes (2n). Humans have 46 chromosomes
Haploid - have one pair of homologous chromosomes. Ex: Human sex cells have 23 chromosomes

45. Diploid Diploid organisms have pairs of chromosomes.
Human somatic cells are diploid.

46. Haploid
Haploid cells have single chromosomes.

47. Meiosis Meiosis is divided into two parts:
Meiosis II
Both Meiosis I and II go through the steps of Mitosis:
Prophase, Metaphase, Anaphase, Telophase

48. Meiosis I During Meiosis I, DNA is copied and CROSSING OVER occurs.
Crossing over allows for genetic variability.
Meiosis I includes: Prophase I, Metaphase I, Anaphase I, Telophase I

49. Prophase I
After the chromosomes condense, the homologous chromosomes come together in pairs (the pairing up of chromosomes is called synapsis)

50. Prophase I
These pairs of chromosomes are known as Sister Chromatids, they are alike but not identical.

51. Prophase I The four chromatids come together form a tetrad.
The chromosomes exchange portions of their DNA. This exchange of DNA is called crossing over.

52. Metaphase I
The centromeres of the chromosomes line up at the equator of the cell.
Each tetrad (group of chromosomes) is attached to a spindle fiber.

53. Anaphase I
During anaphase I, the homologous chromosomes separate, with one going to each pole of the cell.
The sister chromatids do not separate.

54. Anaphase I
The homologous chromosomes separate, NOT the sister chromatids.

55. Telophase I
During telophase I, the cytoplasm divides, forming two daughter cells.
Each daughter cell has half the number of chromosomes as the parent, but each chromosome is double stranded.

56. Meiosis I Overview

57. Meiosis II
After telophase I has ended, Meiosis II begins. NO G1, S, or G2 occurs!
The end product of meiosis II is four haploid daughter cells that are NOT genetically identical.

58. Meiosis II
Meiosis II includes: prophase II, metaphase II, anaphase II, and telophase II.
The process of meiosis II is very similar to mitosis.

59. Prophase II
Each of the daughter cells forms a spindle and the double stranded chromosomes move toward the middle of the spindle.

60. Metaphase II The chromosomes line up at the equator of the cell.
Each chromosome still consists of two chromatids

61. Anaphase II The centromeres divide and the two chromatids separate.
The single stranded chromosomes move toward the opposite ends of the cell.

62. Telophase II
Both daughter cells divide, forming four haploid cells.

63. Meiosis II
At the end of meiosis II, the haploid cells return to their interphase state– the DNA uncoils and the nuclear membrane reappears.

65. Oogenesis Oogenesis is the formation of an egg cell through meiosis.
During oogenesis, the cytoplasm divides unevenly. One cell gets the majority of the cytoplasm, the other three cells are much smaller.
The larger cell becomes the egg, while the others are called “polar bodies” and are disintegrated by the body.

66. Oogenesis

67. Spermatogenesis
Spermatogenesis is the formation of sperm cells through meiosis.
The cytoplasm divides evenly and four haploid sperm cells are generated from one diploid cell.

68. Spermatogenesis
Spermatogenesis