Mitosis & Meiosis

1. 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 • 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 10-20 hours. • During the cell cycle, the DNA and organelles are copied, the cell increases in size, and then divides.

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

16. 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) Interphase

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 I • 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.