Cellular Division

1. Cellular Division

2. When do Cells Divide? • Cells obtain nutrients and eliminate wastes through the cell membrane. There must be enough surface area for all of the exchanges to take place with the environment. • It is more difficult for cells with a larger volume to transport, so cells remain relatively small to survive. • When a cell reaches its maximum size, the nucleus initiates cell division.

3. Why do Cells Divide? • Cell division accounts for 3 essential life processes: • Growth - increase in size • Differentiation - cells develop specialized shapes and functions.

4. Why do Cells Divide? • Cell division accounts for 3 essential life processes: • Repair - heal injuries by cell regeneration at the injury site

5. Why do Cells Divide? • Cell division accounts for 3 essential life processes: • Reproduction - making new organisms • Asexual - offspring produced by one parent; the result of cell division • Sexual - offspring have a combination of genetic material from two parent organisms; requires meiosis, a specialized form of cell division

6. How do Cells Divide? • Phase - a defined period within a cycle of change • Cell cycle - the sequence of phases in the life cycle of a cell

7. The Cell Cycle

8. Interphase • Interphase - growth and preparation • Occurs between divisions • 3 stages • G1 (gap 1) - growth and development • S (synthesis - chromosomes replicate; the cell commits to cell division • G2 (gap 2) - cell synthesizes other organelles and other materials needed for division

9. Interphase • During interphase, the chromosomes are in chromatin form - a tangled bundle • During S phase, each chromosome is replicated, creating sister chromatids • As cell division begins (during prophase), the chromatin coils and condenses to chromosomes and the sister chromatids are clearly visible, joined at the centromere.

10. Chromatin - uncoiled DNA • Chromosome - neatly coiled DNA

11. Cell Division - Mitosis and Cytokinesis • There are 2 stages in cell division: • 1. Mitosis • The cell’s nucleus divides into 2 nuclei. • 2. Cytokinesis • The cell’s cytoplasm divides to make 2 daughter cells

12. Mitosis • Mitosis is the splitting of the cell’s nucleus • It has 4 parts: • Prophase • Metaphase • Anaphase • Telophase

13. Mitosis - Prophase • Chromosomes condense • In animal cells, a pair of centrioles migrates to each side of the cell • The mitotic spindle forms from the centrioles • The nuclear membrane and nucleolus dissappear

14. Mitosis - Prophase

15. Mitosis - Metaphase • Spindle arranges chromosomes along “metaphase plate”

16. Mitosis - Anaphase • Centromeres divide and sister chromatids are pulled apart, so that each daughter cell has an identical set of chromosomes

17. Mitosis - Telophase • Nuclear membrane reforms around each set of chromosomes • Two nucleoli form • Chromosomes uncoil into chromatin • Cleavage furrow starts to form (for cytokinesis)

18. Cytokinesis • The cell’s cytoplasm divides to make 2 daughter cells • The cleavage furrow pinches cell in half

19. Four Mitotic Stages • Prophase • Metaphase • Anaphase • Telophase

20. Identical Daughter Cells Two identical daughter cells Parent Cell

21. Mitosis in Animal Cells • Rat epithelial cells

22. Mitosis in Plant Cells • Plant cells have a cell wall. • During mitosis, a cell plate forms a new cell membrane and cell wall between the two daughter cells.

23. History • Mitosis was first observed microscopically in the 1870s by the German biologist Walter Flemming • He coined the term mitosis • from the Greek word mitos, • meaning thread

24. The Cell Cycle Interphase may be up to 90% of the cell cycle. The time varies for different cell types.

25. The Cell Cycle

28. MeiosisFormation of Gametes(Eggs & Sperm)

29. Facts About Meiosis • Produces sex cells called gametes (eggs & sperm) • Occurs in the testes in males (Spermatogenesis) • Occurs in the ovaries in females (Oogenesis)

30. Facts About Meiosis • The gametes (eggs and sperm) are haploid - which means they have 1 set of chromosomes, not 2 like all of our diploid body cells. • Why we need meiosis --> this is critical for sexual reproduction.

31. The Process of Meiosis • It starts with a diploid cell that has 46 chromosomes (2N). • The DNA is replicated, so there are 92 chromosomes. There are 4 copies of each, making a tetrad.

32. The Process of Meiosis • The first division, very similar to mitosis, occurs, producing two cells with 46 chromosomes each. • A second division occurs, producing 4 haploid daughter cells that each have 23 chromosomes (N)

33. Meiosis I MeiosisII Meiosis: Two Part Cell Division Diploid at start Diploid Haploid

34. from mom from dad child too much! meiosis reduces genetic content Meiosis Forms Haploid Gametes • Meiosis must reduce the chromosome number by half • Fertilization then restores the 2n number The right number!

35. Crossing-Over Pieces of chromosomes are exchanged, which produces genetic recombination in the offpsring

36. Crossing-Over Crossing-over multiplies the number of different gamete types

37. Fertilization 2N = 6 1N = 3

38. Humans • Haploid number (1N) = 23 • Diploid number (2N) = 46 • Somatic cells are diploid. • Gametes are haploid.