The Cell Cycle and Cell Division

1. 7 The Cell Cycle and Cell Division

2. Chapter 7 The Cell Cycle and Cell Division • Key Concepts • 7.1 Different Life Cycles Use Different Modes of Cell Reproduction • 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • 7.3 Cell Reproduction Is Under Precise Control

3. Chapter 7 The Cell Cycle and Cell Division • 7.4 Meiosis Halves the Nuclear Chromosome Content and Generates Diversity • 7.5 Programmed Cell Death Is a Necessary Process in Living Organisms

4. Chapter 7 Opening Question How does infection with HPV result in uncontrolled cell reproduction?

5. Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction • The lifespan of an organism is linked to cell reproduction, or cell division: a parent cell duplicates its genetic material and then divides into two similar cells. • Cell division is important in growth and repair of multicellular organisms and the reproduction of all organisms.

6. Figure 7.1 The Importance of Cell Division

7. Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction • Organisms have two basic strategies for reproducing themselves: • Asexual reproduction • Sexual reproduction

8. Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction • Asexual reproduction • The offspring are clones—genetically identical to the parent • Any genetic variations are due to mutations (changes in DNA sequences due to environmental factors or copying errors)

9. Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction • Single-celled prokaryotes usually reproduce by binary fission • Single-celled eukaryotes can reproduce by mitosis and cytokinesis • Many multicellular eukaryotes can also reproduce by asexual means

10. Figure 7.2 Asexual Reproduction on a Large Scale

11. Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction • Sexual reproduction • Involves fusion of gametes • Results in offspring with genetic variation • Gametes form by meiosis—a process of cell division that reduces genetic material by half

12. Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction • DNA in eukaryotic cells is organized into chromosomes. • Somatic cells: body cells not specialized for reproduction • Each somatic cell contains two sets of chromosomes that occur in homologous pairs. • One homolog came from the female parent and one from the male parent and have corresponding genetic information.

13. Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction • Gametes have only one set of chromosomes—one homolog from each pair. • They are haploid; number of chromosomes = n • Fertilization: two haploid gametes fuse to form a zygote • They are diploid; number of chromosome in zygote = 2n

14. Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction • All sexual life cycles involve meiosis: • Gametes may develop immediately after meiosis • Or each haploid cell may develop into a haploid organism (haploid stage of the life cycle) that eventually produces gametes by mitosis • Fertilization results in a zygote and begins the diploid stage of the life cycle.

15. Figure 7.3 Sexual Life Cycles Involve Fertilization and Meiosis (Part 1)

16. Figure 7.3 Sexual Life Cycles Involve Fertilization and Meiosis (Part 2)

17. Figure 7.3 Sexual Life Cycles Involve Fertilization and Meiosis (Part 3)

18. Concept 7.1 Different Life Cycles Use Different Modes of Cell Reproduction • The essence of sexual reproduction is: • Random selection of half the diploid chromosome set to form a haploid gamete • Followed by fusion of haploid gametes from separate parents to make a diploid cell • This results in shuffling of genetic information in a population, and no two individuals have exactly the same genetic makeup.

19. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Four events in cell division: • Reproductive signals initiate cell division • DNA replication • DNA segregation—distribution of the DNA into the two new cells • Cytokinesis—division of the cytoplasm and separation of the two new cells

20. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Prokaryotes divide by binary fission: results in reproduction of the entire organism. • Reproductive signals may be environmental factors such as nutrient availability.

21. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Replication: • Most prokaryotes have one circular chromosome with two important regions: • ori—where replication starts • ter—where replication ends • Replication occurs as the DNA is threaded through a “replication complex” of proteins at the center of the cell.

22. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Segregation: • As replication proceeds, the ori complexes move to opposite ends of the cell. • DNA sequences adjacent to the ori region actively bind proteins for the segregation, using ATP. • An actin-like protein provides a filament along which ori and other proteins move.

23. Figure 7.4 Prokaryotic Cell Division: Binary Fission

24. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Cytokinesis: • After chromosome segregation, the cell membrane pinches in by contraction of a ring of protein fibers under the surface. • As the membrane pinches in, new cell wall materials are deposited, resulting in separation of the two cells.

25. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Eukaryotic cells divide by mitosis followed by cytokinesis. • Reproductive signals are usually related to functions of the entire organism, not the environment of a single cell. • Most cells in a multicellular organism are specialized and do not divide.

26. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Replication of each chromosome occurs as they are threaded through replication complexes. • DNA replication only occurs during a specific stage of the cell cycle.

27. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • In segregation, one copy of each chromosome ends up in each of the two new cells. • More complex than in prokaryotes: eukaryotes have a nuclear envelope, and there are multiple chromosomes. • Cytokinesis in plant cells (which have cell walls) is different than in animal cells (no cell walls).

28. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • In mitosis, one nucleus produces two daughter nuclei, each containing the same number of chromosomes as the parent nucleus. • Mitosis is continuous, but it is convenient to subdivide it into phases.

29. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • The cell cycle is the period from one cell division to the next, divided into stages in eukaryotes. • M phase: Mitosis (segregation of chromosomes into two new nuclei), followed by cytokinesis. • Interphase: cell nucleus is visible and cell functions occur, including DNA replication.

30. Figure 7.5 The Phases of the Eukaryotic Cell Cycle

31. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells Interphase has three subphases: • G1 (Gap 1)—variable, may last a long time • S phase (synthesis)—DNA is replicated • G2 (Gap 2)—the cell prepares for mitosis; synthesizes microtubules for segregating chromosomes

32. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Prophase: three structures appear • Condensed chromosomes • Reoriented centrosomes • Spindle

33. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Even during interphase, DNA is packaged by winding around specific proteins, and other proteins coat the DNA coils. • In prophase, the chromosomes become much more tightly coiled and condensed.

34. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • After replication, each chromosome has two DNA molecules called sister chromatids, joined at a region called the centromere.

35. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Karyotype: the condensed chromosomes for a given organism can be distinguished by their sizes and centromere positions

36. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Karyotype analysis was used to identify and classify organisms, but DNA sequencing is more commonly used today. • Karotype analysis is still used to identify chromosome abnormalities.

37. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • The centrosome determines orientation of the spindle. • Consists of two centrioles—hollow tubes formed by microtubules. • The centrosome is duplicated during S phase; centrosomes move towards opposite sides of the nucleus at the G2–M transition. • Centrosome position determines the plane of cell division—important in the development of multicellular organisms.

38. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Centrosomes serve as poles toward which the chromosomes move. • The spindle forms between the poles from microtubules: • Polar microtubules overlap in the middle region of the cell and keep the poles apart. • Astral microtubules interact with proteins attached to the cell membrane; also help keep the poles apart.

39. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Kinetochore microtubules attach to kinetochores on the chromatid centromeres. • Sister chromatids attach to kinetochore microtubules from opposite sides so that the two chromatids will move to opposite poles. • Sister chromatids become daughter chromatids after separation.

40. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Prometaphase: the nuclear envelope breaks down and chromatids attach to the kinetochore microtubules. • Metaphase:the chromosomes line up at the midline of the cell. • Anaphase: the chromatids separate, and daughter chromosomes move toward the poles.

41. Figure 7.6 The Phases of Mitosis (1)

42. Figure 7.6 The Phases of Mitosis (2)

43. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Two mechanisms move the chromosomes to opposite poles: • Kinetochores have molecular motor proteins (kinesin and dynein), which move the chromosomes along the microtubules. • The kinetochore microtubules shorten from the poles, drawing the chromosomes toward the poles.

44. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Telophase: nuclear envelopes form around each set of chromosomes and nucleoli appear, and the spindle breaks down and chromosomes become less compact.

45. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • Cytokinesis: • In animal cells, the cell membrane pinches in between the nuclei. • A contractile ring of actin and myosin microfilaments forms on the inner surface of the cell membrane; the two proteins produce a contraction to pinch the cell in two.

46. Figure 7.7 Cytokinesis Differs in Animal and Plant Cells

47. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • In plant cells, vesicles from the Golgi apparatus appear along the plane of cell division. • The vesicles fuse to form a new cell membrane. • Contents of vesicles also contribute to forming the cell plate—the beginning of the new cell wall.

48. Concept 7.2 Both Binary Fission and Mitosis Produce Genetically Identical Cells • After cytokinesis,each daughter cell contains all of the components of a complete cell. • Chromosomes are precisely distributed. • The orientation of cell division is important to development, but there does not appear to be a precise mechanism for distribution of the cytoplasmic contents.

49. Table 7.1

50. Concept 7.3 Cell Reproduction Is Under Precise Control • Cell reproduction must be under precise control. • If single-celled organisms had no control over reproduction, they would soon overrun the environment and starve to death. • In multicellular organisms, cell reproduction must be controlled to maintain body form and function.