Understanding the Cell Cycle: Key Stages, Control Mechanisms, and Cancer Implications

1. THE CELL CYCLE

2. The Cell Cycle Events that occur in the life of a cell. Includes 3 major stages: • Interphase • Mitosis • Cytokinesis

4. 1. Interphase (Cell is not dividing) • G1 Phase – carries out basic functions & performs specialized activities. • duration is extremely variable • contains restriction checkpoint ~ cell “decides” to: • divide • enter a quiescent phase (G0) • die

6. 1. Interphase (Cell is not dividing) • G0 Phase – cell maintains specialized characteristics, but does not divide Ex. neurons & muscle cells

8. 1. Interphase (Cell is not dividing) • S Phase – cell replicates chromosomes & synthesizes proteins • animal cells replicate centrioles as well

10. 1. Interphase (Cell is not dividing) • G2 Phase - cell synthesizes additional proteins (ex. tubulin) & assembles/stores membrane material

12. Mitosis (M phase) – Equal distribution of replicated genetic material. • Five steps: • Prophase • Prometaphase • Metaphase • Anaphase • Telophase

13. Mitosis – Prophase • replicated chromosomes condense • centrosomes separate & migrate toward opposite sides of cell • mitotic spindle forms (microtubules grow out from centrosomes) • nucleolus disappears

15. Mitosis – Prometaphase • nuclear membrane breaks down • spindle fibers attach to centromeres of chromosomes

17. Mitosis – Metaphase • chromosomes are lined up single-file along equator of mitotic spindle

19. Mitosis – Anaphase • Centromeres part, sister chromatids (now called chromosomes) separate • chromosomes move toward opposite poles

21. Mitosis – Telophase • mitotic spindle breaks down • chromosomes decondense • nuclear membranes reform around two nuclei • nucleoli reappear

23. Cytokinesis • Distribution of cytoplasm to daughter cells • begins during anaphase or telophase • differs in animal & plant cells

25. Cytokinesis in animal cells • Cleavagefurrow (slight indentation) forms around equator of cell • Actin & myosin microfilaments act like a drawstring to pinch the cell in two • Usually an equal division

26. Cytokinesis in plant cells • phragmoplast (microtubule structure) forms in cytoplasm & traps vesicles containing cell wall material • vesicles fuse, forming a cell plate across midline of cell • cell plate gives rise to two primary cell walls

28. Review of the M-phase

29. Review of the M-phase

30. Review of the M-phase

31. Review of the M-phase

32. Review of the M-phase

33. Review of the M-phase

34. Review of the M-phase

35. Review of the M-phase

36. Review of the M-phase

37. Review of the M-phase

39. Does cytokinesis always accompany karyokinesis? Karyokinesis in the absence of cytokinesis results in a syncytium(mass of multinucleated cells).

40. Control of the Cell Cycle Checkpoints - groups of interacting proteins that ensure cell cycle events occur in the correct sequence.

41. Shortening of telomeres - loss of telomere DNA signals cell to stop dividing. Some cells produce telomerase(enzyme that continually adds telomere DNA).

42. Contact Inhibition - healthy cells stop dividing when they come in contact with other cells.

43. Hormones - stimulate cell division. Ex. Estrogen stimulates uterine cell division Growth factors - proteins that stimulate local cell division. Ex. Epidermal growth factor (EGF) stimulates epithelial cell division filling in new skin underneath a scab Interaction of kinases & cyclins - activate genes that stimulate cell division.

44. B. Apoptosis Programmed cell death; part of normal development.

45. Steps of Apoptosis:

46. C. Cancer (loss of cell cycle control) Condition resulting from excess cell division or deficient apoptosis. Characteristics of Cancer Cells: • can divide uncontrollably & eternally • are heritable & transplantable • lack contact inhibition • readily metastasize • exhibit angiogenesis • exhibit genetic mutability

47. Causes of Cancer: • Over-expression of oncogenes Oncogenes are genes that trigger limited cell division. • Inactivation of tumor suppressor genes Tumor suppressor genes prevent a cell from dividing or promote apoptosis.

48. Normal functioning of oncogenes & tumor suppressor genes may be affected by environmental factors: • carcinogens • radiation • viruses • diet • exercise habits

49. Meiosis - formation of gametes • Somatic cells – body cells • In contrast to mitosis (occurs in somatic cells), gametes (eggs or sperm) are produced only in gonads (ovaries or testes). • In the gonads, cells undergo a variation of cell division (meiosis) which yields four daughter cells, each with half the chromosomes of the parent. • In humans, meiosis reduces the number of chromosomes from 46 to 23 • Chromosomes #1 through 22 – autosomal • Chromosome #23 – sex

50. Meiosis - formation of gametes • Fertilization fuses two gametes together and doubles the number of chromosomes to 46 again. • Organisms inherit single copy of each gene from each parent • These copies are segregated from each other during formation of the gametes • Homologous – corresponding male and female chromosomes