1. Chapter 17 The Cell Cycle

14. An Overview of the Cell Cycle 1. The eucaryotic cell cycle is divided into four phases 2. Cell-cycle control is similar in all eucaryotes 3. Cell-cycle control system dissected genetically in yeasts 4. Cell-cycle control system analyzed biochemically in animal embryos 5. Cell-cycle progression studied in various ways

15. The major events of the cell cycle

16. Events of eucaryotic cell division as seen under a microscope

17. The four phases of the cell cycle

18. Cell-cycle control system dissected genetically in yeasts

19. Behavior of a temperature-sensitive Cdc mutant room temperature 36°C

20. Morphology of budding yeast cells arrested by a Cdc mutation Normal yeast cells – buds vary in size according to the cell-cycle stage In a Cdc15 mutant, grown at the restrictive temperature, cells complete anaphase but cannot complete the exit from mitosis and cytokinesis. They arrest uniformly with the large buds, which are characteristic of late M phase

21. Cell-cycle control system analyzed biochemically in animal embryos A mature Xenopus egg, ready for fertilization

22. Oocyte growth and egg cleavage in Xenopus

23. Studying cell cycle in a cell-free system

24. Cell-cycle progression studied in various ways Labeling S-phase cells

25. Analysis of DNA content with a flow cytometer

26. The cell-cycle control system 1. Cell-cycle control system triggers the major events of the cell cycle 2. The cell-cycle control system depends on cyclically activated cyclin- dependent protein kinases (Cdks) 3. Inhibitory phosphorylation and Cdk inhibitory proteins (CKIs) can suppress Cdk activity 4. The cell-cycle control system depends on cyclical proteolysis 5. Cell-cycle control also depends on transcriptional regulation 6. The cell-cycle control system functions as a network of biochemical switches

27. Control of the cell cycle

28. Two key components of the cell-cycle control system

29. Cyclin-Cdk complexes of the cell-cycle control system

31. Inhibitory phosphorylation and Cdk inhibitory proteins (CKIs) can suppress Cdk activity The structural basis of Cdk activation

32. The regulation of Cdk activity by inhibitory phosphorylation

33. The inhibition of a cyclin-Cdk complex by a CKI

34. The cell-cycle control system depends on cyclical proteolysis

36. Cell-cycle control also depends on transcriptional regulation In budding yeast, about 10% of the genes encode mRNAs whose levels oscillate during the cell cycle

37. The cell-cycle control system functions as a network of biochemical switches

38. An overview of the cell-cycle control system

39. The two central events of the cell cycle are: - replication of DNA during the S phase - chromosome segregation and cell division during the M phase Both these events are controlled by the cyclin-Cdk complexes

40. S phase 1. S-Cdk initiates DNA replication once per cycle 2. Chromosome duplication requires duplication of chromatin structure 3. Cohesins help hold sister chromatids together

41. Control of chromosome duplication

42. Control of the initiation of DNA replication The ORC remains associated with the ori site throughout the cell cycle. In early G1, Cdc6 and Cdt1 (helicase loading proteins) associate with the ORC and the resulting complex allows the assembly of the Mcm ring and the formation of the prereplicative complex. In the S phase, S-Cdk stimulates the assembly of several additional proteins to form the preinitiation complex. Other proteins are recruited to the origin and replication begins. S-Cdk blocks rereplication by triggering the destruction of Cdc6 and the inactivation of the ORC. The cell is able to assemble the pre-RC only after M-Cdk is inactivated and APC/C is activated at the end of the M-phase

43. S-Cdk activity is high during G2 and early mitosis. This prevents rereplication from occurring after the S phase M-Cdk also prevents rereplication from occurring during mitosis by phosphorylating the Cdc6 and ORC proteins With all the control elements preventing rereplication, how does DNA replication take place in the next cell cycle? At the end of mitosis, APC/C activation leads to the inactivation of Cdk activity and the destruction of geminin. Pre-RC components are dephosphorylated and Cdt1 is activated allowing pre-RC assembly to initiate a new round of replication

44. Mitosis • 1. M-Cdk drives entry into mitosis • 2. Dephosphorylation activates M-Cdk at the onset of mitosis • 3. Condensin helps configure duplicated chromosomes for separation • 4. The mitotic spindle is a microtubule-based machine • Centrosome duplication occurs early in the cell cycle • 6. M-Cdk initiates spindle assembly in prophase • 7. The completion of spindle assembly in animal cells requires nuclear envelope breakdown • 8. The APC/C triggers sister-chromatid separation and the completion of mitosis • 9. Unattached chromosomes block sister-chromatid separation: The spindle assembly checkpoint

45. Activation of M-Cdk drives entry into mitosis

46. The APC/C triggers sister-chromatid separation and the completion of mitosis

47. Control of cell division and cell growth

48. Mechanism controlling cell-cycle entry and S-phase initiation in animal cells

49. How DNA damage arrests the cell cycle in G1