The Cell Cycle

1. The Cell Cycle

2. Every cell is the product of a cell cycle. The cell cycle comprises two alternating events: Cell division & cell growth

4. The Cell Cycle S phase: Doubling of DNA M phase: halving of DNA during mitosis S phase: Doubling of DNA M phase: halving of DNA during mitosis • So, the cell cycle consists of: • G1 = growth and preparation of the chromosomes for replication • S = synthesis of DNA • G2 = preparation for mitosis • M = mitosis • When a cell is in any phase of the cell cycle other than • mitosis, it is said to be in interphase.

5. Cell division in animals • chromosomes move toward a discrete spindle pole

8. The nuclear envelope remains intact during mitosis, and the spindle forms within the nucleus in contrast with higher eucaryotic cells.

9. The behavior of a temperature-sensitive cdc mutant.

10. In a cdc15 mutant grown at the restrictive temperature, cells complete anaphase but cannot complete the exit from mitosis and cytokinesis.

11. >1 mm in diameter Carries 100,000 times more cytoplasm than an average cell in the human body. 212 cells (4096 cells) within 7 hrs without detectable G1 or G2 phases.

12. 1 division cycle every 30 min.

14. fibroblasts

15. 3H-thymidine incorporation Fluorescent anti-BrdU Abs

16. DNA Replication DNA replication is strictly controlled during the cell cycle.

17. Many molecules are involved in controlling S-phase progression.

18. Stained cells with DNA-binding dye (flow cytometer)

21. Checkpoints: Quality control of the cell cycle The cell has several systems for interrupting the cell cycle if something goes wrong.

23. The passage of a cell through the cell cycle is controlled by proteins in the cytoplasm: • Cyclins • G1-cyclins • G1/S- and S-phase cyclins • M-phase cyclins •Their levels in the cell rise and fall with the stages of the cell cycle.

24. Cyclin-dependent kinases (CDKs) • G1 CDKs • A CDK shared by both G1/S- and S-cyclins • M-phase CDK • Their levels in the cell remain fairly stable, but each must bind the appropriate cyclin (whose levels fluctuate) in order to be activated. • They add phosphate groups to a variety of protein substrates that control processes in the cell cycle.

28. Phosphorylation of a CDK inhibits the protein kinase activity of the enzyme

29. CKI (Cdk inhibitor prot.)

30. Mechanisms of cell cycle control Specific kinase complexes advance the cell through the cell cycle.

31. SCF serves as a ubiquitin ligase with the help of E1 and E2

32. APC (anaphase-promoting complex) serves as ubiquitin ligase for M-cyclin.

33. Only G1 cells are competent to initiate DNA replication. Cells that have completed S phase (G2 phase) are not able to rereplicate their DNA.

34. The initiation of DNA replication once per cell cycle.

35. Cdc25 is stimulated in part by Polo kinase and is further stimulated by active M-Cdk

36. The DNA replication checkpoint. Hydroxyurea blocks DNA synthesis  activates checkpoint mechanism Addition of caffeine makes the checkpoint mechanism fail.

37. Mitosis Structural and regulatory molecules are involved in controlling the initiation of mitosis

38. APC (anaphase-promoting complex)

39. The triggering of sister chromatid separation by the APC.

40. Mechanism whereby proteolysis regulates cell cycle progression

41. The anaphase-promoting complex (APC) and other proteolytic enzymes. • The APC • triggers the events leading to destruction of • the cohesins and thus allowing • the sister chromatids to separate. • degrades the mitotic (M-phase) cyclins.

42. Mad2 protein on unattached kinetochores Any kinetochore that is not properly attached to the spindle sends out a negative signal to the cell-cycle control system, blocking Cdc20-APC activation and sister chromatid separation.

43. The creation of a G1 phase by stable Cdk inhibition after mitosis.

44. The control of G1 progression by Cdk activity in budding yeast.

45. The Rb protein acts as a brake in mammalian G1 cells.

46. Cell size control through control of the cell cycle in yeast.

47. A hypothetical model of how budding yeast cells might coordinate cell growth. As the cell grows, the total # of Cln3 molecules increases in parallel with total cell protein.

48. Cell-cycle progression is blocked by DNA damage and p53: DNA damage checkpoints Mdm2: acts as a ubiquitin ligase

50. An overview of the cell-cycle control system. The activity of each cyclin-Cdk complex is influenced by various inhibitory checkpoint mechanisms, which provide information about the extracellular environment, cell damage, and incomplete cell-cycle events.