1. Fundamentals of Cell Biology Chapter 13: The Birth and Death of Cells
Cell cycle and cancer
4/16/2017
Fundamentals of Cell Biology Chapter 13: The Birth and Death of Cells
Dr. Saeb Aliwaini
Dr. Saeb Aliwaini

2. Cell cycle and cancer
4/16/2017
Cell cycle
How cells make the decision to begin moving through the stages of replication, and why some cells never make this journey
How cells decide to die
Dr. Saeb Aliwaini
Dr. Saeb Aliwaini

3. Coordination of cell division
A multicellular organism needs to coordinate cell division across different tissues & organs
critical for normal growth, development & maintenance
coordinate timing of cell division
coordinate rates of cell division
not all cells can have the same cell cycle

4. Frequency of cell division
Frequency of cell division varies by cell type
embryo
cell cycle < 20 minute
skin cells
divide frequently throughout life
12-24 hours cycle
liver cells
retain ability to divide, but keep it in reserve
divide once every year or two
mature nerve cells & muscle cells
do not divide at all after maturity
permanently in G0 G2 S G1 M
metaphase
prophase
anaphase
telophase
interphase (G1, S, G2 phases)
mitosis (M)
cytokinesis (C) C

5. New cells arise from parental cells that complete the cell cycle
Key Concepts :
Cells divide by following carefully scripted program of molecular events collectively called the cell cycle.
The cell cycle is subdivided into five phases named G1, S, G2, M, and G0. Cells not actively dividing reside in G1 or G0 phase.
Progression through the cell cycle is under the control of proteins that form checkpoints to monitor whether the proper sequence of events is taking place. Cells halt at these checkpoints until they complete the necessary steps to continue.
Dr. Saeb Aliwaini

6. Overview of Cell Cycle Control
Two irreversible points in cell cycle
replication of genetic material
separation of sister chromatids
Checkpoints
process is assessed & possibly halted
There’s no turning back,
now!
centromere
sister chromatids
single-stranded
chromosomes
double-stranded  

7. Checkpoint control system
Checkpoints
cell cycle controlled by STOP & GO chemical signals at critical points
signals indicate if key cellular processes have been completed correctly

8. Checkpoint control system
3 major checkpoints:
G1/S
can DNA synthesis begin?
G2/M
has DNA synthesis been completed correctly?
commitment to mitosis
spindle checkpoint
are all chromosomes attached to spindle?
can sister chromatids separate correctly?

9. G1/S checkpoint G1/S checkpoint is most critical
primary decision point
“restriction point”
if cell receives “GO” signal, it divides
internal signals: cell growth (size), cell nutrition
external signals: “growth factors”
if cell does not receive signal, it exits cycle & switches to G0 phase
non-dividing, working state

10. G0 phase G0 phase non-dividing, differentiated state
most human cells in G0 phase
liver cells
in G0, but can be “called back” to cell cycle by external cues
nerve & muscle cells
highly specialized
arrested in G0 & can never divide

11. Activation of cell division
How do cells know when to divide?
cell communication signals
chemical signals in cytoplasm give cue
signals usually mean proteins
activators
inhibitors
experimental evidence: Can you explain this?

12. “Point of no return”
Dr. Saeb Aliwaini

13. The G2/M checkpoint is the trigger for large-scale rearrangement of cellular architecture
MPF
Early experiments characterizing the activity of Mitosis Promoting Factor.
Dr. Saeb Aliwaini

14. New cells arise from parental cells that complete the cell cycle
The G1/S checkpoint, called the restriction point or start point, is where cells commit to completing cell division.
Proteins called cyclins play an important role in advancing cells through checkpoints.
Dr. Saeb Aliwaini

15. Activation of cyclin-CDK complexes begins in G1 phase
Figure 13.04: Scientists discovered the first cyclins when they noted that high cyclin levels with the onset of mitosis in embryos. Cyclin levels drop sharply after this.
Dr. Saeb Aliwaini

16. Cyclins & Cdks
Interaction of Cdk’s & different cyclins triggers the stages of the cell cycle
There are multiple cyclins, each with a specific role. Cyclins are unstable. Some are triggered for destruction by phosphorylation. Others are inherently unstable and are synthesized discontinuously during the cell cycle.
Oscillations in the activities of cyclin-dependent kinases (CDKs) dictate orderly progression through the cell division cycle. In the simplest case of yeast, a progressive rise in the activity of a single cyclin-CDK complex can initiate DNA synthesis and then mitosis, and the subsequent fall in CDK activity resets the system for the next cell cycle.
In most organisms, however, the cell cycle machinery relies on multiple cyclin-CDKs, whose individual but coordinated activities are each thought to be responsible for just a subset of cell cycle events.

17. “Go-ahead” signals Protein signals that promote cell growth & division
internal signals
“promoting factors”
external signals
“growth factors”
Primary mechanism of control
phosphorylation
kinase enzymes
either activates or inactivates cell signals
We still don’t fully understanding the regulation of the cell cycle. We only have “snapshots” of what happens in specific cases.

18. Cell cycle signals Cell cycle controls cyclins Cdk’s
regulatory proteins
levels cycle in the cell
Cdk’s
cyclin-dependent kinases
phosphorylates cellular proteins
activates or inactivates proteins
Cdk-cyclin complex
triggers passage through different stages of cell cycle
inactivated Cdk
activated Cdk

19. Control by cyclin/CDK complexes
Figure 13.05: Distinct cyclin-cdk complexes control progression through cell cycle checkpoints.
Dr. Saeb Aliwaini

20. The cell cycle is divided into five phases
“Resting” cells reside in G0 or G1 phase
Several checkpoints define critical decision-making events in the cell cycle
Dr. Saeb Aliwaini

21. Figure 13. 02: Checkpoints control progression through the cell cycle
Figure 13.02: Checkpoints control progression through the cell cycle. Some of the major checkpoints are shown.
Dr. Saeb Aliwaini

22. To start
You need signals >>>>>
Dr. Saeb Aliwaini

23. Figure 13.07: A simplified MAP kinase signaling pathway.
Cell cycle step 1: Signal transduction initiates cell cycle progression.
Figure 13.06: The family of mitogen activated protein kinases (MAPKs) and their upstream regulatory proteins.
Figure 13.07: A simplified MAP kinase signaling pathway.
Dr. Saeb Aliwaini

24. Dr. Saeb Aliwaini

25. Cell cycle step 2: Changes in gene expression are required for progression through the restriction point
progression through the restriction point in mammalian cells requires activation of at least two cyclin/CDK complexes: cyclin D1/CDK4 (or CDK6) and cyclin E/CDK2
expression of most CDKs does not vary much throughout the cycle, but without their corresponding cyclins, they are not functional
Dr. Saeb Aliwaini

26. Figure 13.08: Summary of the cyclin/cdk activation-inactivation cycle.
Cell cycle step 3: Pro- and anti-growth signaling networks converge at the G1/S cyclin-CDK complexes
Phosphorylation
Binding by inhibitory kinases
Subcellular location
Protein degradation
Figure 13.08: Summary of the cyclin/cdk activation-inactivation cycle.
Dr. Saeb Aliwaini

27. The key function of G1-Cdk complexes in animal cells is to activate a group of gene regulatory factors called the E2F proteins, which bind to specific DNA sequences in the promoters of a wide variety of genes that encode proteins required for S-phase entry, including G1/S-cyclins, S-cyclins, and proteins involved in DNA synthesis and chromosome duplication.
In the absence of mitogenic stimulation, E2F-dependent gene expressionis inhibited by an inter- action between E2F and members of the retinoblastoma protein (Rb) family.
Dr. Saeb Aliwaini

28. Cell cycle step 4: Active cyclin/CDKs phosphorylate pocket proteins, which activate E2Fs
Figure 13.09: The transcription factor E2F is inactivated by Rb binding.
Figure 13.10: Examples of positive (green) and negative (red) feedback loops controlling E2F function.
Dr. Saeb Aliwaini

29. Dr. Saeb Aliwaini

30. How E2Fs enhance expression of some genes while suppressing expression of others remains unclear
Figure 13.11: A model of how E2F transcription factors can suppress or activate gene transcription.
Dr. Saeb Aliwaini

31. Cell cycle step 5: The DNA replication machinery is activated by protein kinases
A key player is a large, multiprotein complex called the origin recognition complex (oRc), which binds to replication origins throughout the cell cycle. In late mitosis and early G1, the proteins cdc6 and other proteins bind to the ORC at origins and help load a group of six related proteins called the Mcm pro-
teins. The resulting large complex is the pre-RC, and the origin is now licensed
for replication.
Figure 13.12: Assembly of the prereplication complex.
Dr. Saeb Aliwaini

32. DNA replication occurs in S phase
3 key steps
Figure 13.13: Activation of the replication complex.
Dr. Saeb Aliwaini

33. Cell cycle step 6: DNA integrity is ensured by the G1/S, S/G2, and G2/M checkpoints
Figure 13.14: A current model for DNA repair.
Figure 13.15: Growth arrest induced by Chk1 and Chk2.
Dr. Saeb Aliwaini

34. Cell cycle step 7: Cells increase in size during G2 phase
Figure 13.17: Wee1 mutation affects cell size. Compared to normal ("wild-type," WT) yeast, Wee1 mutants grow to half normal size before dividing.
Dr. Saeb Aliwaini

35. Cell cycle step 8: Cyclin B/CDK1 activation drives cells through the G2/M checkpoint
Figure 13.18: A model for cell size control of cell growth in yeast in G2.
Figure 13.19: A model for how adhesion to ECM promotes cell growth in mammalian cells.
Dr. Saeb Aliwaini

36. Figure 13.20: Phosphorylation of Cdk1 primes it for activation but also keeps it in an inactive state.
Dr. Saeb Aliwaini

37. Figure 13.21: A model for anaphase promotion by APC/C.
Cell cycle step 9: Chromosome alignment is ensured by the mitotic spindle assembly checkpoint
Figure 13.21: A model for anaphase promotion by APC/C.
Dr. Saeb Aliwaini

38. Cell cycle step 10: Onset of cytokinesis is timed to begin only after mitosis is complete
Cytokinesis requires the contraction of the contractile ring that lies just beneath the plasma membrane, perpendicular to the long axis of the mitotic spindle.
It is important that the myosin motors in the ring not activate until mitosis, including reconstitution of the nuclear membrane, is complete.
Dr. Saeb Aliwaini

39. Multicellular organisms contain a cell self-destruct program that keeps them healthy
Key Concepts:
Cells die either by traumatic injury (necrosis) or by a self-destruct program called apoptosis.
Apoptosis begins through at least two molecular mechanisms, called intrinsic and extrinsic pathways.
The family of proteins called caspases includes proteases that promote the degradation of organelles and cytosolic proteins during apoptosis.
Dr. Saeb Aliwaini

40. Cells die in 2 different ways: necrosis and apoptosis
Figure 13.22: Cellular damage can result in necrosis, as organelles swell and the plasma membrane ruptures.
Dr. Saeb Aliwaini

41. Apoptosis is a property of all animal cells and some plant cells
Dr. Saeb Aliwaini

42. Apotosis is voluntary
Figure 13.23: Sections of the interdigital web show cell death (dark-staining nuclei). This cell death has the characteristics of apoptosis.
Dr. Saeb Aliwaini

43. Apoptosis is induced via at least 2 different pathways
Figure 13.24: Ligation of death receptors causes the recruitment of the adaptor protein FADD to the intracellular region of the death receptor.
Figure 13.25: E2F1 lies at the heart of the growth-versus-death decision making system.
Dr. Saeb Aliwaini

44. Targets of pro- and anti-apoptotic transcription factors are members of bcl-2 family
Figure 13.26: The Bcl-2 family proteins share up to four Bcl-2 homology domains (BH) and can be antiapoptotic or proapoptotic.
Figure 13.27: The Bcl-2 family of proteins compete with members of the antiapoptotoic group to access the apoptotic group in an elaborate hierarchy.
Dr. Saeb Aliwaini

45. Mitochondrial Outer Membrane Permeabilization (MOMP)
Figure 13.28: Signals for the induction of apoptosis trigger changes in the Bcl-2 family proteins, which function to inhibit or promote apoptosis. Activation of caspase 9 by the apoptososme. Insert, three views of apoptosome structure as determined by electron microscopy.
Dr. Saeb Aliwaini

46. Apoptosis triggers the activation of special proteases: the caspases
Figure 13.29: Different types of vertebrate caspases are shown schematically.
Dr. Saeb Aliwaini

47. The final changes
Stereotypical morphological changes take place during apoptosis
karyorrhexis
Apoptotic cells are cleared by phagocytosis
Dr. Saeb Aliwaini

48. The APC/C catalyzesthe ubiquitylation and destruction of two major pro- teins. The first is securln,which normally protects the protein linkages that hold sister chromatid pairs together in early mitosis. Destruction of securin at the metaphase-to-anaphasetransition activatesa proteasethat separatesthe sisters and unleashes anaphase.The S- and M-cyclins are the second major targets of the APC/c. Destroying these cyclins inactivatesmost cdks in the cell
Dr. Saeb Aliwaini