Cell cycle checkpoints play an important role in ensuring the proper progression of the cell cycle and preventing unregulated cell growth. Checkpoints at the G1/S and G2/M transitions verify that the cell is ready to progress prior to DNA replication or mitosis. Loss of checkpoint function can allow mutations to accumulate, potentially leading to carcinogenesis. Cancer arises through genetic changes that disrupt the normal balance between cell proliferation, differentiation, and apoptosis. This includes mutations in oncogenes and tumor suppressor genes such as p53, which normally function to regulate the cell cycle and cell growth.

1. Cell Cycle Checkpoints and
Carcinogenesis
Hussein Sabit, Ph.D
College of Biotechnology, Misr University for
Science & Technology.
hussein.sabit@must.edu.eg
01069737737

2. About 100 trillion cell in
your body

3. Thinking about the cell cycle?
• Cell cycle functions according
to some sort of internal clock,
which would determine how
long a phase should take.
• However, the cell cycle is
depicted like the falling
dominoes.

4. DNA
DNA
DNA
DNA
DNA
DNA
DNA
DNA
DNA
G1
Cell growth
S
DNA replication
G2
Cell growth
preparation for
division
Mitotic Phase (M)
Interphase
Interphase
Interphase
Cell cycle

5. Interphase
Represents 90% of the cell cycle, it
consists of:
• Go phase.
• G1 phase.
• S phase.
• G2 phase.

6. Mitotic Cell Division
 Karyokinesis (mitosis):
consists of prophase, metaphase, anaphase and
telophase.
Cytokinesis (cytoplasmic division):
The cytoplasm and its contents are divided. In animal
cells a cleavage furrow forms that essentially pinches
the cell in two.

7. Prophase
 In this phase, chromatin condenses into
chromosome.
 Prophase accounts for approximately 3% of
the cell cycle's duration (35 min.)

8. Metaphase
 Chromosomes are aligned in the middle of the cell
before being separated into each of the two daughter
cells.
 Metaphase accounts for approximately 4% of the cell
cycle's duration (45 min.).
Early metaphase Late metaphase

9. Anaphase
Chromosomes separate
in an eukaryotic cell.
Each chromatid moves
to opposite poles of the
cell.
Anaphase accounts for
approximately 1% of the
cell cycle's duration (11
min.).

10. Cytokinesis
 It is the division
of the cytoplasm.
 Cytokinesis must
be controlled to
ensure that it
occurs only after
sister anaphase
separation.

11. The final result of mitotic division is
two identical daughter cells.

12. Cell cycle

13. Timing of the cell cycle
• G1 phase takes: 0 to 12 hours.
• S phase takes: 12 to 18 hours.
• G2 phase takes: 18 to 22 hours.
• Mitosis takes: 22 to 24 hours.

14.  This is a .. control mechanisms that
ensure the fidelity of cell division in
eukaryotic cells.
 These checkpoints verify whether the
processes at each phase of the cell cycle
have been accurately completed or not.
 Multiple checkpoints have been
identified.
Cell cycle checkpoints

15. 1
23

16. G1 Checkpoint
 The first checkpoint is
located at the end of the
cell cycle's G1 phase, just
before entry into S phase.
 It makes the key decision
of whether the cell should
divide, delay division, or
enter a resting stage (G0).

17. G1
G0
G1 checkpoint
(a) Cell receives a go-ahead
signal
G1
(b) Cell does not receive a
go-ahead signal

18. G2 Checkpoint
• This checkpoint triggers
the start of the Mitosis.
• In order for this
checkpoint to be passed,
the cell has to check a
number of factors to
ensure the cell is ready for
mitosis

19. Metaphase Checkpoint
• The mitotic spindle
checkpoint occurs at the
point in metaphase where
the chromosomes have
aligned at the mitotic plate
and be under bipolar
tension.
• The tension created by
this bipolar attachment is
what initiates the
anaphase entry.

20. Cont.
• Securin is a protein whose
function is to inhibit
Separase, the protein
composite responsible for the
separation of sister
chromatids.
• Once this inhibitory protein is
degraded via ubiquitination
and subsequent proteolysis,
Separase then causes sister
chromatid separation.
• After the cell has split into its
two daughter cells, the cell
enters G1 again.

21. Carcinogenesis

22. Cancer terms
Benign
▫ Harmless and limited in scope. In cancer, the opposite of
malignant.
Malignant
▫ A cancer whose scope extends beyond the initial tumor.
Characterized by anaplasia, invasion and metastasis.
Anaplasia
▫ Loss of differentiation in cells.
Invasion
▫ Movement of cells from a tumor into immediately surrounding
tissue.
Metastasis
▫ Movement of cancer cells into tissues beyond the original tumor.
Usually involves transit through the blood stream.

23. Angiogenesis
▫ Growth of new blood vessels. Important in cancer growth.
Oncogene
▫ A gene whose product causes uncontrolled cell growth.
Proto-oncogene
▫ A normal gene whose normal product does not cause cancer,
but in which a mutation will cause cancer.
Tumor-suppressor gene
▫ A gene whose product impedes uncontrolled cell growth.
Apoptosis
▫ Programmed cell death.
Cancer terms

24. What is cancer?
• Characterized by uncontrolled cell proliferation
• Arises from:
1. Irreversible genetic damage to cell’s DNA.
2. Block in normal process of differentiation.
3. Block in apoptosis.
• Several proteins contribute to oncogenesis:
▫ p53, p16, and β-catenin.

25. Standard Dogma
 Proto-oncogenes (e.g. Ras – Rb genes),
 Tumor suppressor genes (e.g. p53).
Modified Dogma
 Mutation in a DNA repair gene leads to the
accumulation of unrepaired mutations.
Early-Instability Theory
 Master genes required for adequate cell
reproduction are disabled, resulting in
aneuploidy.
Theories of cancer genesis

26. Proliferation
Differentiation
Death
Transit
Proliferating
Exiting
Renewing
Cellular equilibrium

27. Proliferation Differentiation Death
Cancer: disruption of cellular
equilibrium

28. 28
•Benign tumors
generally do not
spread by invasion
or metastasis.
• Malignant tumors
are capable of
spreading by
invasion and
metastasis.
Malignant versus Benign Tumors

29. Six Hallmarks of Cancer cell
1- Self-sufficient growth signals:
 Constitutively activated growth factor
signalling.
2- Resistance to anti-growth signals:
 Inactivated cell cycle checkpoint.
3- Immortality:
 Inactivated cell death pathway.

30. Cont.
4- Resistance to cell death:
 Activated anti- cell death signalling.
5- Sustained angiogenesis:
 Activated Vascular Endothelial Growth
Factor (VEGF) signalling.
6- Invasion and metastasis:
 Loss of cell-to-cell interactions, etc.

32. metastasis

33. What controls cancer are:
• Tumor suppressor gens
• Oncogenes
• Apoptosis

34. P53 Tumor Suppressor Gene
(as an example)
• The most frequently mutated gene in
human cancers.
• Critical roles:
▫ Prevents mutations and repairs DNA.
▫ Cell cycle arrest in G1.
• Also responsible for the activation of
several proteins involved in apoptosis.
▫ Mutant p53 has lost ability to activate
apoptosis.

35. p53 Tumor Suppressor Gene
• Mutated (switched OFF) in more than
50% of all cancers.
• Activated p53 levels rise rapidly if DNA
is damaged during the cell cycle.

36. Proto-oncogenes
 Normal growth factor genes that
become oncogenes (cancer-causing)
when mutated.
 Stimulates cell growth.
 If switched “ON” can cause cancer.
 Example: RAS, Cyclin D1 and Cyclin
E.

37. Oncogens
Oncogenes are mutated forms of
cellular proto-oncogenes.
Proto-oncogenes code for cellular
proteins which regulate normal cell
growth and differentiation.

38.  Class I: Growth Factors
 Class II: Receptors for Growth Factors
and Hormones
 Class III: Intracellular Signal Transducers
 Class IV: Nuclear Transcription Factors
 Class V: Cell-Cycle Control Proteins
Five types of proteins encoded
by proto-oncogenes

39. Cancer is not easy to happen !!
• In order for an oncogene to cause
cancer, only one copy must be
mutated.
• A tumor suppressor gene must be
completely absent to allow
cancerous cell division to occur.

40. Genes controlling cancer
Normal
Cancer
Proto-oncogenes Cell growth
and
proliferationTumor suppressor genes
+
-
Mutated or “activated”
oncogenes Malignant
transformation
Loss or mutation of
Tumor suppressor genes
++
NormalCancer

41. P53 pathway
It has many anticancer
mechanisms, and plays a
role in apoptosis,
genomic stability.
In its anti-cancer role, p53
works through several
mechanisms:
• It can activate DNA repair proteins
when DNA has sustained damage.
• It can induce growth arrest by
holding the cell cycle at the G1/S.
• It can initiate apoptosis if DNA
damage proves to be irreparable.

42. P53 pathway
It has many anticancer
mechanisms, and plays a
role in apoptosis,
genomic stability.
In its anti-cancer role, p53
works through several
mechanisms:
• It can activate DNA repair proteins
when DNA has sustained damage.
• It can induce growth arrest by
holding the cell cycle at the G1/S.
• It can initiate apoptosis if DNA
damage proves to be irreparable.

43. Apoptosis
(programmed cellular death)
“Life cannot exist without cellular death”
It implies that:

44. Apoptosis and Mitosis.. The balance
“Life cannot exist without cellular proliferation”
It implies that:

45. Apoptosis
• Apoptosis is the default disposal system to
delete cells that have accumulated
unrepairable DNA damage to protect genomic
integrity.
• This process takes about 30 - 60 min in
average.
• Programmed cell death, cell suicide.
• Regulated by Bcl2 and BAX
▫ BAX homodimer promotes apoptosis,
▫ Bcl2 homodimer blocks apoptosis,
▫ Some cancer cells overproduce Bcl2.

46. Where it happens in Human body?

47. Epithelial cells must die to
allow fusion of palate
Mammary epithelium
cells die when deprived
of hormones at end of
lactation
Up to 80% neurons die
in some ganglia
Over 95% of
immature T cells
die in thymus
Cells of
interdigital
webbing die
Cells of müllerian
duct die in males
Prostate cells die when
deprived of hormone

48. Dysregulation of apoptotic signaling can play a role in
various Human diseases.
Insufficient apoptosis leads to:
• Cancer,
• Autoimmunity (failure to eliminate autoreactive
lymphocytes), or
• Persistent infections (failure to eradicate infected cells).
Excessive apoptosis leads to:
• Neurodegeneration (Alzheimers’, Parkinson’s,
Huntington’s disease),
• Autoimmunity,
• AIDS (depletion of T lymphocytes), and
• Ischaemia (stroke, myocardial infarction).
Apoptosis and Human disease

49. Don’t be afraid

51. 51
 90-95% of all
cancers are
sporadic.
 5-10% are
inherited.
Cancer genetics