2. Cell cycle
• Cell cycle checkpoints are control mechanisms
that ensure the fidelity of cell division
in eukaryotic cells
• Consist several checkpoints
– verify whether the processes at each phase of
the cell cycle have been accurately completed
before progression into the next phase.
3. Function of cell cycle
• DNA damage be detected
by sensor mechanisms
• When damage is found checkpoint uses
a signal mechanism to target the cell for
destruction via apoptosis
4.
5.
6. Definition
• The cell cycle, or cell-division cycle, is the
series of events that take place in
a cell leading to
1. doubling of its genome (DNA) in S phase
(synthesis phase) of the cell cycle
2. division and replication that produces two
daughter cell.
10. G0 phase
• The G0 phase (referred to the G zero phase)
or resting phase is a period in the cell cycle in
which cells exist in a quiescent state or
senescent
• Either dormant or apoptosis
• Red blood cell or neurons, become quiescent
when they reach maturity
11. • terminally differentiated cells
• they do not need to divide ever again after
reach maturity
• the cells leave G1 and enter an alternative
state called G0 where they stop dividing
permanently
12.
13. In relation to the cell cycle
• This usually occurs in response to a lack
of growth factor or nutrients
• Or it already terminally differentiated
• Until there is a reason for them to divide
14. Growth factor
• polypeptides that stimulate cell proliferation
and maturation
• usually it is a protein or a steroid hormone
• regulating a variety of cellular processes
• Example
– bone morphogenetic proteins stimulate bone cell
differentiation
– fibroblast growth factors and vascular endothelial
growth factors stimulate angiogenesis.
17. G1 phase
• During G1 phase, the cell grows in size and
synthesizes mRNA and protein
• Preparation for subsequent steps leading
to mitosis.
• G1 phase ends when the cell moves into the S
phase of interphase.
18. Cont. G1 phase
• major control switches for the cell cycle
– G1 cyclins (D cyclins)
– G1 Cdk (Cdk4)- binds with cyclin
• Signal the cell to prepare the chromosome for
replication
19. Cont. G1 phase
• If a cell is signaled to remain undivided,
instead of moving onto the S phase, it will
leave the G1 phase and move into a state of
dormancy
• p27 is a protein that binds to cyclin and cdk –
blocks entry into S phase if
• p53 is a protein that responsible to block the
cell cycle if the DNA is damaged.
22. S phase
• Major event in S-phase is DNA replication
• Create exactly two identical semi-conserved
chromosomes
• Prevents more than one replication from
occurring
26. G2 phase
• Start with cyclin E destroyed
• G2 phase is a period of rapid cell growth
and protein synthesis during which the cell
readies itself for mitosis
• G2 phase is not a necessary part of the cell
cycle
• Some cancer proceed directly from DNA
replication to mitosis.
28. Cont. G2 phase
• M-phase promoting factor into the nucleus
initiates
– assembly of the mitotic spindle
– breakdown of the nuclear envelope
– cessation of all gene transcription
– condensation of the chromosomes
30. Mitosis
• Process separates the chromosomes in its cell
nuclues into two identical sets of
chromosomes, followed immediately by
cytokinesis
• Mitosis is the transferring of the parent
cell's genome into two daughter cells
• Prophase, Metaphase, Anaphase, Telophase
31. Prophase
• Chromatin fibers become tightly coiled,
condensing into discrete chromosomes
• Two sister chromatids, bound together at
the centromere
• Visible at high magnification
through a light microscope
• M-phase promoting factor
32. Prometaphase
• The nuclear membrane disintegrates and
microtubules invade the nuclear space
• Late prometaphase, each chromosome forms
two kinetochores at its centromere, one
attached at each chromatid.
• M-phase promoting factor
33. Metaphase
• Two centrosomes start pulling the
chromosomes through their attached
centromeres towards the two ends of the cell
• Convene along the metaphase plate or
equatorial plane
• This line is called the spindle
equator
• anaphase-promoting complex
34. Anaphase
• Entrance triggered by inactivation of M-phase
promoting factor.
• First, the proteins that bind
sister chromatids together are cleaved,
become separate daughter chromosomes
• Then, the polar
microtubules elongate,
pulling the centrosomes
35. Cont. Anaphase
Anaphase-promoting complex
1. allows the sister chromatids at the metaphase
plate to separate and move to the poles = anaphase
– Cohesin breakdown is caused by
a protease called separase (also known as separin).
– Separase is kept inactive until late metaphase by an
inhibitory chaperone called securin.
– Anaphase begins when the anaphase promoting complex
(APC/C) destroys securin (by tagging it with ubiquitin for
deposit in a proteasome) thus ending its inhibition of
separase and allowing
– separase to break down cohesin.
36. Cont. Anaphase
2. destroys B cyclins. This is also done by
attaching them to ubiquitin which targets
them for destruction by proteasomes.
3. turns on synthesis of G1 cyclins (D) for the
next turn of the cycle.
4. degrades geminin, a protein that has kept the
freshly-synthesized DNA in S phase from
being re-replicated before mitosis.
37. Telophase
• Two sets of daughter chromosome pulled completely
apart
• Reversal of prophase and prometaphase events
• Daughter chromosomes attach at opposite ends of the
cell. A new nuclear membrane, using the membrane
vesicles of the parent cell's old nuclear
membrane, forms around each set of separated
daughter chromosomes
• The nucleoli reappear. Both sets of chromosomes, now
surrounded by new nuclei, begin to "relax" or
decondense back into chromatin
38. Cytokinesis
• Division is also driven by vesicles derived from
the Golgi apparatus
• Each daughter cell has a
complete copy of the
genome of its parent cell.
The end of cytokinesis
marks the end of the
M-phase.
