The Mammalian Cell
Cycle
Rajat Singhania
03/21/2006
“Let the Truth be Told!!”
Overview of the Cell Cycle
Source: www.els.net
Let’s Start with G1!!
 Fact:
Most of the cells in our body are
NOT replicating at any given point of time.
 Instead, most cells in our body remain in
the G0/G1 phase for their entire lifetime.
 Most of the cells that go through the cell
cycle on a regular basis are the adult stem
cells in the bone marrow, skin, and the gut
- continual cell division occurs to replenish
the constantly dying cells in those organs.
CdK Video
G1 restriction point
pRb: Tumor
Suppressor Protein
retinoblastoma
E2F-DP: Transcribes
genes whose products
are used in DNA
synthesis as well as for
the genes for Cyclin E,
Cyclin A, and Cdk2.
Various growth factors
and hormones activate
Cdk4/6 & Cdk2 which
phosphorylate pRb…
Source: www.eurogene.org
DNA Damage Checkpoint
[p21 is a CDKI which binds Cdk2 and Cdk4/6]
Source: www.eurogene.org
Triggering the S phase
Source: www.med.upenn.org

The newly formed CycA-Cdk2 complexes
help “trigger” pre-replication complexes
assembled on the DNA by causing the
assembly of DNA polymerase, thus
initiating the S-phase.
More CycA-Cdk2 S-phase
Regulation
 CycA-Cdk2
phosphorylates E2F to
inactivate its function as a transcription
factor.
 CycA-Cdk2 also keeps the pRb protein
inactive during the S phase.
 CycA-Cdk2 also phosphorylates Cdc6
(part of the pre-replicative complex) and
thus marks it for degradation – thus
ensuring that re-replication does not occur.
G2/M transition (Cdc2=Cdk1)

During the G2 phase of the cell cycle, inactive
cyclin B1/Cdc2 complexes accumulate in
mammalian cells due to inhibitory
phosphorylation of Cdc2 on Thr-14 and Tyr-15
by the Wee1 and Myt1 kinases.
 CyclinB1/Cdc2 activation requires
dephosphorylation on Thr-14 and Tyr-15 by the
phosphatase Cdc25C, and phosphorylation on
Thr-161 by CAK3,4.
Activation of Cdc25C

The mammalian Plx1 homolog Plk1
phosphorylates Cdc25C.
 Active cyclin B1/Cdc2 activates Cdc25C by
phosphorylating the N-terminal region of
Cdc25C in an autocatalytic loop. Thus, it
takes only a small amount of active cyclin
B1/Cdc2 for its concentration to rapidly
explode.
G2 Checkpoint Activation
 In response to genotoxic stress such as
Ionizing Radiation or Ultra Violet light, the
ATM/ATR signaling pathway is activated. These
two kinases are part of the phosphoinositide-3
kinase (PI-3K) family.
 ATM phosphorylates and activates Chk2; ATR,
Chk1.
 Activated Chk1 and Chk2 phosphorylate
Cdc25C on Ser-216, generating a consensus
binding site for 14-3-3 proteins.
 Cdc25C is then exported out of the nucleus
and is sequestered in the cytoplasm by the 14-33 proteins.
 This leads to G2 cell cycle arrest due to the
inhibition of the ability of Cdc25C to activate
cyclin B1/Cdc2.
 Cdc25C phosphorylation happens at a
Source: www.eurekah.com different site than when it is done by Plk1, as
mentioned in the previous slide.
Mitosis Stage 1: Prophase
 The replicated chromosomes condense to become very thick and dense.
 Microtubules start sprouting from the centrosomes which had also replicated
during the S-phase of the cell cycle.
 The centrosomes start to move apart.
Mitosis Stage 2: Prometaphase
 The nuclear envelope abruptly
breaks down.
 Microtubules start attaching to the
kinetochores of the chromosomes.
 Centrosomes go towards opposite
poles; chromosomes start moving
towards center.
Source: www.sparknotes.com
Mitosis Stage 3: Metaphase
 The chromosomes align
at the metaphase plate.
Source:
http://www.stanford.edu/group/f
anglab/science/research_cycle.
html
 The metaphase
checkpoint monitors the
attachment of the mitotic
spindle to kinetochores and
the tension generated by
mitotic spindle attachment.
In the presence of even a
single unattached
kinetochore, the metaphase
checkpoint halts the
separation of sister
chromatids and thereby
provides additional time for
spindle attachment.
Mitosis Stage 4: Anaphase
 The breakdown of the
cohesin protein complex that
holds the sister chromatids
together is done by the
Anaphase Promoting Complex
(APC).
 The APC works by
degrading a protein that is
binding a proteolytic enzyme
that cleaves the cohesin when
it is no longer held inactive by
the now-degraded protein.
Mitosis Stage 5: Telophase
Mitosis Stage 6: Cytokinesis
 Cleavage occurs by the
contraction of a thin ring of
actin filaments that form the
contractile ring.
 If the ring is not positioned
at the center of the cell, an
asymmetrical division takes
place.
Cell Cycle Video
Importance of Cell Size
 As
the cell grows in size, more and more
ribosomes are produced. So, more and
more cyclins are produced, and so more
and more Cyclin-dependent Kinase
complexes form (e.g. CyclinA-Cdk2 going
into S; CyclinB-Cdk1 going into M)…since
the nucleus size doesn’t increase, these
become more and more concentrated ->
more active->carry out the transition into
the next cell cycle phase.