EUKARYOTIC CELL CYCLE :
-Different Phases and Molecular Events
-Control mechanisms: Role of
(A) Cyclins and cyclin-dependent kinases
(B) Retinoblastoma and E2F proteins
-Cytokinesis and cell plate formation
PRESENTED BY
PRASHANT VC
DEPT OF ZOOLOGY
GUK

INTRODUCTION
 Cell cycle is a complex set of cytoplasmic &
nuclear processes performed to accomplish
some targets-
 Production of a pair of genetically identical
daughter cells
 Faithful replication of DNA
 Segregation of replicated chromosomes in these
daughter cells

PHASES OF CELL CYCLE
 Cell cycle basically comprises of two phases-
 Interphase
 Mitosis
 Interphase is again subdivided into
1. G1 phase
2. S phase
3. G2 phase
 Mitosis is divided into
 Prophase, Metaphase,Anaphase,Telophase,
Cytokynesis
*Cells that have temporarily or reversibly stopped dividing are
said to have entered a state of quiescence called G0 phase

Phases of Interphase
 G1 phase is also known as the growth phase. cell
continues to grow, synthesis of enzymes mainly
required for S phase.
 S phase also known as the synthesis phase.
DNA replication occurs i.e. the DNA of the cell
has effectively doubled though the ploidy
remains same.
 Rate of RNA transcription & protein synthesis is
very low.
 G2 phase biosynthesis occurs mainly the
production of microtubules for Mitosis.

Fig: Various phases of cell division cycle
G1
S
G2
M
Propha
ase
Meta-
phase
Ana-
phase
Telo-
phase
Growth &
Prepration
For
mitosis
Growth &
Normal
metabolic
roles
DNA
Replication
MITOTIC PHASE
SECOND
GROWTH
PHASE
FIRST
GROWTH
PHASE
SYNTHESIS PHASE

Outer ring:
I = Interphase
M = Mitosis
Inner ring : M = Mitosis, G1 = Gap 1, G2 = Gap 2,
S = Synthesis ,
Not in ring: G0 = Gap 0/Resting
Start point
Nutrients
Mating Factors
Cell Size
G0
2 Daughter cell
Fig-Phases of the Cell cycle
Outer ring:
I = Interphase
M = Mitosis
Inner ring : M = Mitosis, G1 = Gap 1, G2 = Gap 2,
S = Synthesis ,
Not in ring: G0 = Gap 0/Resting
Start point
Nutrients
Mating Factors
Cell Size
G0
2 Daughter cell
Outer ring:
I = Interphase
M = Mitosis
Inner ring : M = Mitosis, G1 = Gap 1, G2 = Gap 2,
S = Synthesis ,
Not in ring: G0 = Gap 0/Resting
Start point
Nutrients
Mating Factors
Cell Size
G0
2 Daughter cell
Outer ring:
I = Interphase
M = Mitosis
Inner ring : M = Mitosis, G1 = Gap 1, G2 = Gap 2,
S = Synthesis ,
Not in ring: G0 = Gap 0/Resting
Start point
Nutrients
Mating Factors
Cell Size
G0
2 Daughter cell

FIG :-

Phases of Mitosis
 Prophase: centrosome duplicates to form the two
poles of the mitotic spindle. Each centrosome
nucleates a radial array of microtubules, called an
aster. The two asters, which initially lie side by side and
close to the nuclear envelope, move apart.
 Metaphase: the nuclear envelope breaks down,
enabling the spindle microtubules to interact with the
chromosomes. The chromosomes are arranged in an
equatorial plane or metaphase plate.
 Anaphase: the sister chromatids are cleaved & begin
to move towards the opposite ends the poles with
microtubules gradually shortened, which are
elongated at the end of anaphase.

 Telophase: nuclear envelope reappears
around each group of daughter
chromosomes. The nucleoli which had
dissapeared in prophase begin to re-appear.
Both sets of chromosomes now surrounded
by new nuclei.
 Cytokinesis: is basically the division of the
cytoplasm. Each daughter cell has a complete
copy of the genome of the parent cell.
Phases of Mitosis

Resting (G0 phase)
 The term "post-mitotic" is sometimes used to refer
to both quiescent and senescent cells.
Nonproliferative cells in multicellular eukaryotes
generally enter the quiescent G0 state from G1 and
may remain quiescent for long periods of time,
possibly indefinitely (as is often the case
for neurons). This is very common for cells that are
fully differentiated. Cellular senescence is a state
that occurs in response to DNA damage or
degradation that would make a cell's progeny
nonviable; it is often a biochemical alternative to the
self-destruction of such a damaged cell by apoptosis.

Fig: Various phases of cell division cycle

Control mechanisms/Regulation of cell cycle
 Regulation of the cell cycle involves processes
crucial to the survival of a cell, including the
detection and repair of genetic damage as well
as the prevention of uncontrolled cell division.
The molecular events that control the cell cycle
are ordered and directional; that is, each
process occurs in a sequential fashion and it is
impossible to "reverse" the cycle.

 On receiving intracellular & extra cellular signals like
growth factors the cell progresses to the next phase.
 Each & every cell has to pass through the START Point or
the decision point only after the fulfillment of certain
criteria like sufficient size, sufficient nutrients & sufficient
mating factors.
 After fulfilling all the requirements the cell directly
progresses to the S phase of the cell cycle.

Cell cycle checkpoints
 Checkpoints are the control mechanisms that
ensure the fidelity of cell division in
eukaryotic cells.
 They are categorized as:
 G1 checkpoint
 S checkpoint
 G2 checkpoint
 Metaphase checkpoint

 G1 check point
 It allows the repair of the damage to take place
before the cell enter S phase where the damaged
DNA could be replicate.
 S-Phasecheckpoint
 Provides monitoring of the DNA to check that
damaged DNA is repaired before replication.
 It also ensure the repairing of error occurred during
replication e.g.. incorporation of incorrect base or
incomplete replication of segment of DNA.

 G2 checkpoint
 It prevents the migration into mitosis until the DNA
replication S-phase completed. If it only senses only
DNA damage then cell cycles is stopped & continue
only often the damaged DNA repair.
 Metaphase checkpoint
 It ensure the alignment of chromosomes on the
mitotic spindle thus ensuring that a complete set of
chromosomes is distributed accurately to the
daughter.

G1 G1
PHASE
S-PHASE G2 PHASE M PHASE
G1 PHASE
CHECKPOINT
S PHASE
CHECKPOINT
G2 PHASE
CHECKPOINT
METAPHASE
CHECKPOINT
Fig: Cell cycle checkpoints

Role of cyclins & cdks
 Two key classes of regulatory
molecules, cyclins and cyclin-dependent
kinases (CDKs), determine a cell's progress
through the cell cycle

Role of cyclins & cdks
 Cyclins form the regulatory subunits and CDKs the
catalytic subunits of an activated cyclin-cdks
complex (heterodimer).
 CDKs are inactive in the absence of a partner cyclin.
When activated by a bound cyclin, CDKs perform a
common biochemical reaction called
phosphorylation that activates or inactivates target
proteins leading entry into the next phase of the cell
cycle.
 Different cyclin-CDK combinations determine the
downstream proteins targeted.
 CDKs are constitutively expressed in cells whereas
cyclins are synthesised at specific stages of the cell
cycle, in response to various molecular signals.

CYCLIN AND CDK
[Animal cells have atleast 10 different cyclins
(A,B and so forth) and atleast 8 cdks(cdk1
through cdk8), which act in various combinations
at specific points in the cell cycle]

Three classes of cyclins in Eukaryotic
cells
 G1/S cyclins : bind cdks at the end of G1 and
commit the cell to DNA replication
 S-cyclins : Bind cdks during S phase & are
required for the initiation of DNA replication
 M-cyclins : promote the events of mitosis

Families of cyclin and cyclin-dependent kinases
(combination or partner of cyclins and cdks)

General mechanism of cyclin-CDK interaction
 Upon receiving a pro-mitotic extracellular signal,
G1 cyclin-CDK complexes become active to prepare
the cell for S phase, promoting the expression
of transcription factors that in turn promote the
expression of S cyclins and of enzymes required
for DNA replication. The G1cyclin-CDK complexes
also promote the degradation of molecules that
function as S phase inhibitors by targeting them
for ubiquitination. Once a protein has been
ubiquitinated, it is targeted for proteolytic
degradation by the proteasome.

 Active S cyclin-CDK complexes phosphorylate
proteins that make up the pre-replication
complexes assembled during G1 phase on
DNAreplication origins. The phosphorylation serves
two purposes: to activate each already-assembled
pre-replication complex, and to prevent new
complexes from forming. This ensures that every
portion of the cell's genome will be replicated once
and only once. The reason for prevention of gaps in
replication is fairly clear, because daughter cells that
are missing all or part of crucial genes will die.
However, for reasons related to gene copy
number effects, possession of extra copies of certain
genes is also deleterious to the daughter cells.

 Mitotic cyclin-CDK complexes, which are
synthesized but inactivated during S and G2 phases,
promote the initiation of mitosis by stimulating
downstream proteins involved in chromosome
condensation and mitotic spindle assembly. A critical
complex activated during this process is a ubiquitin
ligase known as the anaphase-promoting
complex (APC), which promotes degradation of
structural proteins associated with the
chromosomal kinetochore. APC also targets the
mitotic cyclins for degradation, ensuring that
telophase and cytokinesis can proceed.

Regulation of CDK by phosphorylation and proteolysis
DBRP- destruction box recognizing protein; U- ubiquitin
FIG :-

Retinoblastoma and E2F proteins(Specific action
of cyclin-CDK complexes )
 Cyclin D is the first cyclin produced in the cell cycle, in
response to extracellular signals (e.g. growth factors).
 Cyclin D binds to existingCDK4, forming the active cyclin
D-CDK4 complex. Cyclin D-CDK4 complex in turn
phosphorylates the retinoblastoma susceptibility protein
(pRb).
 The hyperphosphorylated pRb dissociates from the E2F-
pRb complex (which was bound to the E2F responsive
genes, effectively "blocking" them from transcription),
activating E2F.
 Activation of E2F results in transcription of various genes
like cyclin E, cyclin A, DNA polymerase, thymidine
kinase, etc.

 Cyclin E thus produced binds to CDK2,
forming the cyclin E-CDK2 complex, which
pushes the cell from G1 to S phase (G1/S
transition).
(When a DNA damage is detected
retinoblastoma protein pRB – arrest the cell
cycle in G1)

Regulation of E2F by pRB

Regulation of cell cycle transition
from G1 to S phase by pRb
Rb Rb
P
P16
Cell Cycle Blocked
Rb inactive
Cdk4/cyclin D
G1
M G2
S
S
G1
M G2
Cell Cycle Proceeds
Rb active

FIG:- Regulation of passage from G1 to S by phosphorylation of pRB
Source:Lehninger principles of Biochemistry

Cyclin D + cdk4
Cyclin D-cdk4 complex
E2F+pRb-P
Dissociation of pRb
(pRb-P=Inactive pRb)
Activation of E2F
Transcription of genes (cyclin E, cyclin B etc.)
Cyclin E + cdk2
Cyclin E-cdk2 complex
G1toS transition
CyclinB+cdk1
Cyclin B-cdk1 complex
G2 to M transition
CyclinA+cdk1
Cyclin A-cdk 1 complex
S to G2 transition
Fig: Specific action of cyclin-cdk interaction
Phosphorylation of pRb

CYTOKINESIS AND CELL PLATE FORMATION
This is the last stage of the cell cycle & it
completes the process of cell division.
It is process in which cytoplasm divides to two
daughter cells. Organelles are distributed into
each of the new cells
Initiates during late telophase or even in late
anaphase, completes as the next interphase
begins.

CYTOKINESISAND CELL PLATE FORMATION
 In animal cells, cytokinesis occurs by a process known
as cleavage, forming a cleavage furrow and by the
action of the contractile ring.
 Actin and myosin II in the contractile ring generate the
force for cytokinesis.
 In plant cells, a cell plate forms during cytokinesis

Mitotic spindle determines the plane of cytoplasmic cleavage

Actin and myosin II in the contractile ring generate the
force for cytokinesis.

Contractile ring
Made up of non-muscle myosin II and actin
filaments assemble equitorially at cell cortex.
 Myosin uses ATP energy which moves along
actin filament constricting the cell membrane to
form cleavage furrow.
 Ingression continues till midbody structure is
formed abscission physically cleaves in to two.
Contractile ring assembly is dedicated by mitotic
spindle.

Contractile Ring of
Actin and Myosin

 This ring, shown at the
bottom, constricts to divide a
proliferating cell into two. It
is composed of antiparallel
actin filaments, cross-linked
by myosin molecules.
 The process of filament
formation is shown from the
top. First, actin monomers
assemble into trimers
('nucleation'); trimers act as
a seed to which further
monomers are added
 Arp2/3 complex and the
Cdc12 and Cdc3 proteins are
needed for nucleation or
polymerization. The
filaments are then organized
into the contractile ring.

The double ring model of cytokinesis positioning

Local activation of
RhoA triggers the
assembly and
contraction of
contractile ring :
> RhoA,a small GTPase,is
activated at the cell cortex
at future division site.
> activated by RhoGEF
found at the cell cortex by
binding GTP.

The microtubules of the mitotic spindle determine the
plane of animal cell division.
Membrane enclosed organelles must be distributed to the
daughter cells during cytokinesis.

Plant cell Cytokinesis
Due to presence of cell wall plant cell cytokinesis
differ from animal cell.
Formation of cell plate occurs in the centre and
grows outwards to meet the lateral walls.
Phragmoplast, array of microtuble that guides
and support cell plate formation.
Orientation of cell plate is determined by
prophase band.

Cytokinesis in Plant cells

Plantcellcytokinesis

REFERENCES
 Bruce Albert, Alexender Johnson, Julian lewis, Martin raff, Keith
roberts, Peter walter (2002) Molecular biology of the cell fifth edition
published by Garland science,Taylor and Francis group.
 Gerald karp (1996,1999)Cell and Molecular BiologyConcepts and
Experiment Second Edition published by John wiley and Sonc.Inc.
 Harvey Lodish,Arnold Berk, Paul Matsurdaria,Chris A.Kaiser, Monty
Krieger, Matthew, P.Scott, S.Lawrence,Zipursky, James Darnell
(1986,1990,1995,2000,2004) Published byW.W. Freeman and
Company.
 www.google . Com
 http://cytokinesis.wikipedia.encyclopedia

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