Cell Cycle

by Assist. Prof. Görke Gürel Peközer

Yıldız Technical University

Biomedical Engineering Department
Spring 2020
 Last week on BME 1532
 Cell Signaling
 Endocrine
 Paracrine
 Neuronal signaling through synapses via neurotransmitters
 Contact dependent
 Sensing of the signals by the cells through receptors
 Cell surface receptors
 Intracellular receptors: Steroid hormones or gases
 Intracellular signal transduction via signaling molecules
 Activation of signaling molecules via phosphorylation activity of
kinase enzymes
 Molecular cross-talk between different pathways and common
molecules
• A cell reproduces by carrying out an orderly sequence of events in
which it duplicates its contents and then divides in two.
• This cycle of duplication and division, known as the cell cycle, is
the essential mechanism by which all living things reproduce.

 The most basic function of the cell

cycle is to duplicate accurately the vast
amount of DNA in the chromosomes
and then to segregate the DNA into
genetically identical daughter cells
such that each cell receives a complete
copy of the entire genome.

• In most cases, a cell also duplicates its other macromolecules and

organelles and doubles in size before it divides; otherwise, each
time a cell split it would get smaller and smaller. Thus, to maintain
their size, dividing cells coordinate their growth with their division.
 Cell Cycle Phases
 The eukaryotic cell cycle includes 4
phases: G1, S, G2, M.
 The nucleus divides in a process
called mitosis, and the cell later
splits in two in a process called
cytokinesis.
 These two processes together
constitute the M phase of the cycle.
 In a typical mammalian cell, the
whole of M phase takes about an
hour, which is only a small fraction
of the total cell-cycle time.
 The period between one M phase
and the next is called interphase
which consist of G1, S and G2
phases.
• Interphase, is a very busy time for a proliferating cell, and it
encompasses the remaining three phases of the cell cycle.
• During S phase (S = synthesis), the cell replicates its DNA.
• S phase is flanked by two “gap” phases—called G1 and G2—during
which the cell continues to grow.

 During these gap phases, the cell

monitors both its internal state and
external environment.
 This monitoring ensures that
conditions are suitable for
reproduction and that preparations
are complete before the cell commits
to the major upheavals of S phase
(which follows G1) and mitosis
(following G2).

• At particular points in G1 and G2, the cell decides whether to proceed

to the next phase or pause to allow more time to prepare.
 During all of interphase, a cell generally continues to
transcribe genes, synthesize proteins, and grow in
mass.
 Together with S phase, G1 and G2 provide the time
needed for the cell to grow and duplicate its
cytoplasmic organelles.
 Cell Cycle Control
 To ensure that they replicate all their DNA and organelles,
and divide in an orderly manner, eukaryotic cells possess a
complex network of regulatory proteins known as the cell-
cycle control system.
 This system guarantees that the events of the cell cycle—
DNA replication, mitosis, and so on—occur in a set
sequence and that each process has been completed before
the next one begins.
 To accomplish this, the control system is itself regulated at
certain critical points of the cycle by feedback from the
process currently being performed.
 All of the nuclear DNA, for example, must be replicated
before the nucleus begins to divide, which means that a
complete S phase must precede M phase.
 If DNA synthesis is slowed down or stalled, mitosis and cell
division must also be delayed. Similarly, if DNA is
damaged, the cycle must arrest in G1, S, or G2 so that the
cell can repair the damage, either before DNA replication is
started or completed or before the cell enters M phase.
 The cell-cycle control system achieves all of this by
employing molecular brakes, sometimes called
checkpoints, to pause the cycle at certain transition points.
 In this way, the control system does not trigger the next
step in the cycle unless the cell is properly prepared.
 Cell Cycle Check Points
 The cell-cycle control system regulates progression through the cell cycle
at three main transition points.
1. G1-S checkpoint: At the transition from G1 to
S phase, the control system confirms that the
environment is favorable for proliferation
before committing to DNA replication.
2. G2-M check point: At the transition from G2
to M phase, the control system confirms that
the DNA is undamaged and fully replicated,
ensuring that the cell does not enter mitosis
unless its DNA is intact.
3. M check point: Finally, during mitosis, the cell-cycle control machinery
ensures that the duplicated chromosomes are properly attached to a
cytoskeletal machine, called the mitotic spindle, before the spindle pulls
the chromosomes apart and segregates them into the two daughter cells.
 In animals, the transition from G1 to S phase is especially
important as a point in the cell cycle where the control system
is regulated.
 Signals from other cells stimulate cell proliferation when
more cells are needed—and block it when they are not.
 Cell proliferation in animals requires both sufficient nutrients
and specific signal molecules in the extracellular
environment; if these extracellular conditions are
unfavorable, cells can delay progress through G1 and may even
enter a specialized resting state known as G0 (G zero).
 The cell-cycle control system therefore plays a central part in
the regulation of cell numbers in the tissues of the body; if
the control system malfunctions such that cell division is
excessive, cancer can result.
 Two types of machinery are involved in cell division:
one manufactures the new components of the growing
cell, and another hauls the components into their
correct places and partitions them appropriately when
the cell divides in two.
 The cell-cycle control system switches all this
machinery on and off at the correct times, thereby
coordinating the various steps of the cycle.
 The core of the cell-cycle control system is a series of
molecular switches that operate in a defined sequence
and orchestrate the main events of the cycle, including
DNA replication and the segregation of duplicated
chromosomes.
 The cell-cycle control system governs the cell-cycle
machinery by cyclically activating and then inactivating
the key proteins and protein complexes that initiate or
regulate DNA replication, mitosis, and cytokinesis.
 This regulation is carried out largely through the
phosphorylation and dephosphorylation of proteins
involved in these essential processes.
 The phosphorylation reactions that control the cell cycle
are carried out by a specific set of protein kinases, while
dephosphorylation is performed by a set of protein
phosphatases.
 The protein kinases at the core of the cell-cycle control
system are present in proliferating cells throughout the
cell cycle. They are activated, however, only at
appropriate times in the cycle, after which they are
quickly inactivated.
 Thus, the activity of each of these kinases rises and
falls in a cyclical fashion. Some of these protein
kinases, for example, become active toward the end of
G1 phase and are responsible for driving the cell into S
phase; another kinase becomes active just before M
phase and drives the cell into mitosis.
 Switching these kinases on and off at the appropriate
times is partly the responsibility of another set of
proteins in the control system—the cyclins.
 Cyclins have no enzymatic activity themselves, but
they need to bind to the cell-cycle kinases before the
kinases can become enzymatically active. The kinases
of the cell-cycle control system are therefore known as
cyclin-dependent protein kinases, or Cdks.
 The cyclical changes in cyclin concentrations help
drive the cyclic assembly and activation of the cyclin–
Cdk complexes. Once activated, cyclin–Cdk complexes
help trigger various cell-cycle events, such as entry into
S phase or M phase.
 G1 Phase
 In addition to being a bustling period of metabolic activity,
cell growth, and repair, G1 is an important point of
decision-making for the cell.
 Based on intracellular signals that provide information
about the size of the cell and extracellular signals reflecting
the environment, the cell-cycle control machinery can
either hold the cell transiently in G1 (or in a more
prolonged nonproliferative state, G0), or allow it to prepare
for entry into the S phase of another cell cycle.
 Once past this critical G1-to-S transition, a cell usually
continues all the way through the rest of the cell cycle
quickly.
 G0 Phase
 As a general rule, mammalian cells will multiply only if
they are stimulated to do so by extracellular signals, called
mitogens, produced by other cells.
 If deprived of such signals, the cell cycle arrests in G1; if the
cell is deprived of mitogens for long enough, it will
withdraw from the cell cycle and enter a nonproliferating
state (G0), in which the cell can remain for days or weeks,
months, or even for the lifetime of the organism.
 Escape from cell-cycle arrest—or from certain
nonproliferating states— requires the accumulation of
cyclins. Mitogens act by switching on cell signaling
pathways that stimulate the synthesis of cyclins, proteins
involved in DNA synthesis and chromosome duplication.
 Cells can delay progress through the cell cycle at specific
transition points, to wait for suitable conditions or to repair
damaged DNA.
 They can also withdraw from the cell cycle for prolonged
periods—either temporarily or permanently by going into
G0 phase.
 Many cells in the human body permanently stop dividing
when they differentiate. In such terminally
differentiated cells, such as nerve or muscle cells, the cell-
cycle control system is dismantled completely and genes
encoding the relevant cyclins and Cdks are irreversibly shut
down.
 G1-S checkpoint
 DNA damage can signal the cell-cycle control system
to delay progress through the G1-to-S transition,
preventing the cell from replicating damaged DNA.
 The mechanism that operates at the G1-to-S transition,
which prevents the cell from replicating damaged
DNA is called G1-S checkpoint.
 The arrest of the cell cycle in G1 gives the cell time to
repair the damaged DNA before replicating it.
 If the DNA damage is too severe to be repaired, cells
can be induce to kill itself by undergoing a form of
programmed cell death called apoptosis.
 In the absence of appropriate signals, other cell types
withdraw from the cell cycle only temporarily, entering an
arrested state called G0.
 They retain the ability to reassemble the cell-cycle control
system quickly and to divide again. Most liver cells, for
example are in G0, but they can be stimulated to proliferate
if the liver is damaged.
 Liver cells, normally divide only once every year or two,
whereas certain epithelial cells in the gut divide more than
twice a day to renew the lining of the gut continually.
 Many of our cells fall somewhere in between: they can
divide if the need arises but normally do so infrequently.
 S Phase
 Before a cell divides, it must replicate its DNA.
 This replication must occur with extreme accuracy to minimize
the risk of mutations in the next cell generation.
 Of equal importance, every nucleotide in the genome must be
copied once—and only once—to prevent the damaging effects of
gene amplification.
During G1 DNA is first made replication-ready by the
recruitment of proteins to origins of replication.
 Then S-Cdks promote the assembly of DNA replication
machinery and pulll the trigger for initiating DNA replication.
 Since G1-Cdk levels fall when cell cycle proceeds to S phase DNA
synthesis can not be re-initiated so that DNA is replicated only
once.
 G2 Phase
 If the DNA is not replicated correctly cell can delay
entry into M phase.
 For the cell to progress into M phase, phosphorylation
of M-Cdks should be removed by a phosphatase.
 When DNA is incompletely replicated that
phosphatase is inhibited. Thus M-Cdk remains
inactive and M phase is delayed until DNA damage is
repaired or DNA replication is complete.
 Once a cell has successfully replicated its DNA in S
phase, and progressed through G2, it is ready to enter
M phase.
 M Phase
 During this brief period, the cell reorganizes virtually all of its
components and distributes them equally into the two daughter
cells.
 The earlier phases of the cell cycle, in effect, serve to set the stage
for the drama of M phase.
 The central problem for a cell in M phase is to accurately
segregate the chromosomes that were duplicated in the
preceding S phase, so that each new daughter cell receives an
identical copy of the genome.
 Eukaryotes solve this problem in a similar way: they assemble
two specialized cytoskeletal machines, one that pulls the
duplicated chromosomes apart (during mitosis) and another
that divides the cytoplasm into two halves (cytokinesis).
 M-Cdk helps prepare the duplicated chromosomes for
segregation and induces the assembly of the mitotic spindle—
the machinery that will pull the duplicated chromosomes apart.
 Immediately after a chromosome is duplicated during S phase,
the two copies remain tightly bound together. These identical
copies—called sister chromatids—each contain a single, double-
stranded molecule of DNA, along with its associated proteins.
 The sisters are held together by protein complexes called
cohesins, which assemble along the length of each chromatid as
the DNA is replicated. This cohesion between sister chromatids
is crucial for proper chromosome segregation, and it is broken
completely only in late mitosis to allow the sisters to be pulled
apart by the mitotic spindle.
 Defects in sister-chromatid cohesion lead to major errors in
chromosome segregation. In humans, such mis-segregation can
lead to abnormal numbers of chromosomes, as in individuals
with Down Syndrome, who have three copies of chromosome 21.
 Chromosome Condensation
 When the cell enters M phase, the duplicated
chromosomes condense, becoming visible under the
microscope as threadlike structures.
 Protein complexes, called condensins, help carry out
this chromosome condensation, which reduces mitotic
chromosomes to compact bodies that can be more
easily segregated within the crowded confines of the
dividing cell.
 The assembly of condensin complexes onto the DNA is
triggered by the phosphorylation of condensins by M-
Cdk.
 Cohesins tie the two sister chromatids together.
 Condensins assemble on each individual sister
chromatid at the start of M phase and help each of
these double helices to coil up into a more compact
form.
 After the duplicated chromosomes have condensed, two complex cytoskeletal
machines assemble in sequence to carry out the two mechanical processes that
occur in M phase.
 The mitotic spindle carries out nuclear division (mitosis), and, in animal cells
and many unicellular eukaryotes, the contractile ring carries out cytoplasmic
division (cytokinesis).
 The mitotic spindle is composed of microtubules and the various proteins that
interact with them, including microtubule-associated motor proteins.
 In all eukaryotic cells, the mitotic spindle is responsible for separating the
duplicated chromosomes and allocating one copy of each chromosome to each
daughter cell.
 The contractile ring consists mainly of actin filaments and myosin filaments
arranged in a ring around the equator of the cell .
 It starts to assemble just beneath the plasma membrane toward the end of
mitosis. As the ring contracts, it pulls the membrane inward, thereby dividing
the cell in two.
 Stages of Cell Cycle
• Cell cycle includes 4
stages: G1, S, G2 and M.

• G1, S and G2 together

form the interphase.

• Mitosis and cytokinesis

together form the M
phase.
 Interphase
 It includes G1, S and G2 stages of cell cycle.
 During interphase cell increases in size, DNA and the
centrosomes are replicated.
 Stages of M phase
 Although M phase proceeds as a continuous sequence of
events, it is traditionally divided into a series of stages.
 The first five stages of M phase—prophase, prometaphase,
metaphase, anaphase, and telophase— constitute mitosis.
Cytokinesis, which constitutes the final stage of M phase,
becgins before mitosis ends.
 Together, they form a dynamic sequence in which many
independent cycles—involving the chromosomes,
cytoskeleton, and centrosomes—are coordinated to
produce two genetically identical daughter cells.
 Before M phase begins, two critical events
must be completed: DNA must be fully
replicated, and, in animal cells, the
centrosome must be duplicated.
 The centrosome is the principal microtubule-
organizing center in animal cells. It duplicates
so that it can help form the two poles of the
mitotic spindle and so that each daughter cell
can receive its own centrosome.
 Centrosome duplication begins at the same
time as DNA replication.
 As mitosis begins, however, the two
centrosomes separate, and each nucleates a
radial array of microtubules called an aster.
The two asters move to opposite sides of the
nucleus to form the two poles of the mitotic
spindle.
 Prophase
 Duplicated chromosomes begin to condense.
 The mitotic spindle begins to form.
 Centrosomes move apart towards the opposite poles.
 Promethaphase
 Prometaphase starts abruptly with the disassembly of
the nuclear envelope, which breaks up into small
membrane vesicles.
 The spindle microtubules, which have been lying in
wait outside the nucleus, now gain access to the
duplicated chromosomes and capture them.
 The spindle microtubules grab
hold of the chromosomes at
kinetochores, protein complexes
that assemble on the centromere
of each condensed chromosome
during late prophase. Each
duplicated chromosome has two
kinetochores—one on each sister
chromatid—which face in
opposite directions.
 Kinetochores recognize the
special DNA sequence present at
the centromere.
 Metaphase
 During prometaphase, the duplicated chromosomes, now
attached to the mitotic spindle, begin to move about.
 Eventually, they align at the equator of the spindle, halfway
between the two spindle poles, thereby forming the
metaphase plate. This event defines the beginning of
metaphase.
 Anaphase
 Anaphase begins abruptly with the breakage of the cohesin
linkages that hold sister chromatids together .
 This release allows each chromatid—now considered a
chromosome—to be pulled to the spindle pole to which it
is attached.
 This movement segregates the two identical sets of
chromosomes to opposite ends of the spindle.
 Spindle Assembly Checkpoint
(M Checkpoint)
 If a dividing cell were to begin to segregate its chromosomes
before all the chromosomes were properly attached to the
spindle, one daughter cell would receive an incomplete set of
chromosomes, while the other would receive a surplus. Both
situations could be lethal.
 Thus, a dividing cell must ensure that every last chromosome is
attached properly to the spindle before it completes mitosis.
 To monitor chromosome attachment, the cell makes use of a
negative signal: unattached chromosomes send a “stop” signal to
the cell-cycle control system.
 This so-called spindle assembly checkpoint thereby controls
the onset of anaphase, as well as the exit from mitosis.
 Telophase
 During telophase, the final stage of mitosis, the mitotic spindle
disassembles, and a nuclear envelope reassembles around each
group of chromosomes to form the two daughter nuclei.
 The condensed chromosomes decondense into their interphase
state. As a consequence of this decondensation, gene
transcription is able to resume.
 A new nucleus has been created, and mitosis is complete.
 Cytokinesis
 Cytokinesis, the process by which the cytoplasm is cleaved in two,
completes M phase.
 It usually begins in anaphase by furrowing the plasma membrane but
is not completed until the two daughter nuclei have formed in
telophase.
 Whereas mitosis depends on a transient microtubule-based structure,
the mitotic spindle, cytokinesis in animal cells depends on a transient
structure based on actin and myosin filaments, the contractile ring.
 The furrowing of plasma membrane invariably occurs in a
plane that runs perpendicular to the long axis of the
mitotic spindle.
 This positioning ensures that the cleavage furrow cuts
between the two groups of segregated chromosomes, so
that each daughter cell receives an identical and complete
set of chromosomes.
 When the mitotic spindle is located centrally in the cell—
the usual situation in most dividing cells—the two
daughter cells produced will be of equal size.
Control of Cell Survival, Size and Number
 In a multicellular organism, the fate of individual cells is
controlled by signals from other cells. For either tissue growth or
cell replacement, cells must grow before they divide.
 Nutrients are not enough for an animal cell to survive, grow, or
divide. It must also receive chemical signals from other cells,
usually its neighbors. Such controls ensure that a cell survives
only when it is needed and divides only when another cell is
required, either to allow tissue growth or to replace cell loss.
 Most of the extracellular signal molecules that influence cell
survival, cell growth, and cell division are either soluble proteins
secreted by other cells or proteins that are bound to the surface
of other cells or to the extracellular matrix.
 Although most act positively to stimulate one or more of these
cell processes, some act negatively to inhibit a particular process.
Positive Regulators of Cell Growth and Division
 The positively acting signal proteins can be classified, on
the basis of their function, into three major categories:
1. Survival factors promote cell survival, largely by
suppressing apoptosis.
2. Mitogens stimulate cell division, primarily by overcoming
the intracellular braking mechanisms that tend to block
progression through the cell cycle and by stimulating the
progression through S phase.
3. Growth factors stimulate cell growth (an increase in cell
size and mass) by promoting the synthesis and inhibiting
the degradation of proteins and other macromolecules.
Negative Regulators of Cell Growth and Division
 The extracellular signal proteins that we have discussed so far—
survival factors, mitogens, and growth factors—act positively to
increase the size of organs and organisms. Some extracellular
signal proteins, however, act to oppose these positive regulators
and thereby inhibit cell growth and division.
 Positive and negative signals act in balance to control cell size
and cell number in organism.
 Cancers are similarly the products of mutations that set cells free
from the normal “social” controls operating on cell survival,
growth, and proliferation.
 Because cancer cells are generally less dependent than normal
cells on signals from other cells, they can out-survive, outgrow,
and outdivide their normal neighbors, producing tumors that
can kill their host.
Mutations in Myostatin gene which normally inhibits the cell growth
and division in myoblasts (muscle cells) result in big muscles in cattle
(A) and in mouse (B).