The phenomenon of signal transduction, also known as cell signaling, pertains to the intricate mechanisms that facilitate the transfer of biological information between cells. The effective coordination of diverse specialized cell types in various tissues and organs is a prerequisite for the proper functioning of complex multicellular organisms, necessitating intercellular communication. This communication must be continuous and dynamic to maintain coordination. Additionally, cell signaling pathways play a crucial role in the mechanisms of action of numerous drugs, including both local and general anesthetics. Consequently, a fundamental understanding of cell signaling mechanisms is imperative for comprehending various pathophysiologic and pharmacologic mechanisms.

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CELL SIGNLLING
PATHWAYS AND NETWORKS
MOLECULAR EVENTS IN THE CELL CYCLE
CYCLINS AND CDKs
Seminar by
B.JEGADESHWARI
BMS15350
IV YEAR
Msc BIOMEDICAL SCIENCE

2. INTRODUCTION
• Cells enter the cell cycle through two quite distinct processes - fertilization and cell
proliferation activated by growth factors.
• The primary focus will be on the cell cycle events.
• A complex cell cycle network of signalling pathways that interact with each other to
control whether or not cells will grow and divide.
• An extensive cell cycle toolkit that contains both the signalling molecules and the large
number of targets that are engaged as the cell passes through the cell cycle.
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3. CELL CYCLE
• The sequence of events that occur when a cell is stimulated to grow and divide
constitutes the cell cycle.
• The cell cycle is a highly regulated process during which cells replicate themselves to
produce two genetically identical daughter cells as a result of faithful DNA duplication,
and is conserved across species.
• This series of events is controlled by the machinery that is often termed the “cell cycle
clock”.
• It is divided into two phases
• Non dividing or growing phase – Interphase (G₁,S,G₂ Phases )
• Dividing phase – Mitotic/ M phase
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4. CELL CYCLE PHASES
• The cell cycle is comprised of four phases, which can vary in length depending on cell
type and the signals the cell receives:
• G₁ = gap phase for growth and preparation of the chromosomes for replication
• S = synthesis of DNA (and centrosomes)
• G₂ = gap phase for growth and preparation for mitosis
• M = mitosis (nuclear division) and cytokinesis (cell division).
• Cells can enter a “resting” non-proliferative state, which can be very prolonged or even
permanent – termed G₀, which equates with terminal differentiation.
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6. TO PROCEED OR NOT TO PROCEED
• The G1 phase is an important gap phase, when the cell
may receive many signals that can influence cell division.
• Many of these are related to growth factors as well as
cell–cell contact.
• In other words, on the basis of signals received, the cell
will decide whether to enter the S phase or to pause or
arrest in G0, to differentiate or to die.
• Diverse metabolic stress and environmental signals are
integrated and influence the activity of various cell cycle
regulatory proteins and consequently cell cycle
progression. 6

7. CONT……
• Signals could be external – from other cells or from the circulation – or arise within the
cell, as a result of DNA damage caused by genotoxic agents, physical or chemical
stresses, or oncogenic stimuli.
• Given the task of interpreting a flood of signals in order for the correct outcome– cell
cycle progression, arrest, differentiation, or death – it is perhaps not surprising that
mistakes in this process can lead to cancer.
• In fact, it is hard to imagine any cancer developing at all without deregulation of some
aspects of cell cycle control.
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8. The cell cycle engines: cyclins and kinases
• Once the cell has received appropriate signals entry into the G1 phase of the cell cycle,
activation of various regulatory proteins called cyclin-dependent kinases (CDKs) must
occur if the cell is to proceed and enter S phase.
• CDKs are serine/threonine protein kinases that require binding to their regulatory
subunits, called cyclins, in order to become catalytically active.
• Active CDKs are universal regulators of the cell cycle that alter the biological functions
of regulatory proteins through phosphorylation, and have been described as a “cell
cycle engine”.
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9. Cyclins
• Different members of the CDK family are activated by their appropriate cyclins during
certain phases of the cell cycle.
• While levels of CDKs, albeit inactive, remain fairly stable throughout the cell cycle, levels
of cyclins rise and fall (with the exception of cyclin D, which rises early in G1 and
remains constant thereafter), thus determining at what stage of the cycle the CDK
partner is activated.
• Levels of cyclins rise and fall during the progression of the cell cycle, and each cyclin is
categorized with respect to the stage at which it is elevated.
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Regulation by degradation

11. • In general levels of cyclins are
determined by rate of synthesis and
also by degradation by the ubiquin –
proteasome pathway.
• The types of cyclins includes;
• G1 cyclins (cyclin D1, D2, andD3)
• S-phase cyclins (cyclins E1, E2, andA)
• Mitotic cyclins (cyclins B and A)
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13. Cyclin-dependent kinases (CDKs)
• The progression of a cell through the cell cycle is promoted by CDKs, whose periodic
activation is driven by cyclins and negatively regulated by CDK inhibitors (CKI).
• CDKs are categorized numerically.
• CDK protein levels remain relatively stable during the cycle, but activity is dependent on
each CDK forming an active holoenzyme complex by binding to its relevant cyclin.
• Cyclin-CDK complexes in turn promote phosphorylation of target substrates needed for
cell cycle progression.
• Substrate specificity is likely determined by both subcellular localization and structural
factors. 13

14. CONT…..
G1 CDK (CDK4&6) : cyclin D-CDK4/6 complex: key target primarily RB.
S-phase CDK (CDK2) : cyclin E or A-CDK2 complex: key target RB and others such as Cdc6.
M-phase CDK (CDK1 or Cdc2) : cyclin B or A-CDK1: key target not well defined but
include WARTS and PAK1.
• CDK activity during the cell cycle is either high or low; the change in state underlies
temporal control of DNA replication and links it to mitosis.
• Initiation of DNA replication in S phase requires the formation of a prereplication
complex at chromosomal sites known as replication origins, and the activation of DNA
unwinding and polymerase functions 14

15. CONT….
• The formation of a prereplication complex occurs when CDK activity is low whereas
recruitment of DNA helicases (to unwind DNA) and polymerases occur when CDK
activity is high.
• The transition from low to high CDK activity is essential for correct DNA replication.
• Moreover, the high CDK activity prevents the formation of further prereplication
complexes until the completion of mitosis, when CDK activity is reduced.
• These events ensure that DNA will be replicated once and only once per cell cycle.
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16. INHIBITORS OF THE CELL CYCLE ENGINE
• Cyclin–CDK complexes are themselves negatively regulated by two classes of CDK inhibitors(CKI).
• The two families of CKI proteins;
• The INK4 family of CKI (p15INK4b, p16INK4a, p18INK4c, and p19ARF) inhibit cyclin–CDK action by
binding to the CDK.
• The CIP/KIP family members (p21CIP1, p27KIP1, and p57KIP2) bind to the cyclin component of the
cyclin–CDK complex.
• These proteins – many of which are known to be tumor suppressors in human – are able to inhibit
cell cycle entry and progression and prevent replication of abnormal DNA, by allowing cells to stall
at appropriate points during the cell cycle so that DNA damage can be repaired.
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20. • Cell cycle checkpoints reinforce the correct order of cell cycle events by delaying the progression
in the cell cycle in response to biochemical stimuli that signal unmet requirements for progression
into the next phase.
• These mechanisms allow the cell to monitor the completion of a stage and confer the cell cycle
with the fidelity required for innumerable repetitions.
• They coordinate the cell cycle with growth to maintain cell size, ensure that chromosomes are
duplicated only once per cycle and warrant accurate distribution of the duplicated chromosomes
to the daughter cells.
• Formally, checkpoints consist of: a surveillance mechanism, which checks for errors; an error
correction machinery, which acts in response to problems detected by the surveillance
mechanism; a checkpoint enforcement mechanism, which delays progression until successful
completion of the corresponding phase. 20

21. CHECKPOINTS- CELL CYCLE MONITOR
• There are three main checkpoints present
• G1 checkpoint – it control the cells from G1
phase to S phase.
• G2 checkpoint – it will check the DNA
integrity (DNA damage) and DNA replication.
• Mitosis checkpoint – assess the spindle fibre
attachment to centromere. Arrest the cell
cycle from metaphase to anaphase state.
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22. CHECKPOINTS – PUTTING BREAKS ON
THE CELL CYCLE ENGINE
• The cell has several options for arresting the cell cycle if something goes wrong – the so-called
checkpoints.
• For instance, if a cell sustains DNA damage following exposure to radiation or chemical agents,
or replication errors have occurred during the cell cycle, then the duplication of that cell might
pose a potential cancer risk to the organism.
• Far better for the organism is to eliminate the cell by death (apoptosis) or for it to permanently
exit from the cell cycle (senescence), unless the DNA damage can be efficiently repaired.
• For these reasons, there exist various checkpoints throughout the cell cycle.
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23. In cancer, mutations have been
observed in genes encoding CDK,
cyclins, CDK activating enzymes, CKI,
CDK substrates, and checkpoint
proteins.
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