Proliferation and the Cell Cycle

Slide 1 · Proliferation and the Cell Cycle

Cell-cycle progression • Quality-control checkpoints • Tissue homeostasis

From coordinated cell division to control of tissue cell populations

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning objectives

Distinguish G₀, G₁, S, G₂ and M phases and identify their essential events.

Explain the sequential actions of cyclins, CDKs and CDK inhibitors.

Relate checkpoint integrity to DNA repair, senescence, apoptosis and cancer risk.

Connect proliferative control with stem-cell renewal and tissue maintenance.

Slide 3 · Why controlled proliferation matters

Proliferation is an organized biological process, not simply an increase in cell number.

Generates cells during development and organ growth.

Replaces cells lost through normal turnover or injury.

Preserves organ function by coordinating cellular gain and loss.

Requires complete duplication and accurate allocation of cellular contents.

A dividing cell must reproduce both its genome and its cellular machinery.

Slide 4 · Three tasks underlying one successful division

DNA replication

Chromosome separation

Cytokinesis

Distinct coordinated events—not synonyms

DNA must be replicated accurately.

Proteins, membranes and organelles must be synthesized.

Mitosis segregates chromosomes; Cytokinesis separates daughter cells.

A failure in any task can compromise the next cell generation.

Slide 5 · Related processes with distinct meanings

Cell growth: Increase in biosynthetic mass and components.

DNA synthesis: Duplication of genetic information.

Mitosis: Ordered separation of duplicated chromosomes.

Cytokinesis: Physical partitioning into daughter cells.

Growth, genome replication and cell division are coordinated but nonidentical.

Slide 6 · The cell-cycle map

G₁

Cell growth

S

DNA replication

G₂

Preparation

M

Division

G₀

Quiescent pool

G₁, S, G₂ and M form the active cycle.

G₀ is a noncycling quiescent state.

G₁ can follow mitosis or re-entry from G₀.

Each phase depends on successful completion of the preceding steps.

Cell-cycle order safeguards inherited information.

Slide 7 · G₀ versus the proliferative cycle

G₀: Not actively cycling

G₁: Re-entry possible

Loss of proliferative capacity

Quiescence ≠ permanent inability to divide

G₀ cells are not actively replicating.

Some quiescent cells can be recruited back into G₁.

Cells with permanently lost proliferative capacity are distinct from reversibly quiescent cells.

Slide 8 · G₁ phase: Biosynthesis before DNA replication

Cell mass increases and cellular components accumulate.

Growth signals can influence entry and continued progression.

The G₁ restriction point marks commitment beyond a requirement for the initiating mitogenic signal.

The G₁/S surveillance gate assesses DNA integrity before synthesis.

Commitment to cycle progression and DNA-damage checking are conceptually distinct.

Slide 9 · S phase: Duplication of the genome

The defining event is DNA synthesis.

Chromosome duplication must preserve sequence and completeness.

Replication errors can activate cycle-control mechanisms.

Cyclin A–associated kinase complexes act during this phase.

Slide 10 · G₂ phase: Preparing for chromosome segregation

Occurs after DNA synthesis and before mitosis.

The cell prepares its organelles and machinery for division.

The G₂/M checkpoint detects incompletely duplicated or damaged DNA.

Progression into M phase depends on cyclin B–CDK1.

Slide 11 · M phase: Segregation and physical division

One parent cell

Two daughter cells

DNA and organelles distributed

Mitosis separates duplicated chromosomes.

Cytokinesis divides cytoplasm and other cellular components.

Two daughter cells enter G₁ after completion of division.

Successful partitioning maintains genomic continuity.

Slide 12 · Directionality is fundamental

G₀

Quiescence

G₁

Growth

S

DNA synthesis

G₂

Preparation

M

Cell division

Entry into G₁ from quiescence or after mitosis

Progression follows G₁ → S → G₂ → M.

G₀ lies outside active cycling.

Quality-control events delay progression when prerequisites are unmet.

Slide 13 · Restriction point versus G₁/S checkpoint

Mitogen-responsive G₁

Restriction point

G₁/S checkpoint

Growth signals needed

Commitment

DNA integrity gate

Restriction point: Commitment to continue through the cycle without additional initiating growth signal.

G₁/S checkpoint: DNA-integrity surveillance before replication.

These controls occur in related but nonidentical functional contexts.

Slide 14 · Cell-cycle control has opposing forces

Cyclins + CDKs

Checkpoints + CDKIs

Cycle progression

Progression requires both activating signals and permissive surveillance

Cyclin–CDK complexes promote phase transitions.

CDK inhibitors restrain activity of these complexes.

Damage-responsive checkpoints prevent inappropriate progression.

Slide 15 · Two-part logic of cyclin-dependent kinases

Cyclins are dynamically produced and degraded.

CDKs are synthesized more constitutively.

Binding of a relevant cyclin confers CDK kinase activity.

Active complexes phosphorylate protein substrates to advance cell-cycle events.

CDK activity depends on the presence of its phase-appropriate cyclin.

Slide 16 · How a cyclin activates a CDK

Cyclin

CDK

Active cyclin–CDK complex

CDK alone has little relevant kinase activity

Cyclin binding enables substrate phosphorylation

Cyclin binding enables the CDK to phosphorylate substrates.

A temporary cyclin increase raises activity of the cognate kinase.

Cyclin degradation terminates a wave of CDK activity.

Slide 17 · Cyclin abundance is dynamic

Cyclin availability (qualitative)

Time →

Diagram shows oscillation, not measured abundance

Cyclin production rises in a phase-associated sequence.

Regulated degradation lowers associated kinase activity.

The resulting oscillation coordinates ordered transitions.

Slide 18 · The cyclin relay: D → E → A → B

Cyclin D

G₁

Cyclin E

G₁→S

Cyclin A

S

Cyclin B

G₂→M

Sequential activation; Cyclins subsequently degrade

Cyclin D participates in G₁ progression.

Cyclin E regulates the G₁-to-S transition.

Cyclin A functions during S phase.

Cyclin B is essential for G₂-to-M progression.

Slide 19 · Cyclin D and CDK4/6 at early progression

Cyclin D forms active complexes with CDK4 and CDK6.

These complexes help regulate the G₁-to-S transition.

Phosphorylation of retinoblastoma protein (pRb) is an important target event.

INK4-family inhibitors preferentially restrain this module.

Slide 20 · Cyclin E and CDK2 approaching S phase

Cyclin E associates with CDK2.

The complex participates in control of the G₁/S transition.

It contributes to phosphorylation of pRb pathway.

Unchecked activation can undermine orderly cycle regulation.

Slide 21 · PRb links G₁ kinase activity to S-phase entry

Cyclin D/E + CDKs

PRb phosphorylation

G₁ → S

Simplified textbook relationship: CDK-mediated pRb phosphorylation accompanies G₁/S progression

Cyclin D–CDK4/6 and cyclin E–CDK2 phosphorylate pRb.

The source figure locates these actions at the G₁-to-S transition.

Interpret the diagram as a simplified mechanistic relationship.

Slide 22 · Cyclin A: Coordination of S-phase activity

Cyclin A associates with CDK2 during S phase.

Cyclin A can also associate with CDK1 in the S-phase regulatory program.

Different CDK partners contribute to phase-specific activity.

The subsequent cyclin relay prepares the transition toward mitosis.

Slide 23 · Cyclin B and CDK1: G₂-to-M control

Cyclin B partners with CDK1.

This complex is required for transition from G₂ into mitosis.

The kinase program is active only when the appropriate stage is reached.

Checkpoint restraint protects mitotic entry when DNA is incomplete or damaged.

Slide 24 · Cyclin–CDK complexes by cell-cycle stage

Complex

Principal stage / transition

Source-described role

Cyclin D–CDK4/6

G₁ → S

PRb phosphorylation; G₁ progression

Cyclin E–CDK2

G₁ → S

PRb phosphorylation; G₁/S control

Cyclin A–CDK2

S phase

S-phase kinase activity

Cyclin A–CDK1

S phase

S-phase kinase activity

Cyclin B–CDK1

G₂ → M

M-phase entry

Slide 25 · Why cyclical cyclin degradation matters

A cyclin pulse turns on its corresponding CDK function.

Proteolytic removal limits that kinase activity once its task is completed.

Subsequent cyclins introduce the next regulatory program.

Sequential activation prevents sustained use of a single phase program.

Oscillatory regulator abundance gives the cell cycle its temporal order.

Slide 26 · INK4-family CDK inhibitors target CDK4/6

Cyclin D + CDK4/6

INK4 proteins

Cell-cycle progression

INK4 acts at the CDK4/6 node

P15 — CDKN2B.

P16 — CDKN2A.

P18 — CDKN2C.

P19 — CDKN2D.

Slide 27 · The p21-family inhibitors have broader targets

P21 / p27 / p57

CDK4/6

CDK2

CDK1

Broad suppression of cyclin–CDK activity

P21 — CDKN1A.

P27 — CDKN1B.

P57 — CDKN1C.

Collectively inhibit multiple cyclin–CDK complexes.

Slide 28 · Two CDK-inhibitor families: Compare and recall

Feature

INK4 family

Cip/Kip family

Members

P15, p16, p18, p19

P21, p27, p57

Gene symbols

CDKN2B, CDKN2A,

CDKN2C, CDKN2D

CDKN1A, CDKN1B, CDKN1C

Main activity

Selective CDK4/6 inhibition

Broad CDK inhibition

Physiologic meaning

Restraint at G₁ control

Control across multiple phases

Slide 29 · Activation and inhibition form a control network

Cyclins + CDKs

Checkpoints + CDKIs

Cycle progression

Progression requires both activating signals and permissive surveillance

An activator alone does not make progression safe.

CDKI activity can arrest otherwise activated cyclin–CDK machinery.

Checkpoint signals depend on this inhibitory capacity.

Both positive and negative regulators are physiologically necessary.

Slide 30 · Checkpoints are quality-control mechanisms

Detect DNA injury or problems with genetic replication.

Delay progression until required conditions are met.

Create time for corrective processes, including DNA repair.

Help prevent inheritance of defective chromosomes.

Slide 31 · The G₁/S checkpoint checks DNA integrity

DNA status assessed

Checkpoint gate

Proceed

Delay + repair

Intact DNA

Damage

Operates before irreversible commitment of resources to DNA replication.

A detected genetic lesion delays cycle progression.

Repair responses may allow subsequent cycling.

Slide 32 · The G₂/M checkpoint tests replication completion

DNA status assessed

Checkpoint gate

Proceed

Delay + repair

Intact DNA

Damage

Assesses whether the genome has been duplicated accurately.

DNA injury or incomplete replication delays mitotic entry.

This reduces the likelihood of transmitting defective chromosomes.

Slide 33 · G₁/S versus G₂/M: Distinct surveillance tasks

Control

When it operates

Primary question

G₁/S checkpoint

Before DNA synthesis

Is DNA sufficiently intact to replicate?

G₂/M checkpoint

After synthesis, before mitosis

Has DNA been duplicated accurately and completely?

Restriction point

During G₁, before later cycle entry

Is continued cycling committed despite withdrawal of initiating mitogen?

Slide 34 · DNA damage: Cell-cycle delay gives time for repair

Detected DNA abnormality

Checkpoint delay + repair

Repair successful

Apoptosis / senescence

Damage sensors activate control pathways.

Checkpoint enforcement delays an unsafe transition.

Successful repair can restore normal progression.

Severe injury can instead trigger senescence or apoptosis.

Slide 35 · The p53 axis links damage to cell fate

When DNA irregularities are severe, p53-dependent mechanisms are especially important.

Responses include durable growth arrest (senescence).

Alternatively, cells may be eliminated through apoptosis.

The source describes these as safeguards after damage is detected.

The goal is to prevent replication of seriously compromised genomes.

Slide 36 · Severe damage creates diverging cell fates

Detected DNA abnormality

Checkpoint delay + repair

Repair successful

Apoptosis / senescence

Repairable disturbance → temporary arrest and repair.

Unrepairable disturbance → apoptosis or nonreplicative senescence.

The source emphasizes p53 dependence for these terminal responses.

Slide 37 · Senescence is not identical to transient G₀

G₀ designates the noncycling state; Some cells can be reactivated.

Senescence denotes a nonreplicative state after an enduring arrest.

Both may lack active DNA synthesis, but their biologic contexts differ.

Here, senescence is one protective outcome of severe DNA injury.

Slide 38 · What follows loss of checkpoint enforcement?

DNA damage

Checkpoint failure

Mutated progeny

Faulty surveillance permits propagation of genetic defects

CDKI defects can permit divisions despite DNA damage.

Daughter cells may acquire and propagate mutations.

This process as a risk for malignant transformation.

Slide 39 · Cell-cycle dysregulation and oncogenic potential

Normal progression depends on both mitogenic drive and genomic surveillance.

Excessive kinase activity or inadequate inhibitory control weakens this balance.

Checkpoint failure allows faulty replication or segregation products to persist.

Mutation propagation creates conditions for malignant evolution.

Slide 40 · Four different outcomes of a cell-cycle signal

Finding

Cellular response

Meaning

Adequate growth signals

Progression

Replication program can proceed

No mitogenic drive

G₀ / quiescence

Reversible noncycling state in competent cells

Repairable DNA lesion

Checkpoint delay

Protection while damage is corrected

Severe DNA derangement

Senescence / apoptosis

Prevention of defective daughter cells

Slide 41 · Division requires biomass, not DNA alone

Growth factor signal

Cell-cycle activation

Growth metabolism

Daughter-cell production

Membranes + organelles

Two daughter cells need membranes, cytoplasmic proteins and organelles.

Growth signals coordinate both cell-cycle progression and biosynthesis.

Metabolic reprogramming supports an increased synthetic workload.

Slide 42 · Growth signaling also promotes metabolism

Mitogenic signaling does not merely activate cyclin–CDK complexes.

It concurrently promotes the metabolism needed to build cellular components.

The target is successful generation of two functional daughter cells.

This couples biochemical growth capacity to cycle progression.

Slide 43 · Aerobic glycolysis: The Warburg effect

Growth factor signaling

Aerobic glycolysis ↑

Oxidative phosphorylation ↓

Relative metabolic shift

Increased aerobic glycolysis in growing cells.

It occurs despite oxygen availability.

A relative reduction in oxidative phosphorylation is highlighted.

The same metabolic program is important in cancer biology.

Slide 44 · The Warburg effect in its correct context

A metabolic shift supports the cell-growth program.

Aerobic glycolysis does not mean that oxygen is absent.

The source explicitly links this program to cancer-cell growth.

Detailed neoplastic metabolism is deferred by Robbins to Chapter 7.

Avoid equating aerobic glycolysis with obligatory mitochondrial inactivity.

Slide 45 · Different tissues have different division capacities

Labile

Continuous cycling

Epidermis; Intestinal epithelium

Stable

Quiescent; Re-enter cycle

Hepatocytes

Permanent

Proliferative capacity lost

Neurons; Cardiac myocytes

Labile: Normally cycling cells, such as skin and gastrointestinal epithelium.

Stable: Quiescent cells that may re-enter cycle, such as hepatocytes.

Permanent: Cells that have lost proliferative capacity, such as neurons and cardiac myocytes.

Slide 46 · Labile epithelia maintain continuous replacement

Shedding / loss

Basal stem / progenitor zone

Basal stem/progenitor populations produce new epithelial cells.

Their daughters mature as they move away from the proliferative zone.

Differentiated surface cells are ultimately lost by shedding.

Slide 47 · Stable cells: Quiescence permits regenerative entry

Quiescent hepatocyte

Loss / injury

Re-entry to G₁

Stable populations can proliferate in response to demand

Hepatocytes exemplify cells that can remain outside active cycle.

Under appropriate demand, these cells can re-enter proliferative phases.

This differs from continuous division in labile tissues.

Slide 48 · Permanent cells have limited division capacity

The cited examples are neurons and cardiac myocytes.

Their inability to readily re-enter the cycle constrains cellular replacement.

Injury-related recovery therefore differs from high-turnover epithelia.

This is a tissue-level consequence of cell-cycle competence.

Slide 49 · Tissue homeostasis is a population balance

Stem-cell input

Proliferation

Differentiation

Tissue cell population

Cell death (apoptosis)

Stem-cell input and proliferation expand the population.

Differentiation changes the distribution of cellular states.

Apoptotic removal decreases mature cell number.

These rates jointly determine steady-state cellular composition.

Slide 50 · Stem cells have two defining capabilities

Self-renewal: Preserve an undifferentiated stem-cell pool.

Differentiation: Produce more specialized daughter cells.

Both properties sustain long-term tissue replacement.

Homeostasis depends on balancing output with maintenance of the pool.

Slide 51 · Asymmetric stem-cell division

Stem

Stem cell

Differentiating cell

One daughter preserves stem-cell identity.

The other begins a differentiation pathway.

The stem-cell pool is maintained while mature-cell output continues.

Slide 52 · Symmetric self-renewing division

Stem

Stem cell

Stem cell

Both daughter cells retain self-renewal capacity.

This expands the available stem-cell population.

Expansion during embryogenesis and repopulation after stress.

Slide 53 · Symmetric and asymmetric renewal compared

Feature

Asymmetric division

Symmetric self-renewal

Daughter-cell identities

One stem + one differentiating

Two stem daughters

Net stem-cell pool

Maintenance

Expansion

Source example

Normal tissue homeostasis

Embryogenesis; Marrow recovery after stress

Biologic purpose

Sustain output without depleting pool

Rebuild or enlarge stem-cell reserves

Slide 54 · Stem-cell potency narrows during differentiation

Zygote • Totipotent

Inner cell mass / ES • Pluripotent

Adult tissue stem • Lineage limited

Mature cells • Differentiated

Totipotency characterizes the earliest developmental cells.

Embryonic stem cells of the inner cell mass are pluripotent.

Adult stem cells generally have tissue-restricted lineage potential.

Fully differentiated cells typically have specialized function.

Slide 55 · Embryonic versus adult tissue stem cells

Dimension

Embryonic stem cells

Adult tissue stem cells

Location

Inner cell mass of blastocyst

Niches within mature tissues

Differentiation range

Pluripotent: Derivatives of three germ layers

Normally tissue / lineage restricted

Renewal

Extended self-renewal under suitable conditions

Local maintenance and repair

Typical function

Broad developmental potential

Replace cells normally present in their tissue

Slide 56 · Stem-cell niches regulate cell fate

Niche microenvironment

Stem cell

Nearby cells

Soluble signals

Physical context

A niche is a specialized tissue microenvironment.

Signals from neighboring cells and soluble factors regulate behavior.

A niche governs quiescence, expansion and differentiation.

Slide 57 · Where tissue stem cells reside

Tissue

Niche location

Relevance

Bone marrow

Perivascular regions

Hematopoietic cell maintenance

Intestinal epithelium

Crypts

Continual epithelial renewal

Hair follicle

Bulge region

Epithelial stem-cell residence

Cornea

Limbus

Surface-cell renewal

Brain

Subventricular zone

Resident neural stem-cell niche

Slide 58 · Hematopoietic stem cells maintain blood production

Hematopoietic stem cell

Blood-cell renewal

Marrow reconstitution

Bone marrow and mobilized peripheral blood are clinical cell sources

Hematopoietic stem cells replace diverse circulating blood-cell lineages.

They are found in marrow and can be mobilized into peripheral blood.

Their transplantation can reconstitute marrow after intensive therapy.

Slide 59 · Clinical implication: Marrow reconstitution

Hematopoietic stem cells can be collected from marrow.

Selected colony-stimulating factors can mobilize them into peripheral blood.

Transplantation is used to repopulate marrow after cytotoxic treatment.

Robbins also notes use for selected inherited blood-cell disorders.

Slide 60 · Mesenchymal stromal / stem-cell potential

Marrow and adipose tissue contain populations with mesenchymal differentiation potential.

Source-listed stromal derivatives include cartilage, bone, fat and muscle lineages.

Their expansion and immunomodulatory properties motivate regenerative applications.

Such potential should not be confused with unrestricted embryonic pluripotency.

Slide 61 · Application 1: A DNA-integrity decision

A cell has received sufficient mitogenic stimulation to advance through G₁. Molecular surveillance then detects significant DNA damage.

Which control should delay entry into S phase, and why?

Discussion prompt — answer follows

Slide 62 · Application 1: Reasoning

The G₁/S checkpoint monitors integrity before DNA replication.

Damage triggers delay and an opportunity for repair.

Severe unresolved lesions may result in apoptosis or senescence.

Advancing with damaged DNA risks transmitting mutations.

Slide 63 · Application 2: Expanded stem-cell production

A patient needs marrow repopulation after intensive cytotoxic therapy. A surviving stem-cell population must increase the number of undifferentiated precursors.

Which stem-cell division mode directly expands that precursor pool?

Discussion prompt — answer follows

Slide 64 · Application 2: Reasoning

Symmetric self-renewal produces two stem-cell daughters.

The stem-cell pool expands rather than merely being maintained.

Asymmetric division generates one differentiating cell while retaining one stem cell.

The balance of these modes supports tissue restoration.

Slide 65 · Application 3: A proliferative defect

A hypothetical tumor clone loses an inhibitory regulator of the cyclin D–CDK4/6 module.

Which CDKI family directly restrains that kinase pair, and what is the potential consequence?

Discussion prompt — answer follows

Slide 66 · Application 3: Reasoning

The INK4 family preferentially inhibits cyclin D–CDK4/6.

P15, p16, p18 and p19 belong to this family.

Reduced CDKI restraint can permit propagation of DNA-damaged daughter cells.

The source links defective checkpoint proteins to malignant-transformation risk.

Slide 67 · Integrated summary: Mechanism to tissue outcome

Cyclin–CDK oscillations drive the ordered G₁ → S → G₂ → M sequence.

Checkpoints and CDKIs restrain unsafe transitions and genomic instability.

Metabolic growth and organelle synthesis accompany genome duplication.

Stem-cell self-renewal, differentiation and cell death jointly maintain tissue populations.

Loss of proliferative control links basic cell biology to neoplastic risk.

Slide 68 · Principal source and interpretation notes

Primary source

Robbins, Cotran & Kumar: Pathologic Basis of Disease. 11th edition (2026). Chapter 1, “Proliferation and the Cell Cycle” and “Stem Cells,” pp. 23–27.

Interpretive points

All scientific schematics are original editable PowerPoint drawings; no microscopy or data plots were fabricated.

The source text’s “totipotent” description of ES cells conflicts with its own Fig. 1.22; the presentation correctly calls inner-cell-mass ES cells pluripotent.

Stem-cell material is included because it is explicitly grouped with this topic in the supplied Robbins teaching unit.