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Core
The Cell Cycle Map
A proliferating somatic cell (an ordinary body cell, not a sperm or egg cell) does not enlarge continuously and then split. It advances through ordered phases that separate growth, genome duplication and chromosome segregation. G1 is the first gap: mass increases, organelles are replenished and the cell samples mitogens (signals that tell a cell to divide), nutrients and contact signals before any DNA is copied. S phase replicates the nuclear genome once, producing sister chromatids (the two identical copies of a chromosome) tethered by cohesin (a protein ring that holds them together). G2 is the second gap, used to finish mitotic protein synthesis and to confirm that replication is complete. M phase comprises mitosis and cytokinesis (the splitting of the cell body into two daughter cells). The visible stages of mitosis, from prophase to telophase, are developed in Mitosis and Meiosis. This lesson concerns the decisions that allow entry into those stages and the programmes that abort an unsafe cycle.
G0 is not a failed G1. It is a resting state in which cyclin-CDK activity is suppressed and replication origins (the sites where DNA copying starts) are not switched on. Many differentiated cells occupy G0 for long periods yet retain the capacity to re-enter G1 when tissue demand returns, as hepatocytes do after substantial cell loss. Fully post-mitotic cells, including most mature neurons, normally remain outside the cycle for life. Abnormal re-entry of mature neurons into the cycle has been linked to neurodegeneration. The practical map is therefore five states: G0, G1, S, G2 and M. A cell can move between G0 and G1 in either direction, but once DNA synthesis or chromosome segregation has begun it is committed and cannot go back.
That temporal separation is the first safety rule of tissue renewal. DNA is copied only in S. Chromosomes are pulled apart only in M. Checkpoints (control points that halt the cycle until a condition is met) guard the transitions at which an error would become heritable in both daughters.
Ordered phases of the cell cycle with the restriction point and the DNA, replication and spindle checkpoints that gate unsafe progress.
SimpleMed original educational diagram
Cyclin-CDK Control
Forward motion is not the work of a single clock protein. It is produced by cyclin-dependent kinases (CDKs), which are enzymes that add phosphate groups to other proteins (phosphorylation) to change their activity. A CDK becomes active only when bound to the correct cyclin. Cyclin levels rise and fall through gene transcription and through ubiquitin-mediated proteolysis (the cell tags the cyclin with ubiquitin so that its protein-degrading machinery, the proteasome, breaks it down); the CDK proteins themselves are comparatively stable. The pairing therefore creates a temporary kinase whose substrate preference matches the needs of one phase.
A usable medical set of pairs is small. Cyclin D with CDK4 or CDK6 advances early G1 by phosphorylating the retinoblastoma protein (Rb). Progressive Rb phosphorylation releases E2F, a family of transcription factors (proteins that switch genes on), which drive genes required for DNA synthesis. Cyclin E-CDK2 completes late G1 and triggers entry into S phase. Cyclin A-CDK2 supports S-phase progression and helps prevent re-firing of origins that have already been used. Mitotic entry depends on cyclin B-CDK1, which phosphorylates nuclear lamins (the protein mesh lining the inside of the nuclear envelope), regulators of microtubules (the protein tubes that form the spindle) and other mitotic effectors. Destruction of mitotic cyclins by the anaphase-promoting complex/cyclosome (APC/C), a complex that tags proteins with ubiquitin for breakdown, then permits exit from mitosis and reassembly of G1.
CDK output is sharpened by layers beyond cyclin binding. Inhibitory phosphorylation by Wee1 (a kinase that switches CDK1 off) and activating dephosphorylation by Cdc25 (a phosphatase, an enzyme that removes phosphate groups, which switches CDK1 on) create switch-like mitotic entry. Two families of CDK inhibitor proteins provide direct brakes: the INK4 family, which targets CDK4/6, and the CIP/KIP family, which includes p21 and p27. The first-pass principle is simple: the cyclin decides when a CDK can work; phosphorylation state and stoichiometric inhibitors (inhibitors that bind CDKs one-to-one) decide whether that work is allowed.
Restriction and DNA Checkpoints
The restriction point in late G1 is the classic commitment decision in mammalian cells. Before it, mitogen withdrawal or strong contact inhibition (cells stop dividing when they touch neighbours on all sides) can return the cell towards G0. After it, progression becomes largely independent of external growth factors and the cell is committed to complete the cycle unless a later checkpoint intervenes. Molecularly, the restriction point is thought to be crossed when cyclin D- and cyclin E-dependent CDKs have phosphorylated Rb deeply enough that E2F-driven S-phase gene expression becomes self-reinforcing.
DNA damage checkpoints operate mainly in G1 and G2; a related replication checkpoint monitors S phase. Two sensor kinases (enzymes that detect trouble and relay the alarm) act here. ATM (ataxia telangiectasia mutated) responds principally to DNA double-strand breaks (breaks that cut both strands of the helix) and signals strongly through Chk2 (checkpoint kinase 2). ATR (ataxia telangiectasia and Rad3-related) responds principally to replication stress (problems that slow or stall DNA copying) and stretches of exposed single-stranded DNA, which appear when copying stalls, signalling strongly through Chk1 (checkpoint kinase 1). Their shared output is reduced CDK activity until the DNA is repaired and safe to copy. In G1, p53 is central. It is normally broken down quickly, but it builds up (is stabilised) when DNA damage is detected. The p21 protein, made when p53 switches on its gene, inhibits G1 CDKs, enforcing arrest while repair pathways act. The enzymology of those pathways belongs in DNA Repair and Cancer. If damage is extensive or persistent, the same p53 programme can abandon temporary arrest in favour of apoptosis or permanent senescence (a lasting, non-dividing arrest of a living cell) rather than allowing an unrepaired genome into S phase.
During S phase, stalled replication forks (the Y-shaped points where DNA is being unzipped and copied) and unreplicated DNA activate ATR-Chk1 signalling that slows the start of copying at later origins (late origin firing) and protects fork stability. In G2, residual damage or incomplete replication prevents full activation of cyclin B-CDK1, blocking mitotic entry with an unfinished genome. These delays are not passive pauses. They are decision gates that trade time for fidelity before either DNA synthesis or chromosome segregation can fix an error in both daughters.
The Spindle Checkpoint
Once the cell is in mitosis the dominant risk changes. Incomplete replication gives way to unequal segregation. The spindle assembly checkpoint monitors kinetochore-microtubule attachment (how the kinetochores, the protein patches on the chromosomes, are attached to the microtubules, the protein tubes of the spindle) and inter-kinetochore tension (the pulling force between the two kinetochores of one chromosome). Unattached or incorrectly attached kinetochores generate a diffusible wait-anaphase signal (a chemical signal that spreads through the cell and tells it to wait) that keeps the APC/C from ubiquitinating (tagging with ubiquitin for destruction) securin and cyclin B. Securin is a protein that binds and inhibits separase, the enzyme that cuts cohesin, so cohesin remains intact while the checkpoint is active. Only when every chromosome is bi-oriented (attached to spindle fibres from opposite poles) does the checkpoint silence. APC/C-mediated securin destruction releases separase, which cleaves cohesin so sister chromatids separate and anaphase begins.
Checkpoint failure produces aneuploidy: daughters with missing or extra chromosomes. Aneuploidy is not automatically lethal to the cell, but it is a potent source of genomic instability and a feature of most solid tumours (about 90% show chromosome gains or losses). The spindle checkpoint therefore sits between ordinary mitosis and the chromosomal chaos that later pathology lessons classify under neoplasia.
Intrinsic Apoptosis
When repair cannot restore a safe trajectory, programmed cell death is often preferable to division. Apoptosis is ordered, energy-dependent dismantling of the cell with preservation of plasma-membrane integrity until late. The intrinsic, or mitochondrial, pathway is a major route for genotoxic (DNA-damaging) stress, for activation of oncogenes (genes that drive uncontrolled growth when over-active) and for many developmental cues.
BCL-2 family proteins set the apoptotic threshold at the outer mitochondrial membrane: the family has protective and pro-death members, and apoptosis goes ahead when the pro-death members outweigh the protective ones. Anti-apoptotic members such as BCL-2 and BCL-XL restrain membrane permeabilisation (the membrane becoming leaky). Effectors BAX and BAK, once activated, oligomerise (join together in groups) and create pores. BH3-only sensors (stress-sensing members of the family, of which BIM, PUMA and NOXA are the best known) integrate stress signals and tip the balance towards permeabilisation by inhibiting the protectors or helping activate the effectors. Cytochrome c released into the cytosol binds APAF-1 (apoptotic protease activating factor 1), assembling the apoptosome, a protein complex that activates initiator caspase-9. Caspases are protein-cutting enzymes that carry out the death programme. Caspase-9 cleaves and activates executioner caspases-3 and -7.
The executioner caspases dismantle the nuclear lamina (the protein mesh lining the nucleus), enable DNA cleavage by CAD (caspase-activated DNase, an enzyme that cuts DNA), expose phosphatidylserine (a lipid normally on the inner face of the plasma membrane) on the outer leaflet and package the corpse into membrane-bound apoptotic bodies. Phosphatidylserine on the outside acts as an eat-me signal: neighbouring phagocytes (cells that engulf debris) recognise it and clear the fragments. Because contents are not spilled early, inflammation remains minimal compared with necrotic rupture. Apoptosis is therefore tissue housekeeping as much as cell suicide.
Extrinsic Apoptosis
The extrinsic pathway begins at the plasma membrane. Death receptors of the tumour necrosis factor (TNF) receptor superfamily, including Fas and TRAIL receptors, bind their matching ligands (the signalling molecules that fit them) and recruit adaptor proteins such as FADD (Fas-associated death domain protein). The death-inducing signalling complex (the protein cluster that forms at an activated death receptor) activates initiator caspase-8 and, in some cells, caspase-10. Cells differ in how much the death receptor signal needs the mitochondria. In type I cells, caspase-8 can process executioners directly, without help from the mitochondria. In type II cells, which rely mostly on the mitochondrial route, caspase-8 cleaves BID (a BH3-only protein) to tBID (its active, cut form), which engages mitochondria and amplifies the signal through cytochrome c release. The two routes are therefore not sealed compartments. They converge on the same executioner caspases and the same apoptotic morphology of shrinkage, chromatin condensation and apoptotic-body formation followed by phagocytic clearance.
Immune-mediated killing of infected or transformed (cancerous) target cells uses two main routes: the receptor route, in which Fas ligand or TRAIL on the killer cell engages death receptors on the target, and the release of perforin and granzymes (toxic granule contents that bring about the death of the target cell). Genotoxic stress more often uses the intrinsic route. The examination point is the logic chain: ligand to receptor to initiator caspase, optional mitochondrial amplification, shared executioners, silent clearance.
Intrinsic mitochondrial and extrinsic death-receptor pathways converge on initiator then executioner caspases, producing apoptotic bodies for phagocytic clearance.
SimpleMed original educational diagram
When Control Fails
Controlled proliferation and controlled death are complementary defences of tissue integrity. Cells that ignore the restriction point continue to cycle without receiving adequate mitogen signals. Cells that disable p53 lose a major coupling between DNA damage and either CDK inhibition through p21 or apoptotic elimination, so mutations can be fixed through S phase and mitosis. Overexpression of anti-apoptotic BCL-2 family members raises the threshold for mitochondrial permeabilisation; damaged cells survive signals that should have removed them. Loss of death-receptor signalling or caspase function blunts extrinsic clearance.
None of these defects causes a tumour on its own. Each removes a quality-control step that would otherwise limit clonal expansion (growth of a group of cells descended from one altered cell) of an altered genome. Checkpoint failure allows error-prone genomes to be copied and segregated. Apoptotic failure allows those genomes to persist when death would have been safer than repair. Together they convert the cell cycle from a programme of safe renewal into a vehicle for propagating damage. The clinical taxonomy of dysplasia, invasion and metastasis is developed in Introduction to Neoplasia. The mechanistic reading for this lesson is narrower and sufficient. Cyclin-CDK engines drive the cycle. Checkpoints decide whether progress is safe. Apoptosis removes the cell when safety cannot be restored.
Reviewed by: Dr. Marcus Judge
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