Cell Cycle, Mitosis, and Cancer

The eukaryotic cell cycle is divided into four sequential phases. G1 phase (Gap 1) is the primary growth period: the cell increases in size, synthesizes proteins and organelles, and carries out its differentiated functions. G1 is also the phase where the cell commits to division at the Restriction Point (in animal cells). Cells that exit the cycle enter a quiescent state called G0 — some reversibly (e.g., hepatocytes, lymphocytes), others permanently (e.g., neurons, skeletal muscle). S phase (Synthesis) is devoted entirely to DNA replication; each chromosome is duplicated to produce two identical sister chromatids held together by cohesin rings at the centromere. The centrosome also duplicates during S phase. G2 phase (Gap 2) is a second growth and preparation period: the cell synthesizes proteins required for mitosis (e.g., tubulin for spindle fibers), checks for DNA damage, and verifies that replication is complete. M phase (Mitosis) encompasses both nuclear division (mitosis proper: prophase, prometaphase, metaphase, anaphase, telophase) and cytoplasmic division (cytokinesis). The entire cycle duration varies by cell type — mammalian cells in culture typically complete one cycle in 18–24 hours, with mitosis taking only ~1 hour. The relative length of G1 is the most variable and determines overall cycle duration.

Mitosis partitions duplicated chromosomes into two genetically identical daughter nuclei. Prophase: Chromatin condenses into visible chromosomes (each consisting of two sister chromatids joined at the centromere). The centrosomes migrate to opposite poles and begin nucleating microtubules to form the mitotic spindle. Prometaphase: The nuclear envelope breaks down, allowing spindle microtubules to access chromosomes. Kinetochores — protein complexes assembled at each centromere — attach to microtubules extending from opposite spindle poles (bipolar attachment). Chromosomes congress toward the metaphase plate. Metaphase: All chromosomes align at the metaphase plate (equatorial plane), with sister kinetochores attached to microtubules from opposite poles. The spindle assembly checkpoint (SAC) delays anaphase until every kinetochore is properly attached and under tension. Anaphase: Begins when separase cleaves the cohesin rings holding sister chromatids together. Sister chromatids separate and are pulled toward opposite poles (anaphase A, kinetochore microtubules shorten) while the spindle poles move apart (anaphase B, polar microtubules elongate and slide via kinesin motors). Telophase: Chromosomes decondense, nuclear envelopes reassemble around each set of separated chromatids using ER membrane fragments and lamins, and the mitotic spindle disassembles. Cytokinesis (overlapping late anaphase and telophase): In animal cells, a contractile ring of actin and myosin II assembles beneath the plasma membrane at the cleavage furrow, constricting to pinch the cell into two daughters. In plant cells, vesicles from the Golgi deliver cell-wall material to form a cell plate that grows outward to divide the cell.

The cell-cycle engine is driven by cyclin-dependent kinases (CDKs), which are constitutively present but inactive. Their activity is controlled by binding to cyclins — regulatory subunits whose abundance oscillates through the cell cycle. G1/S cyclins (cyclin D) accumulate in G1 and activate CDK4/6, which phosphorylate the retinoblastoma (Rb) protein to release E2F transcription factors, driving expression of genes needed for S phase entry. S cyclins (cyclin E, cyclin A) bind CDK2 to initiate and sustain DNA replication. M cyclins (cyclin B) bind CDK1 (also called Cdc2) to drive mitotic entry: CDK1-cyclin B phosphorylates lamins (nuclear envelope breakdown), condensins (chromosome condensation), and microtubule-associated proteins (spindle assembly). Cyclin levels are controlled by regulated synthesis (transcription) and destruction (ubiquitin-proteasome pathway). The anaphase-promoting complex/cyclosome (APC/C) is an E3 ubiquitin ligase that targets securin and M cyclins for degradation at the metaphase-to-anaphase transition and during G1, respectively. CDK activity is further regulated by inhibitory phosphorylation (Wee1 kinase phosphorylates CDK1 at Tyr15; Cdc25 phosphatase removes this phosphate) and by CDK inhibitor proteins (CKIs) — the INK4 family (p16, p15) inhibits CDK4/6, while the Cip/Kip family (p21, p27) inhibits a broader range of CDK-cyclin complexes.

Checkpoints are surveillance mechanisms that halt the cell cycle when conditions are unfavorable or errors are detected. G1/S checkpoint (Restriction Point): Assesses cell size, nutrient availability, growth factor signals, and DNA integrity. The key molecular decision is whether to phosphorylate Rb. If conditions are insufficient, Rb remains hypophosphorylated and sequesters E2F, blocking S phase entry. DNA damage at this point activates p53, which induces p21 — a CKI that inhibits CDK-cyclin complexes and enforces G1 arrest. G2/M checkpoint: Detects unreplicated or damaged DNA before the cell commits to mitosis. Damage activates ATM/ATR kinases, which signal through Chk1/Chk2 to inhibit Cdc25 phosphatase. Without Cdc25 activity, CDK1 remains phosphorylated (inactive) by Wee1, and the cell arrests in G2. Spindle assembly checkpoint (SAC): Operates during prometaphase and metaphase. Unattached kinetochores or kinetochores lacking tension generate a wait signal by recruiting Mad2, BubR1, and other SAC proteins, which inhibit Cdc20, the activator of APC/C. When all kinetochores achieve proper bipolar attachment, the SAC is satisfied, Cdc20 activates APC/C, securin is ubiquitinated and degraded, and separase is released to cleave cohesin — triggering anaphase onset.

Cancer arises from mutations in genes that regulate cell division, differentiation, and death. Proto-oncogenes are normal genes whose protein products promote cell-cycle progression (growth factors, receptors, signal transducers, transcription factors). Gain-of-function mutations (point mutations, gene amplification, chromosomal translocation) convert them into oncogenes that drive unregulated proliferation. Classic examples: Ras (GTPase — mutations lock it in GTP-bound active state, found in ~30% of human cancers); Myc (transcription factor — amplified in neuroblastoma, translocated in Burkitt lymphoma); HER2/ErbB2 (RTK amplified in breast cancer); BCR-ABL (fusion kinase from Philadelphia chromosome translocation in CML, constitutively active). Oncogenes are dominant at the cellular level. Tumor suppressor genes encode proteins that restrain cell-cycle progression, promote DNA repair, or induce apoptosis. Loss-of-function mutations in both alleles (Knudson's two-hit hypothesis) are typically required. Key tumor suppressors: Rb — the gatekeeper of the Restriction Point; p53 — guardian of the genome, mutated in over 50% of human cancers, induces p21 (arrest), Bax/Puma (apoptosis); BRCA1/BRCA2 — homologous recombination repair; APC — negative regulator of Wnt signaling; PTEN — antagonizes PI3K-Akt signaling.

Apoptosis is programmed cell death executed by caspases — cysteine proteases that dismantle the cell in an orderly fashion (membrane blebbing, chromatin condensation, DNA fragmentation, formation of apoptotic bodies). Two pathways: Intrinsic (mitochondrial) pathway — activated by DNA damage or growth factor withdrawal; pro-apoptotic BH3-only proteins (Bim, Puma) activate Bax/Bak, which release cytochrome c; cytochrome c binds Apaf-1 to form the apoptosome, activating caspase-9 then executioner caspases-3/7. Extrinsic (death receptor) pathway — triggered by extracellular death ligands (FasL, TRAIL) binding receptors, recruiting caspase-8 via DISC. Cancer cells evade apoptosis by upregulating anti-apoptotic Bcl-2 family proteins or mutating p53. Cellular senescence is irreversible proliferative arrest triggered by telomere shortening, oncogene activation (OIS), or severe damage — a potent tumor-suppressive mechanism. Cancer cells bypass senescence via telomerase (hTERT) upregulation (~90% of cancers) or ALT pathway. The hallmarks of cancer (Hanahan & Weinberg): sustained proliferative signaling, evasion of growth suppressors, resistance to cell death, replicative immortality, induction of angiogenesis, and activation of invasion and metastasis — enabled by genome instability and tumor-promoting inflammation.

How it works

CDK-Cyclin Regulation: The Molecular Engine

1. G1 (growth factor sensing): Mitogenic signals → Ras-MAPK and PI3K-Akt → cyclin D transcription. Cyclin D-CDK4/6 phosphorylates Rb → partial E2F release → cyclin E transcription.
2. G1/S transition: CDK2-cyclin E hyperphosphorylates Rb → full E2F release → S phase genes (polymerases, histones, cyclin A). CDK2-cyclin E also targets p27 for degradation — positive feedback.
3. S phase: CDK2-cyclin A phosphorylates replication licensing factors; cohesin stabilizes sister chromatids.
4. G2/M (bistable switch): Cyclin B accumulates; CDK1-cyclin B held inactive by Wee1 (Tyr15-P). Cdc25 removes the phosphate → CDK1 active → phosphorylates and activates more Cdc25 while inhibiting Wee1 → all-or-none mitotic commitment.
5. Anaphase onset: APC/C-Cdc20 ubiquitinates securin → separase released → cohesin cleaved → chromatids separate. APC/C then destroys cyclin B → CDK1 inactivation → mitotic exit.
6. G1 reset: APC/C-Cdh1 maintains low cyclin levels until the next cycle.

Checkpoint Mechanisms: p53 and Rb Pathways

Rb pathway: Hypophosphorylated Rb binds E2F — block. CDK4/6-cyclin D, then CDK2-cyclin E, sequentially phosphorylate Rb → E2F released. DNA damage, TGF-β, or senescence upregulate CKIs (p16, p21), inhibiting CDKs and keeping Rb active. p53 pathway: Mdm2 ubiquitinates p53 for degradation under normal conditions. DNA damage activates ATM/ATR → phosphorylate p53 → stabilize and tetramerize → induce p21 (arrest), Bax/Puma (apoptosis), GADD45 (repair). p53 also induces Mdm2 — negative feedback ensures transient pulses unless damage persists.

Comparisons

• Biochemistry: CDK enzyme kinetics — ATP binding, serine/threonine phosphorylation, allosteric cyclin regulation. Ubiquitin-proteasome pathway (E1-E2-E3 cascade, APC/C as E3 ligase). Ras as a GTPase switch — oncogenic mutations lock it in GTP-bound active state.
• Genetics/Molecular Biology: Oncogene activation — point mutation (Ras G12V), gene amplification (HER2, Myc), translocation (BCR-ABL). Tumor suppressor inactivation — LOH, promoter methylation. Knudson's two-hit model (hereditary vs. sporadic retinoblastoma).
• Organ Systems: Tissue proliferative classification — labile (continuously dividing: skin, intestinal epithelium, bone marrow), stable (quiescent but can re-enter: liver, kidney), permanent (non-dividing: neurons, cardiac/skeletal muscle). Explains differential cancer susceptibility.
• Evolution/Mutation: Cancer as somatic evolution — sequential acquisition of driver mutations, clonal expansion, tumor heterogeneity as therapeutic challenge.
• Pharmacology: CDK4/6 inhibitors (palbociclib) for ER+ breast cancer; PARP inhibitors (olaparib) for BRCA-mutant cancers (synthetic lethality); imatinib targeting BCR-ABL in CML; venetoclax targeting Bcl-2 in CLL.

Common confusions

• Cyclin levels oscillate dramatically; CDK levels remain constant. Students often reverse this. MCAT shows protein-level graphs and asks which is a cyclin.
• Mitosis stage order: Prophase (condensation) → Prometaphase (nuclear envelope breakdown) → Metaphase (alignment at plate) → Anaphase (sister chromatids separate — separase cleaves cohesin) → Telophase (nuclear envelope reassembles). Drug-arrest questions test morphology at each stage.
• Oncogene = dominant (one mutant allele suffices for transformation); tumor suppressor = recessive at the cellular level (both alleles must be lost — Knudson's two-hit). MCAT tests this with familial cancer inheritance patterns.
• p53 is the damage sensor (induces p21); Rb is the direct G1/S gate (binds E2F). p53 loss removes the alarm; Rb loss removes the gate directly. Mutations in either cause cancer but via different mechanisms.
• Apoptosis = programmed, ATP-dependent, orderly, non-inflammatory (blebbing, apoptotic bodies). Necrosis = pathological, ATP-independent, chaotic, inflammatory (membrane rupture). DNA laddering vs. random smear distinguishes them electrophoretically.
• Sister chromatids separate in mitosis (anaphase) and meiosis II. Homologous chromosomes separate in meiosis I. Know which structure separates when before answering ploidy questions.

Quick review

• Cell cycle: G1 (growth + Restriction Point) → S (DNA replication + centrosome duplication) → G2 (growth + damage check) → M (mitosis + cytokinesis). G0 = quiescent exit.
• Mitosis (PPMAT): Prophase (condensation), Prometaphase (nuclear envelope breakdown), Metaphase (alignment at plate), Anaphase (cohesin cleaved by separase, chromatids separate), Telophase (nuclear envelope reassembles). Cytokinesis: actin-myosin ring (animals) or cell plate (plants).
• Cyclins oscillate; CDK levels constant. G1/S: cyclin D-CDK4/6. G1→S: cyclin E-CDK2. S: cyclin A-CDK2. M: cyclin B-CDK1 (Cdc2).
• CDK regulation: cyclin binding (+), Wee1 phosphorylation (− at Tyr15), Cdc25 phosphatase (+ removes inhibitory phosphate), CKIs: p16 (CDK4/6), p21/p27 (broad CDK inhibition).
• APC/C (E3 ubiquitin ligase): APC/C-Cdc20 targets securin (triggers anaphase); APC/C-Cdh1 targets cyclin B (mitotic exit), maintains G1.
• G1/S checkpoint: Rb binds E2F (block); CDKs phosphorylate Rb → E2F released → S phase. p53 induces p21 → inhibits CDKs → Rb stays active.
• G2/M checkpoint: ATM/ATR → Chk1/Chk2 → inhibit Cdc25 → CDK1 stays inactive (Wee1 phosphorylation). SAC: unattached kinetochores → Mad2/BubR1 → inhibit APC/C-Cdc20.
• p53: activated by DNA damage (ATM/ATR); induces p21 (arrest), Bax/Puma (apoptosis), GADD45 (repair). Degraded by Mdm2 in unstressed cells. Mutated in >50% of cancers.
• Oncogenes (dominant, gain-of-function): Ras (GTPase stuck ON), Myc (amplified), BCR-ABL (fusion kinase). Tumor suppressors (recessive, two-hit): Rb, p53, BRCA1/2, APC, PTEN.
• Apoptosis: intrinsic (cytochrome c → Apaf-1 → apoptosome → caspase-9 → caspase-3/7) and extrinsic (death receptor → DISC → caspase-8 → caspase-3/7). Bcl-2 family: anti-apoptotic (Bcl-2, Bcl-xL) vs. pro-apoptotic (Bax, Bak, Bim, Puma). Hallmarks of cancer: 6 core capabilities + 2 enabling characteristics.