# Lecture 14: Cell Cycle and Mitosis ## Unit 1.1: Foundations of Medicine & Medical Sciences --- ## Learning Objectives By the end of this lecture, students will be able to: 1. Describe the phases of the cell cycle and the key events that occur in each phase 2. Explain the molecular mechanisms of cell cycle regulation including cyclins, CDKs, and checkpoints 3. Identify the stages of mitosis and describe the key events in each stage 4. Describe the roles of oncogenes and tumor suppressor genes in cell cycle control 5. Explain how cell cycle dysregulation contributes to cancer development 6. Identify therapeutic targets in the cell cycle for cancer treatment --- ## Overview of the Cell Cycle ### Definition and Purpose The cell cycle is the ordered sequence of events through which a cell duplicates its contents and divides into two daughter cells. This fundamental process underlies growth, development, tissue renewal, and wound healing. For cell division to be successful, the cell must accurately replicate its genome and then precisely distribute the duplicated chromosomes to each daughter cell. Errors in this process can lead to cell death or, if the cell survives with genomic abnormalities, to cancer. ### Cell Cycle Phases The cell cycle consists of four main phases. The first gap phase (G1) is a period of growth during which the cell prepares for DNA synthesis by accumulating the raw materials and enzymes needed for replication. G1 is highly variable in duration—it can last hours in rapidly dividing cells or extend indefinitely in cells that have exited the cycle. The synthesis phase (S) is when DNA replication occurs, producing an exact copy of each chromosome. S phase typically lasts 6-8 hours and requires precise coordination to ensure that every region of the genome is replicated exactly once. The second gap phase (G2) follows, during which the cell continues to grow and prepares for mitosis by synthesizing proteins needed for chromosome segregation. G2 typically lasts 3-4 hours. Finally, the mitotic phase (M) is when the cell divides its duplicated chromosomes and splits into two daughter cells. Mitosis itself is rapid, typically lasting only about an hour. The three phases G1, S, and G2 are collectively called interphase, during which the cell carries out its normal functions and the chromosomes exist as diffuse chromatin rather than condensed structures. Interphase comprises approximately 95% of the total cell cycle time. A complete cell cycle in a rapidly dividing human cell takes approximately 24 hours, though this varies widely among cell types. ### The G0 Phase Not all cells are actively cycling. Many cells exit the cell cycle from G1 and enter a quiescent state called G0. These cells are metabolically active and perform their specialized functions but do not prepare for division. Some cells remain in G0 temporarily and can re-enter the cycle when stimulated by appropriate signals—this is the case for many resting cells that can be recruited to divide during tissue repair. Other cells, such as neurons and skeletal muscle cells, are terminally differentiated and have permanently exited the cycle; they will never divide again. Understanding G0 is clinically important because many cancers arise from cells that have escaped normal quiescence signals. Panel A: Circular clock-like cell cycle diagram divided into colored sectors: G1 (largest, light blue, ~40-50%), S phase (green, ~30%), G2 (yellow, ~15%), and M phase (red, ~5%), with time durations for each phase. Panel B: Arrow pointing from G1 to a separate smaller circle labeled "G0 (Quiescence)" showing exit from the active cycle, with a legend indicating interphase encompasses G1+S+G2. Panel C: Three checkpoints marked with stop-sign symbols: the G1/S checkpoint (also labeled "Restriction Point") and the G2/M checkpoint at their respective boundaries. Panel D: The Spindle Assembly Checkpoint within M phase, with all three checkpoint positions highlighted on the circular diagram to show their surveillance roles. --- ## Cell Cycle Regulation ### Cyclins and CDKs The cell cycle is driven forward by a family of protein kinases called cyclin-dependent kinases (CDKs). As their name implies, CDKs require binding to regulatory proteins called cyclins for their catalytic activity. CDKs are serine/threonine kinases that, when active, phosphorylate specific target proteins to trigger the molecular events that advance the cell through each phase of the cycle. A key insight is that CDK protein levels remain relatively constant throughout the cell cycle—what changes is the availability of their cyclin partners. Cyclin levels oscillate dramatically, rising and falling in a predictable pattern as each cyclin is synthesized, activates its CDK partner, and is then destroyed by ubiquitin-mediated proteolysis. This cyclic synthesis and destruction creates the periodic CDK activity that drives the cell forward through sequential phases. Different cyclin-CDK complexes dominate at different phases. In G1, cyclin D pairs with CDK4 and CDK6, responding to mitogenic signals and initiating the commitment to divide. At the G1/S boundary, cyclin E activates CDK2 to trigger entry into S phase. During S phase, cyclin A takes over as the CDK2 partner, orchestrating DNA replication. Finally, cyclin B accumulates and activates CDK1 (also called CDC2) to drive the cell into mitosis at the G2/M transition. Panel A: Graph with x-axis showing cell cycle phases (G1, S, G2, M) and y-axis showing cyclin protein levels, with Cyclin D (orange) expressed at low levels throughout G1 in response to growth factors. Panel B: Cyclin E (blue) peaking sharply at the G1/S transition then falling, and Cyclin A (green) rising during S phase and G2 then dropping in mitosis. Panel C: Cyclin B (purple) accumulating during G2 and peaking in mitosis, then falling abruptly at the metaphase-to-anaphase transition. Panel D: Bar below the graph indicating corresponding CDK partners for each cyclin: CDK4/6 for Cyclin D, CDK2 for Cyclins E and A, CDK1 for Cyclin B. ### CDK Activation Full activation of CDK complexes requires more than just cyclin binding. After cyclin association, the CDK must be phosphorylated on a specific threonine residue by a kinase called CAK (CDK-activating kinase). Additionally, inhibitory phosphorylations on tyrosine and threonine residues (added by kinases Wee1 and Myt1) must be removed by the Cdc25 family of phosphatases. Only when the cyclin is bound, the activating phosphate is in place, and the inhibitory phosphates are removed does the CDK achieve full activity. This multi-step activation allows multiple levels of control. The cell can accumulate cyclin-CDK complexes but keep them inactive by maintaining inhibitory phosphorylations; then, by activating Cdc25 phosphatases, it can rapidly switch on the CDK activity when the time is right. This is particularly important at the G2/M transition, where sudden, switch-like activation of CDK1 commits the cell to mitosis. ### CDK Inhibitors Two families of CDK inhibitor proteins (CKIs) provide additional control by blocking CDK activity. The INK4 family (inhibitors of CDK4, including p16, p15, p18, and p19) specifically binds CDK4 and CDK6, preventing their association with cyclin D. Loss of p16 function is one of the most common alterations in human cancer, reflecting the importance of restraining G1 progression. The CIP/KIP family (p21, p27, and p57) inhibits a broader range of CDK complexes. Notably, p21 is a transcriptional target of p53 and mediates cell cycle arrest in response to DNA damage. When DNA damage activates p53, p21 levels rise, inhibiting CDK complexes and halting the cell cycle to allow repair. This connection links the cell cycle machinery to genome surveillance. --- ## Cell Cycle Checkpoints ### Purpose of Checkpoints Checkpoints are surveillance mechanisms that monitor critical processes and halt cell cycle progression if something is wrong. They function as quality control, ensuring that each phase is completed successfully before the next begins. If problems are detected—DNA damage, incomplete replication, improper chromosome attachment—checkpoints delay or arrest the cycle, allowing time for repair or, if damage is too severe, triggering cell death. Checkpoint failure allows cells with damaged or incompletely replicated DNA to divide, propagating errors that can lead to cancer. ### The G1/S Checkpoint (Restriction Point) The most critical decision point in the cell cycle is the G1/S checkpoint, also called the restriction point. Before this point, progression depends on external signals—growth factors must be present, nutrients must be adequate, and the cell must reach a minimum size. After passing the restriction point, the cell is committed to complete the cycle regardless of external conditions. The G1/S checkpoint assesses several parameters. Is the cell large enough? Are nutrients sufficient? Are growth factor signals present? Most importantly, is the DNA intact? If DNA damage is detected, the checkpoint blocks S phase entry to prevent replication of damaged templates. The key molecular players are the tumor suppressor proteins p53 and Rb, along with the CDK inhibitor p21. DNA damage stabilizes p53, which induces p21, which inhibits cyclin-CDK complexes, preventing Rb phosphorylation and blocking S phase entry. ### The G2/M Checkpoint Before entering mitosis, the cell must ensure that DNA replication is complete and that any DNA damage has been repaired. The G2/M checkpoint monitors these conditions. The ATM and ATR kinases are master sensors of DNA damage; they activate checkpoint kinases Chk1 and Chk2, which in turn inhibit the Cdc25 phosphatases that would otherwise activate CDK1. As a result, cyclin B-CDK1 remains inactive, and the cell arrests in G2. This checkpoint is clinically relevant because many cancer therapies work by damaging DNA. Cancer cells that have lost checkpoint function (for example, through p53 mutation) may proceed into mitosis with damaged DNA, leading to catastrophic mitotic errors and cell death—a phenomenon called mitotic catastrophe that contributes to the therapeutic efficacy of DNA-damaging drugs. ### The Spindle Assembly Checkpoint The spindle assembly checkpoint (SAC) operates during mitosis to ensure that all chromosomes are properly attached to the mitotic spindle before sister chromatids separate. Even a single unattached kinetochore generates a "wait" signal that prevents anaphase onset. The checkpoint proteins Mad1, Mad2, BubR1, and others localize to unattached kinetochores and inhibit the anaphase-promoting complex (APC/C), the ubiquitin ligase that triggers sister chromatid separation. Only when every chromosome achieves bipolar attachment—with sister kinetochores attached to microtubules from opposite spindle poles—is the SAC satisfied. The checkpoint then switches off, APC/C becomes active, and anaphase proceeds. Defects in the SAC lead to chromosome missegregation and aneuploidy, a hallmark of cancer cells. Panel A: G1/S checkpoint showing a cell in G1 with DNA damage (lightning bolt symbol), with the pathway: DNA damage leads to ATM/ATR activation, p53 stabilization, p21 induction, CDK inhibition, and STOP, with outcome of cell cycle arrest and DNA repair. Panel B: G2/M checkpoint showing a cell in G2 with incomplete replication or damage, with the pathway: DNA damage/incomplete replication leads to ATM/ATR, Chk1/Chk2, Cdc25 inhibition, CDK1 inactive, and STOP at the G2/M boundary. Panel C: Spindle assembly checkpoint showing a mitotic cell with chromosomes, some properly attached (green) and one unattached (red) to spindle microtubules, with Mad2 at the unattached kinetochore inhibiting APC/C. Panel D: Checkpoint satisfaction showing all chromosomes properly attached (all green), APC/C activated, and anaphase proceeding, with each checkpoint displaying a "GO" versus "STOP" decision outcome. --- ## The Rb Pathway ### The Retinoblastoma Protein The Rb protein, named for its discovery through the childhood eye cancer retinoblastoma, is the master regulator of the G1/S transition. Rb functions as a tumor suppressor by restricting cell cycle progression until appropriate conditions are met. Its mechanism involves controlling the E2F family of transcription factors, which activate genes required for S phase. In its active, hypophosphorylated state, Rb binds E2F and prevents it from activating transcription. Rb does more than simply sequester E2F—it actively represses E2F target genes by recruiting chromatin-modifying enzymes that establish a repressive chromatin state. This double-negative control ensures tight restriction of S phase gene expression. ### The Rb Pathway Mechanism When growth factors stimulate the cell, signaling pathways activate expression of cyclin D. The resulting cyclin D-CDK4/6 complexes begin phosphorylating Rb. This partial phosphorylation weakens Rb's grip on E2F, allowing some E2F to become active and induce expression of cyclin E. Cyclin E-CDK2 then extensively phosphorylates Rb, completely inactivating it and releasing the remaining E2F. Free E2F drives transcription of genes encoding the machinery for DNA synthesis—DNA polymerases, nucleotide biosynthesis enzymes, origin licensing factors—as well as more cyclin E (positive feedback) and cyclin A (for S phase). This creates a switch-like commitment to S phase: once Rb is sufficiently phosphorylated, the positive feedback loop drives full E2F activation, and the cell proceeds through S phase even if external growth signals are removed. The Rb pathway explains why loss of Rb, overexpression of cyclin D, loss of p16 (which normally restrains CDK4/6), or amplification of CDK4/6 all have similar oncogenic effects—they all lead to inappropriate E2F activity and uncontrolled S phase entry. Panel A: Growth-inhibited cell showing Rb (large gray oval) hypophosphorylated and bound to E2F (smaller blue shape), with S phase genes silenced (red X) and CKIs (p16, p21) restraining CDK complexes. Panel B: Growth factor signals (arrow from outside the cell) leading to Cyclin D expression, with Cyclin D-CDK4/6 (yellow complex) partially phosphorylating Rb (small P symbols), releasing some E2F to activate Cyclin E transcription. Panel C: Cyclin E-CDK2 (green complex) fully phosphorylating Rb (many P symbols), completely freeing E2F to activate target genes (Cyclin E, Cyclin A, DNA pol, thymidine kinase) leading to S phase entry. Panel D: Positive feedback loop showing E2F inducing Cyclin E, which drives more Rb phosphorylation, creating a switch-like commitment to S phase that becomes independent of external growth signals. --- ## The p53 Pathway ### p53: Guardian of the Genome The p53 protein has been called the "guardian of the genome" because of its central role in preventing propagation of cells with damaged DNA. It is a transcription factor that responds to various cellular stresses—DNA damage, hypoxia, oncogene activation, nucleotide depletion—by activating genes that cause cell cycle arrest, promote DNA repair, or trigger apoptosis. The TP53 gene is the most commonly mutated gene in human cancer, altered in more than 50% of tumors, underscoring its importance as a tumor suppressor. ### Activation of p53 Under normal conditions, p53 is kept at low levels through rapid turnover. The protein MDM2 (murine double minute 2) binds p53, inhibits its transcriptional activity, and targets it for ubiquitin-mediated degradation. The half-life of p53 is normally only about 20 minutes. When DNA damage occurs, the ATM and ATR kinases are activated and in turn activate the checkpoint kinases Chk1 and Chk2. These kinases phosphorylate p53 at sites that prevent MDM2 binding. Unable to be degraded, p53 accumulates and activates transcription of its target genes. Additionally, ARF (an alternative reading frame product of the CDKN2A locus) can sequester MDM2 in response to oncogenic signals, providing another route to p53 activation. ### Outcomes of p53 Activation The cellular response to p53 activation depends on the severity and type of stress. For repairable damage, p53 induces cell cycle arrest, primarily through activation of p21, which inhibits CDK complexes and prevents S phase entry or mitosis. p53 also activates DNA repair genes like GADD45. Given time, the damage can be repaired, p53 levels fall (partly through MDM2, which is itself a p53 target—negative feedback), and the cell resumes cycling. For severe or irreparable damage, p53 tips the balance toward apoptosis by activating pro-apoptotic genes including BAX, PUMA, and NOXA. These proteins trigger mitochondrial outer membrane permeabilization and caspase activation, leading to cell death. This eliminates cells that might otherwise propagate dangerous mutations. A third possible outcome is senescence—a permanent state of growth arrest in which the cell remains metabolically active but never divides again. Senescent cells may accumulate with aging and contribute to tissue dysfunction and inflammation. Panel A: Stress signals (DNA damage with lightning bolt, hypoxia symbol, oncogene activation) converging on p53 regulation, with unstressed cells showing p53 (brown oval) bound by MDM2 (black shape) adding ubiquitin chains for proteasomal degradation. Panel B: Stressed cells showing ATM/ATR activating Chk1/Chk2 to phosphorylate p53, preventing MDM2 binding, allowing free p53 to accumulate and enter the nucleus as a tetramer bound to DNA at p53 response elements. Panel C: Three outcome pathways branching from active p53: cell cycle arrest (p53 induces p21 inhibiting Cyclin-CDK), DNA repair (p53 induces GADD45), and apoptosis (p53 induces BAX, PUMA, NOXA at mitochondria releasing cytochrome c and activating caspases). Panel D: MDM2 negative feedback loop showing p53 inducing MDM2 transcription, which in turn targets p53 for degradation, creating autoregulatory control of p53 levels. --- ## Mitosis ### Overview Mitosis is the process by which a cell divides its duplicated chromosomes equally between two daughter cells. Although it comprises only about 5% of the total cell cycle time, mitosis is a dramatic period of intense activity in which the cell's architecture is completely reorganized. The chromosomes condense, the nuclear envelope breaks down, a microtubule-based spindle apparatus assembles, the chromosomes align and then separate, and finally the cytoplasm divides. Errors during mitosis can lead to aneuploidy—cells with abnormal chromosome numbers—which is a hallmark of cancer. ### Prophase Mitosis begins with prophase, during which the replicated chromosomes, each consisting of two identical sister chromatids joined at the centromere, condense from diffuse chromatin into compact, visible structures. This condensation, driven by condensin complexes, is essential for the chromosomes to move without tangling. Meanwhile, the two centrosomes (duplicated during S phase) begin migrating to opposite sides of the cell, organizing microtubules into the beginnings of the mitotic spindle. The nuclear envelope remains intact early in prophase but begins to break down near the end. ### Prometaphase Prometaphase is marked by complete breakdown of the nuclear envelope, accomplished by phosphorylation of nuclear lamins by CDK1. This allows spindle microtubules to access the chromosomes. Each sister chromatid pair has a kinetochore assembled at its centromere—a protein structure that serves as the attachment point for spindle microtubules. Microtubules emanating from the spindle poles search the cytoplasm and capture chromosomes by binding their kinetochores. This is a dynamic process: chromosomes are pulled back and forth as attachments form and correct. ### Metaphase During metaphase, all chromosomes achieve bipolar attachment and align at the metaphase plate, an imaginary plane equidistant from the two spindle poles. Each chromosome is attached to microtubules from both poles—one sister kinetochore connected to one pole, the other to the opposite pole. The tension created by this bipolar attachment is sensed by the spindle assembly checkpoint. Only when all chromosomes are properly attached does the checkpoint allow progression to anaphase. ### Anaphase Anaphase begins abruptly when the spindle assembly checkpoint is satisfied and the anaphase-promoting complex (APC/C) is activated. APC/C triggers destruction of securin, releasing the protease separase, which cleaves the cohesin proteins that have been holding sister chromatids together. The sisters separate and move toward opposite poles. Anaphase A involves shortening of kinetochore microtubules, pulling chromosomes poleward. Anaphase B involves elongation of the spindle as polar microtubules slide past each other, pushing the poles apart. By the end of anaphase, a complete set of chromosomes has reached each pole. ### Telophase and Cytokinesis During telophase, the separated chromosomes arrive at the poles and begin to decondense. Nuclear envelopes reform around each chromosome set, reassembling from membrane vesicles and involving dephosphorylation of lamins. The mitotic spindle disassembles. Cytokinesis, the physical division of the cytoplasm, typically begins during anaphase and completes during telophase. A contractile ring composed of actin filaments and myosin motors assembles at the cell equator and constricts, forming a cleavage furrow that deepens until the cell is pinched in two. The result is two daughter cells, each with a complete diploid genome. Panel A: Prophase showing chromosomes condensing as X-shaped structures (sister chromatids joined at centromere), two centrosomes (paired purple dots with radiating microtubules) migrating apart, and the nuclear envelope (dotted line) fragmenting; followed by prometaphase with the nuclear envelope gone, spindle microtubules (green lines) extending from poles and attaching to kinetochores (red dots), and chromosomes scattered. Panel B: Metaphase showing all chromosomes aligned at the metaphase plate (center of cell) with bipolar attachment (microtubules to both poles) and the spindle fully formed. Panel C: Anaphase showing sister chromatids separated and moving toward opposite poles, with the cell elongating, kinetochore microtubules shortening, and polar microtubules lengthening. Panel D: Telophase showing separated chromosomes reaching poles and decondensing, nuclear envelopes reforming (dotted ovals around each chromosome mass), and cytokinesis with two daughter cells nearly separated by a cleavage furrow, each containing a reformed nucleus with decondensed chromatin. ### The Mitotic Spindle The mitotic spindle is a remarkable machine built from microtubules and associated proteins. Three types of microtubules contribute to spindle function. Kinetochore microtubules extend from the poles to the kinetochores of chromosomes, providing the tracks along which chromosomes move. Polar (or interpolar) microtubules extend from each pole toward the cell center, where they overlap with microtubules from the opposite pole; motor proteins walking along these overlapping microtubules push the poles apart during anaphase B. Astral microtubules radiate outward from the poles to the cell cortex, helping position and anchor the spindle. The dynamic instability of microtubules—their constant growth and shrinkage—is essential for the search-and-capture mechanism by which chromosomes become attached. Motor proteins including dynein and various kinesins move along microtubules and generate the forces that position chromosomes and separate chromatids. ### Cytokinesis in Detail The cleavage furrow that divides the cytoplasm is generated by a contractile ring assembled from actin filaments and myosin II motor proteins. The ring assembles at the cell equator, perpendicular to the spindle axis, with its position specified by signals from the central spindle (the overlapping polar microtubules between the separating chromosome masses). Myosin motors slide actin filaments past each other, constricting the ring like a purse string. The furrow progressively deepens until only a thin intercellular bridge remains, which is eventually severed by a process called abscission. --- ## Oncogenes and Tumor Suppressors ### Oncogenes Proto-oncogenes are normal genes that encode proteins promoting cell growth and division. When mutated or abnormally expressed, they become oncogenes that drive uncontrolled proliferation. Oncogenic mutations are typically gain-of-function—they produce a protein that is hyperactive, overexpressed, or active at inappropriate times. Because only one mutant allele is sufficient to produce the abnormal protein, oncogene mutations are dominant at the cellular level. RAS proteins are small GTPases that transmit growth factor signals; mutations locking RAS in its active GTP-bound state cause continuous signaling even in the absence of growth factors. MYC is a transcription factor that drives expression of genes promoting proliferation; its overexpression (often through gene amplification or translocation) is common in many cancers. HER2/ERBB2 is a growth factor receptor amplified in approximately 20% of breast cancers; the excess receptors activate signaling pathways even without ligand. The BCR-ABL fusion kinase, created by the Philadelphia chromosome translocation, has constitutive kinase activity that drives chronic myelogenous leukemia. ### Tumor Suppressors Tumor suppressor genes encode proteins that restrain cell proliferation, promote DNA repair, or trigger cell death in damaged cells. Loss of tumor suppressor function removes these brakes on growth. Most tumor suppressors follow the "two-hit hypothesis" proposed by Alfred Knudson: both alleles must be inactivated for tumor suppressor function to be lost. The first hit may be inherited (germline mutation, causing cancer predisposition) or acquired; the second hit (somatic mutation, deletion, or silencing) then eliminates the remaining functional allele. RB1, the prototype tumor suppressor, controls the G1/S checkpoint; its loss unleashes E2F activity and uncontrolled proliferation. TP53, the most commonly mutated gene in cancer, is the guardian of the genome; its loss allows cells with DNA damage to proliferate. BRCA1 and BRCA2 are essential for homologous recombination repair of DNA double-strand breaks; their loss leads to genomic instability and breast/ovarian cancer predisposition. APC regulates the Wnt signaling pathway; its loss in familial adenomatous polyposis leads to colorectal cancer. p16 (encoded by CDKN2A) inhibits CDK4/6; its loss accelerates G1 progression. Panel A: "Oncogenes = Stuck Accelerator" showing a normal car with the accelerator (proto-oncogene) at rest alongside a mutant car with the accelerator jammed down (oncogene), racing forward uncontrollably. Panel B: Oncogene characteristics: "Gain-of-function mutation," "One mutant allele sufficient (dominant)," with examples listed (RAS, MYC, HER2, BCR-ABL). Panel C: "Tumor Suppressors = Broken Brakes" showing a normal car with functional brakes alongside a mutant car with failed brakes (both copies lost), unable to stop, with characteristics: "Loss-of-function mutation," "Both alleles must be lost (two hits)," and examples (RB1, TP53, BRCA1/2, APC, p16). Panel D: A crashed car representing cancer as the result of stuck accelerator and/or broken brakes, illustrating that both oncogene activation and tumor suppressor loss contribute to malignant transformation. --- ## Cancer and Cell Cycle Dysregulation ### Hallmarks of Cancer Related to Cell Cycle Several of the hallmarks of cancer directly relate to cell cycle control. Sustaining proliferative signaling means that cancer cells no longer require external growth factors—they may produce their own, have hyperactive receptors, or have mutations in downstream signaling pathways. Evading growth suppressors means inactivation of checkpoint pathways involving Rb and p53. Resisting cell death involves suppression of apoptotic pathways that would normally eliminate damaged cells. Enabling replicative immortality typically requires telomerase reactivation to prevent replicative senescence. Genome instability results from defective DNA repair and checkpoint function, allowing mutations to accumulate. ### Common Cell Cycle Alterations The specific molecular alterations found in cancers frequently target cell cycle components. TP53 mutations occur in more than half of all human cancers. RB1 loss, though directly observed in some tumors (retinoblastoma, small cell lung cancer), is functionally equivalent to the more common alterations of cyclin D overexpression, p16 deletion, or CDK4/6 amplification—all of which lead to Rb hyperphosphorylation and E2F release. These alterations are so common that the "Rb pathway" is considered disrupted in the majority of human cancers. --- ## Therapeutic Targets ### CDK4/6 Inhibitors The recognition that the cyclin D-CDK4/6-Rb pathway is central to cancer cell proliferation has led to development of CDK4/6 inhibitors. Palbociclib, ribociclib, and abemaciclib block CDK4 and CDK6, preventing Rb phosphorylation and causing G1 arrest. These drugs have been approved for estrogen receptor-positive breast cancer, where they are combined with hormonal therapy. Importantly, CDK4/6 inhibitors require functional Rb to be effective—tumors that have lost Rb cannot respond because the downstream pathway is already derepressed. ### Aurora Kinase Inhibitors Aurora kinases (Aurora A and B) are critical regulators of mitosis, involved in centrosome maturation, spindle assembly, and chromosome segregation. Aurora kinase inhibitors cause mitotic defects and cell death in cancer cells, which often have elevated Aurora kinase activity. Several Aurora kinase inhibitors are in clinical development for various malignancies. ### Spindle Poisons Drugs that disrupt microtubule function have been mainstays of cancer chemotherapy for decades. Taxanes (paclitaxel, docetaxel) stabilize microtubules, preventing the dynamic instability required for spindle function. Vinca alkaloids (vincristine, vinblastine) destabilize microtubules, causing spindle collapse. Both classes trap cells in mitosis, where the spindle assembly checkpoint is chronically activated. Prolonged mitotic arrest leads to cell death, either during mitosis or in the subsequent G1 phase. --- ## Summary - The cell cycle consists of G1, S, G2, and M phases with G0 as quiescent state - Cyclins and CDKs drive progression; CKIs restrain it - Checkpoints ensure fidelity at G1/S, G2/M, and spindle assembly - Rb controls G1/S transition; p53 responds to DNA damage - Mitosis occurs in prophase, prometaphase, metaphase, anaphase, telophase - Oncogenes (accelerators) and tumor suppressors (brakes) regulate cell division - Cancer results from dysregulation of cell cycle control --- ## Key Terms | Term | Definition | |------|------------| | Cyclin | Regulatory protein with oscillating levels controlling CDK activity | | CDK | Cyclin-dependent kinase; drives cell cycle transitions | | Checkpoint | Control point ensuring cycle fidelity before proceeding | | Rb | Retinoblastoma protein; master regulator of G1/S transition | | p53 | Tumor suppressor responding to DNA damage | | Aneuploidy | Abnormal chromosome number | --- *This content is subject to the [MIT License](https://opensource.org/licenses/MIT). © 2024–2026 Hibbert School of Medicine.*