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Mitotic Spindle Checkpoint and Chromosome Segregation

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Mitosis Cell Cycle Checkpoints and Cancer Prevention→→Kinetochore Structure and Function

Core Idea

The spindle assembly checkpoint (SAC) monitors kinetochore attachment to spindle microtubules, preventing anaphase until all chromosomes are bi-oriented (one sister chromatid pulled to each pole). Unattached or improperly attached kinetochores recruit Mad1/Mad2 complexes, which inhibit Cdc20 and prevent activation of the anaphase-promoting complex (APC), blocking cyclin B degradation and mitotic exit. Once all kinetochores are attached, Cdc20 is released, APC ubiquitinates cyclin B and securin, and anaphase proceeds with accurate chromosome segregation.

Explainer

From your study of mitosis, you know that the goal of cell division is to deliver exactly one copy of every chromosome to each daughter cell. You also know from cell cycle checkpoints that the cell pauses at critical moments to verify that conditions are safe before proceeding. The spindle assembly checkpoint (SAC) is the specific checkpoint that operates during mitosis to answer one question: are all chromosomes properly attached to the spindle before we pull them apart? If even a single chromosome is misattached, the answer is no, and the cell halts at the metaphase-to-anaphase transition until the problem is corrected.

The mechanism works through a "wait" signal generated by unattached kinetochores — the protein structures on each chromosome where spindle microtubules grab hold. Each sister chromatid pair must achieve bi-orientation: one kinetochore attached to microtubules from one spindle pole, the other kinetochore attached to microtubules from the opposite pole. When a kinetochore lacks attachment or is attached to the wrong pole, it recruits checkpoint proteins — particularly Mad1 and Mad2. These proteins undergo a conformational change at the unattached kinetochore that converts Mad2 into an active inhibitor. Active Mad2 then sequesters Cdc20, a key activator of the anaphase-promoting complex (APC/C). Without Cdc20, the APC/C cannot function, and the cell stays locked in metaphase.

Think of it as a voting system where every kinetochore must signal "attached" before the cell proceeds. A single unattached kinetochore continuously produces the Mad2 inhibitory signal, which is amplified and spread throughout the cell. This means the checkpoint is exquisitely sensitive — one unattached chromosome out of 46 in a human cell is enough to block anaphase entirely. The signal is catalytic: the unattached kinetochore acts as a template that converts inactive Mad2 into active Mad2, which then diffuses away to inhibit Cdc20 throughout the cytoplasm.

Once all kinetochores achieve proper bi-oriented attachment and are under tension from opposing spindle poles, the "wait" signal stops. Cdc20 is released from inhibition and activates the APC/C, a ubiquitin ligase that tags two critical substrates for destruction: securin and cyclin B. Securin normally holds the enzyme separase in check — once securin is degraded, separase cleaves the cohesin rings that physically hold sister chromatids together, allowing them to be pulled to opposite poles. Cyclin B degradation inactivates Cdk1, which triggers mitotic exit: the spindle disassembles, chromosomes decondense, and the nuclear envelope reforms. When this checkpoint fails — through mutations in Mad or Bub proteins, for instance — cells can mis-segregate chromosomes, producing aneuploid daughter cells. This is a hallmark of many cancers, which is why the SAC is considered a critical tumor suppressor mechanism.

Practice Questions 5 questions

Prerequisite Chain

Understanding Zero → The Number Zero → Counting to Five → Counting to 10 → Counting to 20 → Counting a Set of Objects Up to 20 → Cardinality: The Last Number Counted → Matching Numerals to Quantities → Subitizing Small Quantities → Addition Within 10 → Number Bonds to 10 → Addition Within 20 → Doubles and Near Doubles → Doubles Facts Within 10 → Near Doubles Facts Within 20 → Mental Math Strategies for Addition → Mental Math: Adding and Subtracting Tens → Addition Within 100 → Repeated Addition as Multiplication → Multiplication as Equal Groups → Multiplication: Arrays → Basic Multiplication Facts (0s, 1s, 2s, 5s, 10s) → Multiplication Facts Within 100 → Division as Equal Sharing → Division as Grouping (Measurement Division) → Division: Grouping (Repeated Subtraction) Model → Division: Fair Sharing Model → Division as Equal Sharing → Division as Grouping → Basic Division Facts → Division Facts Within 100 → Multiplication and Division Fact Families → Relationship Between Multiplication and Division → Division Facts as Inverse of Multiplication → Remainders and Quotients in Division → Division Word Problems → Multi-Step Word Problems → Solving Multi-Step Word Problems → Multiplication Word Problems → Division Word Problems → Introduction to Long Division → Factors and Multiples → Prime and Composite Numbers → Equivalent Fractions → Relating Fractions and Decimals → Decimal Place Value → Integers and the Number Line → Comparing and Ordering Integers → Absolute Value → Adding Integers → Subtracting Integers → Multiplying Integers → Dividing Integers → Unit Rates → Proportions → Percent Concept → Converting Between Fractions, Decimals, and Percents → Operations with Rational Numbers → Two-Step Equations → Solving Multi-Step Equations → Equations with Variables on Both Sides → Angle Pairs: Complementary, Supplementary, and Vertical → Parallel Lines and Transversals → Corresponding Angles → Alternate Interior Angles → Triangle Angle Sum Theorem → Exterior Angle Theorem → Triangle Inequality Theorem → Similar Triangles: AA Similarity → Similar Triangles: SSS and SAS Similarity → Proportions in Similar Triangles → Right Triangle Trigonometry Introduction → Sine, Cosine, and Tangent Ratios → Trigonometric Ratios Review → Radian Measure → Converting Between Degrees and Radians → The Unit Circle → Graphing Sine and Cosine → Graphing Tangent and Reciprocal Trigonometric Functions → Derivatives of Trigonometric Functions → Antiderivatives → Indefinite Integrals → Basic Integration Rules → Riemann Sums → Definite Integral Definition → Double Integrals: Definition and Setup → Iterated Integrals and Fubini's Theorem → Double Integrals over Rectangular Regions → Double Integrals over General Regions → Applications of Double Integrals: Area, Mass, and Moments → Triple Integrals in Cartesian Coordinates → Triple Integrals in Cylindrical and Spherical Coordinates → Change of Variables and the Jacobian Determinant → Applications of Triple Integrals: Volume and Mass → Vector Fields and Their Representations → Line Integrals of Vector Fields → Work and Circulation → Line Integrals of Scalar and Vector Functions → Fundamental Theorem for Line Integrals → Conservative Vector Fields → Conservative Vector Fields and Potential Functions → Curl and Divergence of Vector Fields → Curl and Divergence → Divergence Theorem → Electric Flux and Divergence Theorem → Gauss's Law: Integral Form and Meaning → Solving Problems with Gauss's Law → Conductors in Electrostatic Equilibrium → Capacitance and Capacitors → Dielectrics → Dielectric Constant and Relative Permittivity → Electric Field Inside Dielectric Materials → Dielectric Materials and Polarization → Dielectric Susceptibility and Permittivity → Energy Density in Electric Fields → Electric Current and Current Density → Electrical Resistance and Resistivity → Ohm's Law and Circuit Elements → Electromotive Force (EMF) and Batteries → Kirchhoff's Circuit Laws: Voltage and Current → DC Circuit Network Analysis Methods → Transient Response in RC Circuits → RC Circuits → LC and RLC Circuits → AC Circuits: Fundamentals → Impedance and Reactance → AC Power and Resonance → Electromagnetic Waves → Postulates of Special Relativity → Time Dilation → Length Contraction → Lorentz Transformation → Relativistic Velocity Addition → Relativistic Momentum and Energy → Mass-Energy Equivalence and E=mc² → Photons as Particles with Energy and Momentum → Planck-Einstein Relation: Energy and Frequency → Photoelectric Effect → The Photon: Light as Quanta → Compton Scattering → Wave-Particle Duality → de Broglie Wavelength → The Schrödinger Equation → State Vectors and Wavefunctions → Quantum Superposition → Quantum Entanglement → Bell Theorem and Bell Inequalities → Postulates of Quantum Mechanics → Observables and Quantum Operators → Commutators and Commutation Relations → Quantum Angular Momentum → Quantum Mechanical Treatment of Hydrogen → Solving the Schrödinger Equation for Hydrogen Atom → Quantum Numbers → Electron Configuration → Periodic Trends → Covalent Bonding → Electronegativity and Bond Polarity → Ionic Bonding → Lewis Structures → VSEPR Theory and Molecular Geometry → Molecular Geometry and Electron Pair Geometry → Molecular Polarity and Dipole Moments → Intermolecular Forces → States of Matter and Phase Changes: Melting, Boiling, and Sublimation → Gas Laws and the Ideal Gas Equation → Gas Stoichiometry and Volume-Volume Calculations → Thermochemistry and Enthalpy → Heat Capacity and Calorimetry → Entropy and Molecular Disorder → Spontaneity and ΔG → Entropy and Gibbs Free Energy → Chemical Equilibrium → Acid-Base Chemistry → Weak Acid Ionization → Weak Base Ionization → Acid and Base Strength: Ka, Kb, and Ionization → Leaving Groups and Nucleofugality → SN2 Substitution Reactions → SN1 Substitution Reactions → E1 Elimination Reactions → Alcohols and Ethers: Structure, Properties, and Nomenclature → Reactions of Alcohols → Aldehydes and Ketones: Structure and Reactivity → Oxidation Reactions in Organic Chemistry → Oxidation of Alcohols to Aldehydes and Ketones → Aldehyde and Ketone Structure and Nomenclature → Nucleophilic Addition to Aldehydes and Ketones → Carboxylic Acids and Their Derivatives → IUPAC Nomenclature of Carbonyls and Carboxylic Acids → IUPAC Nomenclature of Alkenes → Electrophilic Addition to Alkenes → Aromaticity and Benzene → Electrophilic Aromatic Substitution (EAS) → Nucleophilic Aromatic Substitution (SNAr) → Nucleophilic Acyl Substitution → Amines: Structure, Basicity, and Reactions → Amine Reactivity: Nucleophilicity and Basicity → Amino Acid Structure and Properties → Peptide Bonds and Polypeptide Formation → Protein Primary Structure → Protein Secondary Structure → Protein Tertiary Structure → Enzyme Structure and Function → Transcription: DNA to RNA → RNA Types and Structure → RNA Structure and Intramolecular Base Pairing → RNA Processing and Splicing → Translation: RNA to Protein → Ribosomes: Protein Synthesis Machines → Translation: Initiation and Elongation → Post-Translational Modifications → Proteasomal Degradation and Ubiquitin-Mediated Marking → Cell Cycle Regulation and Checkpoints → Cell Cycle Checkpoints: Ensuring Genome Integrity → Cell Cycle Checkpoints and Cancer Prevention → Mitotic Spindle Checkpoint and Chromosome Segregation

Longest path: 217 steps · 1172 total prerequisite topics

Prerequisites (2)

Mitosishard Cell Cycle Checkpoints and Cancer Preventionsoft

Leads To (1)

Kinetochore Structure and Functionhard