Mitosis Study Guide

📖 Core Concepts Mitosis – equational (same‑number) division of a eukaryotic cell; separates replicated chromosomes into two genetically identical nuclei. Karyokinesis – division of the nucleus; cytokinesis – division of the cytoplasm that follows. Sister chromatids – two identical copies of a chromosome joined at the centromere. Cell cycle phases – Interphase (G₁, S, G₂) → Mitotic phase (prophase → prometaphase → metaphase → anaphase → telophase) → Cytokinesis. Spindle Assembly Checkpoint (SAC) – monitors kinetochore attachment & tension; blocks anaphase onset until all chromosomes are correctly attached. Open vs. closed mitosis – Open: nuclear envelope breaks down (typical animal cells). Closed: envelope stays intact (some fungi). Endoreduplication / endocycle – DNA replication without mitosis → polyploid cells. 📌 Must Remember Outcome of mitosis: 2 daughter cells, same chromosome number as parent. Key checkpoint: SAC → prevents premature anaphase; unattached kinetochores generate “wait‑anaphase” signal. Anaphase A: kinetochore microtubule shortening pulls chromosomes to poles. Anaphase B: interpolar microtubules elongate, pushing poles apart. Cytokinesis mechanisms: Animal cells → actin‑myosin contractile ring → cleavage furrow. Plant cells → Golgi‑derived vesicles → cell plate → new cell wall. Common mitotic errors: Nondisjunction → one daughter gets both chromatids (trisomy) and the other none (monosomy). Anaphase lag → chromosome lags, may be lost → monosomy. Multipolar division → >2 poles → severe aneuploidy. Mitotic Index = (Number of mitotic cells / total cells) × 100 %; high index = aggressive tumor. 🔄 Key Processes Prophase – chromatin condenses; centrosomes duplicate → spindle formation; nucleolus disappears. Prometaphase – nuclear envelope fragments; microtubules attach to kinetochores; motor proteins generate pulling forces. Metaphase – chromosomes line up on metaphase plate; SAC verifies proper attachment/tension. Anaphase – cohesin cleaved → sister chromatids separate (Anaphase A); polar microtubules push poles apart (Anaphase B). Telophase – nuclear envelopes re‑form; nucleoli reappear; chromosomes decondense. Cytokinesis (Animal) – RhoA activates actin‑myosin ring → contractile ring constricts → cleavage furrow forms. Cytokinesis (Plant) – vesicles fuse at cell centre → cell plate forms → new cell wall separates daughters. 🔍 Key Comparisons Open vs. Closed Mitosis Open: nuclear envelope disassembles → microtubules access chromosomes directly. Closed: envelope remains → spindle forms within nucleus. Anaphase A vs. Anaphase B A: kinetochore microtubule shortening pulls chromosomes. B: interpolar microtubule sliding elongates spindle. Animal vs. Plant Cytokinesis Animal: contractile actin‑myosin ring → cleavage furrow. Plant: vesicle‑derived cell plate → new cell wall. ⚠️ Common Misunderstandings Mitosis ≠ Cytokinesis – they are separate; cytokinesis can fail while karyokinesis succeeds. All mitosis is “open.” Only animal cells typically use open mitosis; many organisms use closed or semi‑closed forms. Nondisjunction only occurs in meiosis. It also occurs in mitosis, producing somatic aneuploidy. Anaphase lag = nondisjunction. Lag is a distinct error where a chromosome is left behind, often leading to monosomy rather than a full‑chromosome pair error. 🧠 Mental Models / Intuition “Pull‑and‑Push” model: Imagine chromosomes as beads on strings; pull (Anaphase A) shortens the string, push (Anaphase B) stretches the whole scaffold. Checkpoint as a “traffic light”: Green (go) only when every car (kinetochore) is correctly positioned and moving; red (stop) if any car is missing or mis‑aligned. Cell rounding = “balloon” – a spherical shape gives the spindle equal space to orient, preventing mis‑attachments. 🚩 Exceptions & Edge Cases Closed mitosis – spindle assembly occurs without nuclear envelope breakdown; kinetochores still attach, but microtubules grow through nuclear pores. Endoreduplication – genome duplication without mitosis; yields polyploid cells (e.g., hepatocytes). Multipolar spindles – can arise from centrosome amplification; leads to >2 daughter cells, often lethal to the cell. 📍 When to Use Which Diagnosing tumor aggressiveness: use Mitotic Index rather than just counting total cells. Predicting error type: If a whole chromosome pair is missing from one daughter: suspect nondisjunction. If a single chromosome lags behind: suspect anaphase lag. Choosing a model system for studying nuclear envelope dynamics: pick an organism with closed mitosis (e.g., yeast) for intact‑envelope studies; use animal cells for open‑mitosis mechanisms. 👀 Patterns to Recognize Chromosome alignment → SAC satisfied → rapid transition to anaphase. Any delay or mis‑alignment signals checkpoint activation. Presence of a cleavage furrow → animal cell undergoing cytokinesis; cell plate formation → plant cell cytokinesis. Rounded cell shape at mitotic entry → proper actomyosin cortex activity; flattened or elongated cells often have spindle orientation defects. 🗂️ Exam Traps Distractor: “Mitosis produces haploid cells.” – Wrong; that describes meiosis. Distractor: “The spindle checkpoint monitors DNA replication.” – It monitors kinetochore attachment/tension, not replication status. Distractor: “Anaphase B is the only mechanism that separates chromosomes.” – Both A and B cooperate; A moves chromosomes, B separates poles. Distractor: “Plant cells undergo cytokinesis by contractile ring.” – Plants use a cell plate, not a contractile ring. Distractor: “Closed mitosis occurs in all eukaryotes.” – Only certain organisms (e.g., many fungi) use closed mitosis; most animal cells use open mitosis.