This MCQ module is based on: Mitosis
TOPIC 42 OF 45
Mitosis
🎓 Class 11
Biology
CBSE
Theory
Ch 10 – Cell Cycle and Cell Division
⏱ ~14 min
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Mitosis
10.2 M Phase — Mitosis
Mitosis is also called equational division because the daughter cells have the SAME chromosome number as the parent (no halving). It is the basis of growth, repair, and asexual reproduction in eukaryotes.
Mitosis occurs in somatic cells (body cells) — skin epidermis, intestinal lining, bone marrow, plant meristems. It is essentially the "copy-and-distribute" step: the cell has already copied its DNA in S phase; now it distributes the copies equally into two daughter cells.
Karyokinesis — Four Phases
The nuclear division (karyokinesis) happens in four sequential phases: Prophase → Metaphase → Anaphase → Telophase. After this, the cytoplasm divides (cytokinesis).
10.2.1 Prophase — Condensation
Early in mitosis, the cell prepares its chromosomes for movement:
- Chromosomes condense — chromatin coils and folds repeatedly into compact rod-shaped structures, now visible under light microscope.
- Each chromosome has two sister chromatids (produced in S phase) attached at the centromere.
- Centrioles (in animal cells), already duplicated in S phase, separate and move to opposite poles. From them, the spindle apparatus begins to form.
- Nucleolus disappears by end of prophase.
- Nuclear envelope begins to break down (completed in prometaphase, sometimes considered part of prophase in plants).
- The Golgi apparatus and endoplasmic reticulum fragment.
10.2.2 Metaphase — Alignment
The most photogenic phase:
- Nuclear envelope is fully disassembled — chromosomes have access to the spindle.
- Spindle fibres attach to kinetochores at each centromere — fibres from opposite poles pull on the two sister chromatids in opposite directions.
- Chromosomes align at the metaphase plate — the equator of the cell, halfway between the poles.
- The spindle assembly checkpoint verifies that every chromosome is properly attached to fibres from BOTH poles. If even one is mis-attached, anaphase is delayed.
This is the ideal phase for chromosome study (karyotyping): chromosomes are most condensed and visible. Hospitals use colchicine to arrest cells in metaphase for chromosome counting and structural analysis.
10.2.3 Anaphase — Separation
The shortest mitotic phase but most dramatic:
- The centromere of each chromosome splits — sister chromatids are no longer joined.
- The separated sister chromatids (now called daughter chromosomes) are pulled toward opposite poles by shortening spindle fibres.
- Movement uses motor proteins (dynein, kinesin) and depolymerisation of microtubules. Speed: ~1 µm per minute.
- The result: at end of anaphase, each pole has a complete set of chromosomes (e.g., 46 in human cell).
10.2.4 Telophase — Reformation
The reverse of prophase:
- Daughter chromosomes arrive at their respective poles and begin to decondense (uncoil).
- The nuclear envelope reforms around each set of chromosomes from ER fragments.
- Nucleolus reappears.
- Spindle apparatus disassembles.
- The cell now has two daughter nuclei, but is still a single cell.
10.2.5 Cytokinesis — Cell Division
Cytokinesis is the actual physical splitting into two cells:
| Feature | Animal Cells | Plant Cells |
|---|---|---|
| Mechanism | Cleavage furrow — actin-myosin ring contracts, pinching cell in two | Cell plate — vesicles fuse at equator, building new cell wall outward |
| Direction | From outside inward | From inside outward |
| Reason for difference | No rigid cell wall — soft cells can be pinched | Rigid cell wall prevents pinching; new wall must be built |
10.2.6 Significance of Mitosis
- Growth: A single fertilised egg becomes a multicellular body of trillions of cells, all through mitosis.
- Repair: Wound healing, replacement of dead cells (skin shedding, gut lining renewal every 2–3 days).
- Maintenance: Continuous replacement of short-lived cells (RBCs live 120 days; new ones from bone marrow mitosis).
- Asexual reproduction: Single-celled eukaryotes (yeast, Amoeba) and many plants reproduce by mitosis.
- Cellular constancy: Maintains identical chromosome number and genetic content across all body cells.
🎯 Interactive: Mitosis Phase Identifier
Match the description with the correct mitosis phase:
Phase: —
Choose a description to identify the phase.
📐 Activity 10.2 — Observe Mitosis in Onion Root Tip
Setup: A classic NCERT lab. You need: onion bulb (rooted in water), 1 N HCl, acetocarmine stain, slide, coverslip, microscope.
- Cut 1 cm tip of an actively growing root.
- Hydrolyse in 1 N HCl at 55–60°C for ~10 min (softens cells).
- Place tip on a slide, stain with acetocarmine for 5 min.
- Cover with a coverslip; gently squash with thumb (chromosomes spread out in single layer).
- Examine under microscope. Look at hundreds of cells.
Predict: Will you see all phases of mitosis in equal proportion? Why or why not? Where in the root will mitosis be most active?
No — phases are NOT in equal proportion. The fraction of cells in each phase reflects how long that phase lasts (recall Activity 10.1):
- Most cells in interphase (~95%): large round nuclei with no visible chromosomes.
- Prophase cells: condensing thread-like chromosomes; nuclear envelope still intact.
- Metaphase: chromosomes aligned on a clear equatorial plate. Best for counting (onion has 2n=16).
- Anaphase: rare; V-shaped chromosomes moving to poles.
- Telophase: two new nuclei in one cell with cell plate forming between.
Location: Mitosis is most active in the meristematic region just behind the root cap — the "growth zone". Below it is the cap (no division); above it is the elongation zone (no division but cells stretching).
Why squash? Plant cells are stacked in 3-D. Squashing spreads them in a single layer so all chromosomes lie in the focal plane of the microscope.
Worked Examples
Worked Example 1: Counting Chromosomes
An onion cell has 16 chromosomes in G₁. How many chromosomes and chromatids does it have at: (a) end of G₂, (b) metaphase, (c) end of anaphase, (d) telophase of each daughter cell?
Reminder: Chromosome number is counted by centromeres; chromatids are individual DNA molecules.
Key concept: At anaphase, when centromeres split, the chromosome count momentarily doubles in the parent cell. After cytokinesis, each daughter inherits exactly the original parent number.
| Stage | Chromosomes | Chromatids | DNA |
| G₁ | 16 | 16 (one per chromosome) | 2C |
| (a) end of G₂ | 16 | 32 (two per chromosome) | 4C |
| (b) metaphase | 16 | 32 | 4C |
| (c) end of anaphase | 32 (sister chromatids now count as independent chromosomes since centromeres split) | 32 (one per chromosome now) | 4C (total in cell, before cytokinesis) |
| (d) telophase, each daughter | 16 | 16 | 2C |
Worked Example 2: Mitosis and Generations
A diploid cell (2n=20) undergoes 3 successive mitotic divisions. (a) How many daughter cells form? (b) What is the chromosome number in each? (c) Are they genetically identical?
(a) After 1 mitosis → 2 cells; after 2 → 4 cells; after 3 → 2³ = 8 daughter cells.
(b) Mitosis is EQUATIONAL — chromosome number unchanged. Each daughter cell has 2n = 20 chromosomes, identical to the original parent.
(c) Yes — genetically IDENTICAL (clones). DNA replication is essentially error-free, and mitosis precisely partitions sister chromatids. So all 8 cells are genetically the same as the original.
Exceptions:
(b) Mitosis is EQUATIONAL — chromosome number unchanged. Each daughter cell has 2n = 20 chromosomes, identical to the original parent.
(c) Yes — genetically IDENTICAL (clones). DNA replication is essentially error-free, and mitosis precisely partitions sister chromatids. So all 8 cells are genetically the same as the original.
Exceptions:
- Spontaneous mutations during DNA replication (rare).
- Defects in cell division (aneuploidy, polyploidy).
- Epigenetic differences (DNA methylation patterns may drift) — same DNA but different gene expression.
🎯 Competency-Based Questions
Q1. The metaphase plate is formed at the: L1 Remember
Answer: (c) Equator of the cell. The metaphase plate (equatorial plate) is the imaginary plane midway between the two spindle poles where chromosomes align during metaphase.
Q2. Distinguish between karyokinesis and cytokinesis. L2 Understand
Karyokinesis = division of the nucleus. It comprises prophase, metaphase, anaphase, telophase. Result: two daughter nuclei within ONE cell.
Cytokinesis = division of the cytoplasm. Begins in late anaphase/telophase, completed after karyokinesis. Result: two separate daughter cells.
Key insight: Karyokinesis without cytokinesis = multinucleate cell (e.g., skeletal muscle fibres, some fungi). Cytokinesis without karyokinesis would be impossible (cells need a nucleus).
Cytokinesis = division of the cytoplasm. Begins in late anaphase/telophase, completed after karyokinesis. Result: two separate daughter cells.
Key insight: Karyokinesis without cytokinesis = multinucleate cell (e.g., skeletal muscle fibres, some fungi). Cytokinesis without karyokinesis would be impossible (cells need a nucleus).
Q3. Why is mitosis called "equational division"? L3 Apply
"Equational" because the chromosome number remains the SAME (equal) in parent and daughter cells.
Mechanism that ensures equational division:
- Parent: 2n chromosomes (e.g., 46 in human)
- After mitosis: each daughter has 2n chromosomes (still 46)
Mechanism that ensures equational division:
- DNA replicates ONCE in S phase (2C → 4C, but n stays same).
- In anaphase, centromeres split — sister chromatids (identical copies) separate to opposite poles.
- Each daughter receives ONE copy of every chromosome → same total number as parent.
Q4. Analyse: What would happen if anaphase didn't occur properly and one sister chromatid failed to move to its pole? L4 Analyse
Consequence: Aneuploidy (abnormal chromosome number).
Cellular safeguard: The spindle assembly checkpoint normally prevents anaphase if attachment is wrong. Failure of this checkpoint underlies many cancers.
- One daughter cell ends up with an EXTRA chromosome (trisomy for that chromosome).
- The other daughter cell ends up MISSING a chromosome (monosomy).
- Trisomy 21 → Down Syndrome (3 copies of chromosome 21).
- Trisomy 18 → Edward Syndrome.
- Trisomy 13 → Patau Syndrome.
- XXY → Klinefelter Syndrome.
- X0 → Turner Syndrome (single X).
Cellular safeguard: The spindle assembly checkpoint normally prevents anaphase if attachment is wrong. Failure of this checkpoint underlies many cancers.
Q5. HOT (Create): Design a poster summarising the four phases of mitosis. Include: name, key event, visual marker, and one mnemonic. L6 Create
Poster Concept — "The 4 P-M-A-T Steps":
Mnemonic: "Please Make A Taco" or "PMAT — Pretty Much A Test" (favourite of biology students!). Add a final box for Cytokinesis (cell pinches/plates).
| Phase | Key Event | Visual Marker | Mnemonic |
| Prophase | Chromosomes condense; nuclear envelope breaks | Thread-like dark structures visible | Pack and Prepare |
| Metaphase | Chromosomes align at equator | Straight line of chromosomes at middle | Middle of cell |
| Anaphase | Sister chromatids separate to poles | V-shaped chromosomes moving outward | Apart to poles |
| Telophase | Two new nuclei form; chromosomes uncoil | Two clear nuclei with developing membranes | Two new cells |
🧠 Assertion–Reason Questions
Choose: (A) Both true, R explains A. (B) Both true, R doesn't explain A. (C) A true, R false. (D) A false, R true.
A: Plant cells form a cell plate, but animal cells form a cleavage furrow during cytokinesis.
R: Plant cells have rigid cell walls that prevent pinching; animal cells have flexible membranes that can be constricted by an actin-myosin ring.
Answer: (A). Both true; R explains A. The presence/absence of cell wall fundamentally changes the mechanics of cytokinesis.
A: Sister chromatids separate during anaphase, not metaphase.
R: The spindle assembly checkpoint must verify that all kinetochores are properly attached before allowing chromatid separation.
Answer: (A). Both true; R explains A. The spindle assembly checkpoint is a critical safety mechanism. Premature separation would cause aneuploidy.
A: Daughter cells produced by mitosis are genetically identical to the parent cell.
R: DNA replication in S phase is semi-conservative, and mitosis precisely segregates sister chromatids.
Answer: (A). Both true; R explains A. Semi-conservative replication produces two identical DNA molecules; precise chromatid segregation distributes them equally. This is why your skin cells are genetically identical to your other body cells, even after billions of divisions.
Frequently Asked Questions - Mitosis
What is the main concept covered in Mitosis?
In NCERT Class 11 Biology Chapter 10 (Cell Cycle and Cell Division), "Mitosis" covers the core biological structures, functions, and classifications students need for board exam success. The MyAiSchool lesson explains the topic with definitions, labelled diagrams, comparison tables, and interactive simulations. Scientific terminology and ecological/physiological significance are highlighted throughout to build conceptual depth aligned with CBSE 2025-26 syllabus.
How is Mitosis useful in real-life or applied biology?
Real-life applications of "Mitosis" from NCERT Class 11 Biology Chapter 10 include medical diagnostics, agriculture, food preservation, biotechnology, ecological monitoring, and public health. The MyAiSchool lesson links every biological concept to a tangible application so students see biology as a problem-solving framework for living systems, not just textbook content.
What are the key terms students should memorize for Mitosis?
Key terms in "Mitosis" (NCERT Class 11 Biology Chapter 10 Cell Cycle and Cell Division) are tabulated in the MyAiSchool key-terms grid. Students should memorize each term with its precise definition, function, and example. Terminology is high-yield in CBSE board exams — 1-mark MCQs and 2-mark short answers test definitions directly. The Summary section provides a printable quick-reference card.
How does this part connect to other parts of Chapter 10?
NCERT Class 11 Biology Chapter 10 (Cell Cycle and Cell Division) is structured so each part builds biological understanding sequentially. "Mitosis" connects to neighbouring parts via shared classifications, structural hierarchies, and physiological processes. The MyAiSchool lesson cross-references related concepts with internal links so students can navigate the whole chapter as one connected biological story rather than disconnected fragments.
What types of CBSE board questions come from Mitosis?
CBSE board questions from "Mitosis" typically include: (1) 1-mark MCQs on definitions and classification, (2) 2-mark short-answer differences/comparisons, (3) 3-mark labelled-diagram questions, (4) 5-mark long-answer essays combining structure + function + significance. The MyAiSchool lesson tags each Competency-Based Question (CBQ) with Bloom level (L1-L6) so students know how to study for each weight.
How can students use the interactive simulation effectively?
The interactive simulation in the "Mitosis" lesson allows students to explore biological structures, classifications, or processes using selectors and sliders, with live visual feedback. To use it effectively: (1) explore each option/state, (2) compare with textbook diagrams, (3) note the function changes, (4) try the integrated practice quiz. The simulation reinforces visual-spatial understanding that pure text-based study cannot.
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