Class 11 : Biology (In English) – Lesson 10: Cell Cycle and Cell Division
EXPLANATION & SUMMARY
🌿✨ Introduction
🧠 All living organisms grow, repair, and reproduce by cell division.
Each new cell arises from a pre-existing cell (Virchow’s principle: Omnis cellula e cellula).
🌿 The cell cycle is a series of events a cell undergoes from its formation to the next division. It includes growth, DNA replication, and division.
💡 Concept:
Cell cycle = life span of a cell
Cell division = actual splitting to form new cells
Essential for growth, repair, reproduction, and continuity of life
🧭 Phases of the Cell Cycle
🧬 Cell cycle = Interphase + M Phase (Mitotic phase)
🕓 Duration varies by cell type (e.g., human cell ≈ 24 hours)
🌱 1️⃣ Interphase — “Resting Phase” (but metabolically active)
🧠 Represents 95% of cycle; cell prepares for division.
Divided into three sub-phases:
🧪 (a) G₁ Phase (Gap 1)
🌿 Intense metabolic activity
⚙️ Synthesis of RNA, proteins, organelles
🧬 Cell grows; decides to divide or not
🧫 (b) S Phase (Synthesis Phase)
🧬 DNA replication occurs
📈 Chromosomes duplicate
💧 Amount of DNA doubles, chromosome number constant
⚙️ (c) G₂ Phase (Gap 2)
🧪 Synthesis of proteins for division
🧠 Cell checks DNA, prepares for mitosis
🌿 Centriole duplication completes (in animal cells)
✏️ Note: Some cells exit cycle after G₁ → enter G₀ (quiescent stage) — no division (e.g., neurons).
🌸 2️⃣ M Phase (Mitotic Phase)
🧠 Actual cell division: nucleus and cytoplasm divide.
Divided into:
1️⃣ Karyokinesis – division of nucleus
2️⃣ Cytokinesis – division of cytoplasm
💡 Two main types:
Mitosis – equational, growth
Meiosis – reductional, gamete formation
🌿 Mitosis — “Equational Division”
🧠 Occurs in somatic cells; daughter cells identical to parent (same chromosome number).
Ensures growth and repair.
🧪 Phases of Mitosis:
1️⃣ Prophase
🌸 Chromatin → chromosomes (visible)
🧠 Each chromosome = 2 sister chromatids joined at centromere
🧬 Nuclear membrane, nucleolus disappear
⚙️ Spindle fibres form from centrioles (in animals)
2️⃣ Metaphase
🌿 Chromosomes align at equatorial plane
🧪 Spindle attaches to centromeres
💡 Best stage for karyotype study
3️⃣ Anaphase
⚡ Centromeres split; chromatids move to opposite poles
🌾 Each chromatid = new chromosome
4️⃣ Telophase
🌸 Chromosomes uncoil → chromatin
🧠 Nuclear membrane & nucleolus reappear
🌿 Two nuclei formed
🌊 Cytokinesis
💧 Cytoplasm divides → 2 daughter cells
🌿 In animal cells – cleavage furrow
🌱 In plant cells – cell plate formation
💡 Significance of Mitosis:
Growth of organism
Cell replacement & repair
Asexual reproduction
Genetic stability
🌾 Meiosis — “Reductional Division”
🧬 Occurs in germ cells; chromosome number halved (2n → n).
Ensures genetic variation and gamete formation.
Divided into two stages:
1️⃣ Meiosis I – reductional
2️⃣ Meiosis II – equational
🌿 Meiosis I
🧠 Prophase I — longest, complex
Substages: Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis
🌀 Leptotene – chromosomes condense
🌸 Zygotene – homologous chromosomes pair (synapsis) → bivalents
🧬 Pachytene – crossing over (exchange of genes)
💫 Diplotene – chiasmata visible
⚡ Diakinesis – terminalisation; spindle forms
🧭 Metaphase I
Bivalents align at equator; spindle attaches
⚙️ Anaphase I
Homologous chromosomes separate to opposite poles (chromatids remain joined)
🌾 Telophase I
Nucleus reforms; cytokinesis → 2 haploid cells
🌸 Meiosis II (Like Mitosis)
Prophase II – chromosomes condense again
Metaphase II – chromosomes align
Anaphase II – sister chromatids separate
Telophase II – nuclei reform
🌿 Final result: 4 haploid daughter cells (gametes)
💡 Significance of Meiosis:
Maintains chromosome number across generations
Introduces variation through crossing over
Essential for sexual reproduction
🧬 Control of Cell Cycle
⚙️ Regulated by cyclins and cyclin-dependent kinases (CDKs)
🧠 Checkpoints at G₁, G₂, and M phase ensure accuracy.
✏️ Note: Faulty control → cancer (uncontrolled division)
🧠 Comparison
Feature Mitosis Meiosis
Type Equational Reductional
Divisions One Two
Daughter cells 2 (diploid) 4 (haploid)
Similarity to parent Identical Genetically different
Function Growth, repair Gamete formation
🌍 Why This Lesson Matters
🌱 Explains how organisms grow and reproduce
🧬 Provides foundation for genetics and evolution
🧠 Essential for understanding cancer and fertility
⚡ Important for NEET and board exams
📝 Quick Recap
🧠 Cell cycle = Interphase + M Phase
🌿 Interphase: G₁ (growth), S (DNA replication), G₂ (preparation)
🧬 Mitosis: Prophase, Metaphase, Anaphase, Telophase → 2 identical cells
🌾 Meiosis: 2 divisions → 4 haploid gametes; introduces variation
⚙️ Checkpoints ensure accuracy; errors cause disease
📘 Summary
The cell cycle describes the sequence of events by which a cell grows, duplicates its DNA, and divides.
Interphase prepares the cell; M phase executes division.
Mitosis produces identical cells for growth and repair.
Meiosis halves chromosome number, forming gametes and ensuring variation.
Accurate regulation ensures organismal health; disruptions lead to disorders like cancer.
Understanding cell cycle and division connects molecular control, genetics, and development, forming a key pillar of biology 🌿.
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QUESTIONS FROM TEXTBOOK
🔵 Question 1. What is the cell cycle?
🟢 Answer:
🧬 The cell cycle is the sequence of events by which a cell grows, prepares for division, and divides to form two daughter cells.
It consists of two main phases:
🌿 Interphase (period of growth)
G₁ phase: Cell growth and metabolism.
S phase: DNA replication.
G₂ phase: Preparation for mitosis.
🌸 M phase (Mitotic phase): Includes nuclear division (karyokinesis) and cytoplasmic division (cytokinesis).
✔️ Result: Formation of two genetically identical daughter cells.
🔵 Question 2. Describe the various phases of the cell cycle.
🟢 Answer:
🧫 Phases of the cell cycle:
G₁ phase (Gap 1):
➡️ Cell grows in size, synthesizes RNA and proteins.
S phase (Synthesis):
➡️ DNA replication takes place; amount of DNA doubles.
G₂ phase (Gap 2):
➡️ Further growth, preparation for division, duplication of organelles.
M phase (Mitotic):
➡️ Division of nucleus and cytoplasm → two daughter cells.
💡 Duration: Interphase occupies about 95% of total cycle time.
🔵 Question 3. Describe the events taking place during interphase.
🟢 Answer:
🌿 Interphase is the metabolically active phase between two successive cell divisions.
➡️ G₁ phase:
Cell grows
Synthesis of proteins and RNA
Cell decides to divide or not
➡️ S phase:
DNA replication occurs
Centrosome duplication
➡️ G₂ phase:
Cell prepares for mitosis
Synthesis of tubulin and mitotic proteins
✔️ Interphase ensures cell is ready for division.
🔵 Question 4. What is G₀ (quiescent phase)?
🟢 Answer:
🧠 Some cells exit the cell cycle after G₁ and enter a resting stage called G₀ phase.
💡 Features:
Cells are metabolically active but do not divide.
Remain in this phase for variable periods.
Seen in nerve cells and heart muscle cells.
✔️ Important for differentiation and maintenance.
🔵 Question 5. Why is mitosis called equational division?
🟢 Answer:
🧬 Mitosis produces two daughter cells with chromosome number identical to the parent cell.
➡️ DNA replicates once, and nucleus divides once.
➡️ Each daughter cell receives equal and identical set of chromosomes.
✔️ Hence, called equational division.
💡 Occurs in somatic cells for growth and repair.
🔵 Question 6. Name the stage of the cell cycle at which one of the following events occurs:
(a) Chromosomes are moved to spindle equator
(b) Centromere splits and chromatids separate
(c) Pairing between homologous chromosomes takes place
(d) Crossing over between homologous chromosomes takes place
🟢 Answer:
(a) 🌸 Metaphase — chromosomes align on equator
(b) 🌿 Anaphase — centromeres split, chromatids move apart
(c) 🧬 Zygotene (prophase I of meiosis) — homologous pairing (synapsis)
(d) ⚗️ Pachytene (prophase I of meiosis) — crossing over occurs
🔵 Question 7. Describe the following:
(a) Metaphase (b) Anaphase (c) Telophase
🟢 Answer:
(a) 🌸 Metaphase:
Chromosomes align on equatorial plate.
Spindle fibres attach to centromeres.
(b) 🌿 Anaphase:
Centromeres divide.
Sister chromatids move to opposite poles.
(c) 🧫 Telophase:
Chromatids reach poles, decondense into chromatin.
Nuclear envelope reforms; nucleolus reappears.
Cytokinesis begins.
✔️ Marks end of karyokinesis.
🔵 Question 8. Describe the significance of mitosis.
🟢 Answer:
🧬 Significance of mitosis:
🌱 Growth of multicellular organisms.
🧠 Maintenance and repair of tissues.
🌸 Asexual reproduction (in unicellular organisms).
⚖️ Genetic stability — identical daughter cells.
🧫 Cell replacement (e.g. skin cells, blood cells).
✔️ Maintains chromosome number constant.
🔵 Question 9. What are the main differences between mitosis and meiosis?
🟢 Answer:
Feature Mitosis Meiosis
Division One division Two divisions
Daughter cells Two Four
Chromosome number Same as parent Half of parent
Occurs in Somatic cells Reproductive cells
Crossing over Absent Present
Genetic composition Identical Different
✔️ Mitosis: Growth Meiosis: Gamete formation.
🔵 Question 10. What is meiosis? What are the stages of meiosis I?
🟢 Answer:
🧠 Meiosis is a type of cell division producing four haploid cells from one diploid cell.
➡️ Occurs in germ cells to form gametes.
➡️ Reduces chromosome number by half.
🌿 Meiosis I (Reductional division):
Prophase I: Longest phase, subdivided into:
Leptotene: Chromosomes condense.
Zygotene: Homologous chromosomes pair (synapsis).
Pachytene: Crossing over occurs.
Diplotene: Chiasmata visible.
Diakinesis: Terminalisation, spindle formation.
Metaphase I: Homologous pairs align on equator.
Anaphase I: Homologous chromosomes move to poles.
Telophase I: Nuclear membrane reforms, cytokinesis follows.
✔️ Result: Two haploid cells with half chromosome number.
🔵 Question 11. What is the significance of meiosis?
🟢 Answer:
🧬 Significance:
🌸 Formation of gametes (sperms and eggs).
⚖️ Maintains chromosome number constant across generations.
🌿 Genetic variation due to crossing over and independent assortment.
🧠 Basis of evolution and heredity.
✔️ Essential for sexual reproduction.
🔵 Question 12. Differentiate between cytokinesis in plant and animal cells.
🟢 Answer:
Feature Plant Cell Animal Cell
Method Formation of cell plate Formation of cleavage furrow
Location At centre At periphery
Involvement Golgi vesicles form plate Microfilaments constrict
Result New cell wall forms Two daughter cells formed
✔️ Both ensure division of cytoplasm equally.
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OTHER IMPORTANT QUESTIONS FOR EXAMS
(CBSE MODEL QUESTIONS PAPER)
ESPECIALLY MADE FROM THIS LESSON ONLY
🔴 Question 1:
The sequence of events by which a cell duplicates its genome and divides is called:
🔴1️⃣ Cytokinesis
🟢2️⃣ Cell Cycle
🟡3️⃣ Karyokinesis
🔵4️⃣ Mitosis
🟢 Answer: 2️⃣ Cell Cycle
🔴 Question 2:
Which is the longest phase of the cell cycle?
🔴1️⃣ G₁ phase
🟢2️⃣ S phase
🟡3️⃣ G₂ phase
🔵4️⃣ M phase
🟢 Answer: 1️⃣ G₁ phase
🔴 Question 3:
During which phase does DNA replication occur?
🔴1️⃣ G₁ phase
🟢2️⃣ S phase
🟡3️⃣ G₂ phase
🔵4️⃣ M phase
🟢 Answer: 2️⃣ S phase
🔴 Question 4:
Karyokinesis refers to division of:
🔴1️⃣ Cytoplasm
🟢2️⃣ Nucleus
🟡3️⃣ Organelles
🔵4️⃣ Cell membrane
🟢 Answer: 2️⃣ Nucleus
🔴 Question 5:
Cytokinesis refers to division of:
🔴1️⃣ Nucleus
🟢2️⃣ Cytoplasm
🟡3️⃣ Chromosomes
🔵4️⃣ DNA
🟢 Answer: 2️⃣ Cytoplasm
🔴 Question 6:
Which phase is called the resting phase?
🔴1️⃣ G₀ phase
🟢2️⃣ G₁ phase
🟡3️⃣ G₂ phase
🔵4️⃣ S phase
🟢 Answer: 1️⃣ G₀ phase
🔴 Question 7:
In which phase of mitosis do chromosomes align on equator?
🔴1️⃣ Prophase
🟢2️⃣ Metaphase
🟡3️⃣ Anaphase
🔵4️⃣ Telophase
🟢 Answer: 2️⃣ Metaphase
🔴 Question 8:
During which phase do sister chromatids separate?
🔴1️⃣ Prophase
🟢2️⃣ Metaphase
🟡3️⃣ Anaphase
🔵4️⃣ Telophase
🟢 Answer: 3️⃣ Anaphase
🔴 Question 9:
In which phase do nuclear envelope and nucleolus reappear?
🔴1️⃣ Prophase
🟢2️⃣ Metaphase
🟡3️⃣ Anaphase
🔵4️⃣ Telophase
🟢 Answer: 4️⃣ Telophase
🔴 Question 10:
The number of chromosomes in daughter cells after mitosis is:
🔴1️⃣ Half of parent cell
🟢2️⃣ Double of parent cell
🟡3️⃣ Same as parent cell
🔵4️⃣ Variable
🟢 Answer: 3️⃣ Same as parent cell
🔴 Question 11:
Name the two main phases of cell cycle.
🟢 Answer:
1️⃣ Interphase – Period of growth and DNA replication.
2️⃣ M phase (Mitotic phase) – Period of division (karyokinesis + cytokinesis).
🔴 Question 12:
Define mitosis.
🟢 Answer:
Mitosis is an equational division in which one parent cell divides into two identical daughter cells with same chromosome number as parent cell.
🔴 Question 13:
What are the main phases of the cell cycle? Describe each briefly.
🟢 Answer:
The cell cycle has two main phases:
1️⃣ Interphase:
• Period of growth and preparation for division.
• Sub-phases:
🔹 G₁ phase: Cell grows, proteins and organelles synthesized.
🔹 S phase: DNA replication occurs; chromosome number constant.
🔹 G₂ phase: Final preparation, enzymes and energy produced.
2️⃣ M phase (Mitotic phase):
• Includes karyokinesis (nuclear division) and cytokinesis (cytoplasmic division).
💡 Result: Two genetically identical daughter cells.
🔴 Question 14:
Differentiate between G₁, S, and G₂ phases of interphase.
🟢 Answer:
Phase Events
G₁ phase Cell grows in size, RNA and proteins synthesized, organelles duplicated.
S phase DNA replication, each chromosome forms sister chromatids.
G₂ phase Preparation for mitosis, synthesis of mitotic proteins, spindle formation.
💡 Interphase = G₁ + S + G₂
🔴 Question 15:
Describe the stages of mitosis.
🟢 Answer:
1️⃣ Prophase:
• Chromatin condenses into chromosomes.
• Nuclear envelope and nucleolus disappear.
• Spindle fibres form.
2️⃣ Metaphase:
• Chromosomes align on equatorial plate.
• Spindle attaches to centromere.
3️⃣ Anaphase:
• Centromeres split.
• Sister chromatids move to opposite poles.
4️⃣ Telophase:
• Chromatids uncoil to chromatin.
• Nuclear envelope and nucleolus reappear.
💡 Followed by cytokinesis → two identical cells.
🔴 Question 16:
Define cytokinesis. Explain cytokinesis in plant and animal cells.
🟢 Answer:
Definition: Division of cytoplasm after karyokinesis.
In animal cells 🧫:
• Occurs by cleavage furrow formation from periphery to center.
In plant cells 🌿:
• Cell plate forms in center by fusion of Golgi vesicles, later becomes cell wall.
💡 Both produce two daughter cells with equal cytoplasm.
🔴 Question 17:
Write any three significance of mitosis.
🟢 Answer:
1️⃣ Maintains chromosome number constant.
2️⃣ Produces genetically identical cells.
3️⃣ Helps in growth, repair, and regeneration.
4️⃣ Maintains nucleus-cytoplasm ratio.
5️⃣ Basis of asexual reproduction.
🔴 Question 18:
What is meiosis? How is it different from mitosis?
🟢 Answer:
Definition: Division that reduces chromosome number by half, forming haploid cells.
Feature Mitosis Meiosis
Division One Two (I & II)
Chromosome number Same as parent Half of parent
Genetic composition Identical Different
Occurs in Somatic cells Germ cells
💡 Result: Four haploid cells.
🔴 Question 19:
What is synapsis? In which phase does it occur?
🟢 Answer:
Definition: Pairing of homologous chromosomes during meiosis.
Occurs in: Zygotene stage of Prophase I.
Feature:
• Homologous chromosomes form bivalents.
• Prepares for crossing over.
🔴 Question 20:
Explain crossing over and its significance.
🟢 Answer:
Definition: Exchange of genetic material between non-sister chromatids of homologous chromosomes.
Occurs in: Pachytene of Prophase I.
Significance:
1️⃣ Creates genetic variation.
2️⃣ Helps in evolution.
3️⃣ Maintains chromosomal integrity.
🔴 Question 21:
Name and describe the stages of Prophase I of meiosis.
🟢 Answer:
1️⃣ Leptotene: Chromosomes become visible.
2️⃣ Zygotene: Synapsis and bivalent formation.
3️⃣ Pachytene: Crossing over occurs.
4️⃣ Diplotene: Chiasmata appear.
5️⃣ Diakinesis: Terminalisation of chiasmata, spindle formation.
💡 Longest and most complex phase.
🔴 Question 22:
Differentiate between Meiosis I and Meiosis II.
🟢 Answer:
Feature Meiosis I Meiosis II
Type Reductional Equational
Chromosome number Reduced to half Remains same
Crossing over Occurs Absent
Homologous chromosomes Separate Sister chromatids separate
💡 End result: 4 haploid cells.
🔴 Question 23:
Explain the phases of mitosis in detail with labelled diagram description.
🟢 Answer:
Mitosis is an equational division producing two identical daughter cells. It includes:
1️⃣ Prophase:
• Chromatin condenses into visible chromosomes.
• Each chromosome has two sister chromatids joined at a centromere.
• Nuclear envelope and nucleolus disappear.
• Spindle fibres form from centrioles.
2️⃣ Metaphase:
• Chromosomes align on equatorial plate.
• Spindle fibres attach to centromeres.
3️⃣ Anaphase:
• Centromeres divide, sister chromatids separate.
• Move to opposite poles pulled by spindle fibres.
4️⃣ Telophase:
• Chromatids uncoil to form chromatin.
• Nuclear membrane and nucleolus reappear.
• Spindle fibres disappear.
5️⃣ Cytokinesis:
• Division of cytoplasm — cleavage furrow in animals 🧫; cell plate in plants 🌿.
💡 Result: Two diploid daughter cells identical to parent.
🔴 Question 24:
Describe the stages of meiosis I.
🟢 Answer:
1️⃣ Prophase I — Longest phase, subdivided into:
• Leptotene: Chromosomes condense.
• Zygotene: Synapsis → bivalent formation.
• Pachytene: Crossing over between non-sister chromatids.
• Diplotene: Chiasmata visible.
• Diakinesis: Terminalisation, spindle formed.
2️⃣ Metaphase I: Bivalents align on equator; spindle attaches to centromeres.
3️⃣ Anaphase I: Homologous chromosomes move to opposite poles; reduction in chromosome number.
4️⃣ Telophase I: Nuclear membrane reforms; cytokinesis → two haploid cells.
💡 Result: Two haploid daughter cells, chromosomes still duplicated.
🔴 Question 25:
Explain the stages of meiosis II.
🟢 Answer:
Similar to mitosis but starts with haploid cells:
1️⃣ Prophase II:
• Chromosomes condense, spindle forms, nuclear envelope disappears.
2️⃣ Metaphase II:
• Chromosomes align on equator.
3️⃣ Anaphase II:
• Centromeres split; sister chromatids separate and move to opposite poles.
4️⃣ Telophase II:
• Nuclear membrane reappears; cytokinesis occurs.
💡 Result: 4 haploid daughter cells genetically different.
🔴 Question 26:
Discuss the significance of mitosis and meiosis.
🟢 Answer:
Mitosis:
1️⃣ Maintains chromosome number.
2️⃣ Produces genetically identical cells.
3️⃣ Helps in growth, repair, regeneration.
4️⃣ Basis of asexual reproduction.
Meiosis:
1️⃣ Produces haploid gametes.
2️⃣ Introduces genetic variation via crossing over.
3️⃣ Maintains chromosome number across generations.
4️⃣ Essential for sexual reproduction.
🔴 Question 27:
Explain cell cycle checkpoints and their importance.
🟢 Answer:
Cell cycle checkpoints regulate orderly progression:
1️⃣ G₁ Checkpoint:
• Checks cell size, nutrients, DNA damage before S phase.
2️⃣ G₂ Checkpoint:
• Ensures complete DNA replication and repair before mitosis.
3️⃣ M Checkpoint:
• Verifies spindle attachment before anaphase.
💡 Importance: Prevents mutations, maintains genetic stability, controls uncontrolled cell division (cancer).
🔴 Question 28:
Describe Prophase I of meiosis in detail.
🟢 Answer:
Prophase I subdivided into five stages:
1️⃣ Leptotene: Chromosomes become visible.
2️⃣ Zygotene: Synapsis of homologous chromosomes → bivalents.
3️⃣ Pachytene: Crossing over at chiasmata.
4️⃣ Diplotene: Homologous chromosomes separate except at chiasmata.
5️⃣ Diakinesis: Terminalisation of chiasmata; spindle formed.
💡 Significance: Crossing over ensures genetic variation.
🔴 Question 29:
What is crossing over? Explain its steps and significance.
🟢 Answer:
Definition: Exchange of genetic material between non-sister chromatids of homologous chromosomes.
Occurs in: Pachytene of Prophase I.
Steps:
1️⃣ Synapsis → Bivalent formation.
2️⃣ Chiasma formation.
3️⃣ Exchange of DNA segments.
4️⃣ Recombination of genes.
Significance:
✔️ Genetic recombination.
✔️ Variation in offspring.
✔️ Evolutionary adaptation.
🔴 Question 30:
Compare mitosis and meiosis in tabular form.
🟢 Answer:
Feature Mitosis Meiosis
Number of divisions One Two
Chromosome number Maintained (2n → 2n) Halved (2n → n)
Daughter cells 2 4
Genetic composition Identical Different
Crossing over Absent Present
Occurs in Somatic cells Germ cells
Function Growth, repair Gamete formation
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