Exercise Solutions Available
Step-by-step answers for all textbook questions in this chapter
Chapter Introduction
Cell Cycle and Cell Division is Chapter 10 of the NCERT Class 11 Biology textbook and belongs to the Cell Biology unit. This chapter explains how cells divide — the sequence of events in the cell cycle, the mechanics of mitosis and meiosis, and the biological significance of each type of division. These concepts are essential for understanding genetics, reproduction, and development in later chapters.
This is a high-weightage chapter for CBSE and NEET, with frequent questions on the comparison of mitosis and meiosis, prophase I stages, and the significance of meiosis.
Key Definitions
| Term | Definition |
|---|---|
| Cell cycle | The sequence of events from the formation of a cell to its division into daughter cells |
| Interphase | The longest phase of the cell cycle (G₁, S, G₂) during which the cell grows and DNA is replicated |
| Mitosis | Equational division producing two genetically identical diploid daughter cells |
| Meiosis | Reductional division producing four genetically distinct haploid daughter cells |
| Chromosome | A thread-like structure of DNA and proteins that carries genetic information |
| Chromatid | One of the two identical copies of a replicated chromosome, joined at the centromere |
| Centromere | The region where sister chromatids are held together; spindle fibres attach here |
| Synapsis | The pairing of homologous chromosomes during zygotene of prophase I |
| Crossing over | Exchange of genetic material between non-sister chromatids of homologous chromosomes during pachytene |
| Chiasmata | The X-shaped points where crossing over occurred; visible during diplotene |
| Karyokinesis | Division of the nucleus |
| Cytokinesis | Division of the cytoplasm |
| G₀ phase | A quiescent stage where cells exit the cell cycle and stop dividing |
Detailed Concepts
The Cell Cycle
The cell cycle is the period from one cell division to the next. It has two main phases: Interphase (preparation) and M phase (division).
Phases of the cell cycle:
| Phase | Duration (typical human cell) | Key Events |
|---|---|---|
| G₁ (Gap 1) | ~11 hours | Cell growth, normal metabolism, organelle duplication, preparation for S phase |
| S (Synthesis) | ~8 hours | DNA replication — each chromosome duplicates into two sister chromatids; histone synthesis |
| G₂ (Gap 2) | ~4 hours | Preparation for mitosis; protein synthesis, spindle fibre precursors assembled |
| M (Mitotic) | ~1 hour | Karyokinesis (prophase, metaphase, anaphase, telophase) followed by cytokinesis |
- G₀ phase: Cells that are not dividing exit the cell cycle at G₁ and enter G₀. Some cells (e.g. neurons, heart muscle cells) remain in G₀ permanently; others (e.g. liver cells) can re-enter the cycle when stimulated.
Mitosis
Mitosis is called equational division because the chromosome number in daughter cells equals that of the parent cell.
Stages of mitosis:
| Stage | Key Events |
|---|---|
| Prophase | Chromatin condenses into visible chromosomes (each with two sister chromatids); centrioles move to opposite poles; spindle fibres form; nucleolus and nuclear envelope disintegrate |
| Metaphase | Chromosomes align at the metaphase plate (equator); spindle fibres attach to kinetochores of centromeres; this is the best stage to observe chromosome morphology |
| Anaphase | Centromere splits; sister chromatids separate and are pulled to opposite poles by shortening spindle fibres; chromatids are now called daughter chromosomes |
| Telophase | Chromosomes decondense to chromatin; nuclear envelope reforms around each set; nucleolus reappears; spindle fibres disappear |
| Cytokinesis | Cytoplasm divides; in animal cells, a cleavage furrow deepens from the periphery inward; in plant cells, a cell plate forms from the centre outward |
Cytokinesis in animal vs plant cells:
| Feature | Animal Cell | Plant Cell |
|---|---|---|
| Mechanism | Cleavage furrow (centripetal) | Cell plate (centrifugal) |
| Direction | Periphery to centre | Centre to periphery |
| Structure involved | Actin ring contracts | Vesicles from Golgi form phragmoplast |
| Rigid cell wall | Absent — flexible membrane | Present — requires new wall material |
Meiosis
Meiosis involves two successive divisions (Meiosis I and Meiosis II) without DNA replication between them. It reduces the chromosome number from 2n to n.
Meiosis I — Reductional Division:
| Stage | Key Events |
|---|---|
| Prophase I | Longest phase; five sub-stages (see below) |
| Metaphase I | Homologous chromosome pairs (bivalents) align at the metaphase plate; spindle fibres attach to one chromosome of each pair |
| Anaphase I | Homologous chromosomes separate and move to opposite poles; sister chromatids remain attached |
| Telophase I | Two haploid nuclei form; chromosomes may briefly decondense; cytokinesis may follow (diatoms) or not (most organisms) |
Sub-stages of Prophase I:
| Sub-stage | Key Features |
|---|---|
| Leptotene | Chromosomes appear as thin, long threads; not yet distinguishable as pairs |
| Zygotene | Synapsis — homologous chromosomes pair forming bivalents (tetrads); synaptonemal complex forms |
| Pachytene | Crossing over occurs between non-sister chromatids at chiasmata; exchange of genetic material |
| Diplotene | Synaptonemal complex dissolves; homologous chromosomes begin to separate but remain attached at chiasmata |
| Diakinesis | Chromosomes fully condensed; chiasmata terminalise; nucleolus and nuclear envelope disappear; spindle forms |
Meiosis II — Equational Division (similar to mitosis):
| Stage | Key Events |
|---|---|
| Prophase II | Chromosomes recondense; nuclear envelope breaks down; spindle forms |
| Metaphase II | Chromosomes align at metaphase plate; spindle fibres attach to kinetochores |
| Anaphase II | Centromeres split; sister chromatids separate and move to opposite poles |
| Telophase II | Chromosomes decondense; nuclear envelope reforms; cytokinesis follows |
Significance of Mitosis
1. Growth: Increases cell number during development (e.g. zygote to embryo to adult). 2. Repair: Replaces dead or damaged cells (e.g. skin, gut lining, blood cells). 3. Asexual reproduction: Produces genetically identical offspring (e.g. budding in yeast, binary fission in Amoeba). 4. Maintains chromosome number: Daughter cells have the same number and kind of chromosomes as the parent cell. 5. Genetic stability: No crossing over — exact copies of chromosomes are distributed.
Significance of Meiosis
1. Maintains chromosome number: Produces haploid gametes, so fertilisation restores the diploid number. Without meiosis, chromosome number would double each generation. 2. Genetic variation: Crossing over during pachytene and independent assortment during metaphase I create new combinations of alleles. 3. Basis of evolution: Genetic variation is the raw material for natural selection. 4. Gamete formation: Essential for sexual reproduction — spermatogenesis and oogenesis.
Comparison of Mitosis and Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Divisions | One | Two (Meiosis I + II) |
| Daughter cells | Two | Four |
| Ploidy of daughter cells | Diploid (2n) | Haploid (n) |
| Genetic composition | Identical to parent | Genetically different |
| Crossing over | Absent | Occurs in prophase I |
| Homologous pairing | Does not occur | Occurs (bivalent formation) |
| Metaphase plate | Individual chromosomes | Bivalents (Metaphase I), then chromosomes (Metaphase II) |
| Anaphase separation | Sister chromatids separate | Homologous chromosomes separate (Anaphase I); sister chromatids separate (Anaphase II) |
| Occurs in | Somatic cells | Germ cells |
| Function | Growth, repair, asexual reproduction | Gamete formation, genetic variation |
Important Diagrams to Practise
1. Cell cycle diagram — show G₁, S, G₂, and M phases with relative durations, plus G₀ branch. 2. Stages of mitosis — prophase, metaphase, anaphase, telophase with labelled chromosomes, spindle fibres, centrioles, and metaphase plate. 3. Stages of meiosis I and II — all 8 stages showing reduction in chromosome number. 4. Prophase I sub-stages — especially zygotene (synapsis) and pachytene (crossing over) with bivalent/tetrad and chiasmata labelled. 5. Cytokinesis — cleavage furrow in animal cell vs cell plate in plant cell.
Common Mistakes to Avoid
1. Confusing Anaphase I with Anaphase II — in Anaphase I, homologous chromosomes separate (sister chromatids stay together); in Anaphase II, sister chromatids separate. 2. Saying meiosis produces two cells — meiosis produces four haploid cells after two divisions. 3. Placing crossing over in prophase of mitosis — crossing over occurs only in prophase I of meiosis. 4. Confusing chromatid and chromosome — before S phase, each chromosome is one chromatid; after replication, each chromosome has two sister chromatids. 5. Forgetting that DNA replication occurs before meiosis I but not between meiosis I and II — this is why the chromosome number is halved. 6. Omitting synapsis from zygotene — the specific pairing of homologous chromosomes is the defining feature of zygotene. 7. Confusing the cell plate direction — plant cell plate forms from centre outward (centrifugal), not from periphery inward.
Exam Tips
- For VSA (1 mark): Know the five sub-stages of prophase I in order, the phase where DNA replication occurs (S phase), and the definition of synapsis and crossing over.
- For SA (2–3 marks): Prepare a comparison table of mitosis vs meiosis, or explain the significance of meiosis.
- For LA (5 marks): Describe all stages of mitosis with diagrams, or explain prophase I sub-stages in detail. A "compare mitosis and meiosis" question is almost certain in board exams.
- The mnemonic for prophase I: Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis — "Lazy Zebras Play Doubtful Diaries" or "Let Zoologists Paint Different Diagrams".
- Frequently asked NCERT lines: "Metaphase is the best stage to study chromosome morphology" and "Meiosis enables the maintenance of chromosome number across generations."
How to Use This Guide
This guide is structured for layered revision:
- First pass: Read Chapter Introduction and Key Definitions to orient yourself.
- Second pass: Study Detailed Concepts section by section, matching with your NCERT textbook.
- Third pass: Use the tables and Common Mistakes for quick comparison and self-testing.
- Final pass: Use Quick Revision and Exam-Day Recall for last-minute review.
Additional Detail: Key Numerical Facts for Exams
| Fact | Value |
|---|---|
| Typical human cell cycle duration | ~24 hours |
| G₁ phase duration | ~11 hours (longest) |
| S phase duration | ~8 hours |
| G₂ phase duration | ~4 hours |
| M phase duration | ~1 hour (shortest) |
| Interphase as % of cell cycle | ~95% |
| Number of cells produced by mitosis | 2 |
| Number of cells produced by meiosis | 4 |
| Ploidy after mitosis | 2n (diploid, same as parent) |
| Ploidy after meiosis | n (haploid, half of parent) |
| DNA content after S phase | Doubled (4C for a diploid cell) |
| DNA content after mitosis | Restored to 2C per daughter cell |
| DNA content after Meiosis I | 2C per cell (chromosomes halved but each still has 2 chromatids) |
| DNA content after Meiosis II | 1C per cell (each chromosome is a single chromatid) |
Additional Detail: Synaptonemal Complex
- The synaptonemal complex is a protein structure that forms between homologous chromosomes during the zygotene stage.
- It consists of two lateral elements (along each homologous chromosome) connected by a central element with transverse filaments.
- It holds homologous chromosomes in precise alignment, facilitating crossing over during pachytene.
- The synaptonemal complex disassembles during diplotene, allowing homologous chromosomes to begin separating.
- Without the synaptonemal complex, proper synapsis and crossing over cannot occur, leading to errors in chromosome segregation.
Additional Detail: Significance of Crossing Over
- Crossing over occurs during pachytene of prophase I.
- It involves the exchange of genetic material between non-sister chromatids of homologous chromosomes.
- The enzyme recombinase mediates the breakage and rejoining of DNA strands.
- Crossing over produces recombinant chromosomes with new combinations of alleles — this is the primary source of genetic variation in sexually reproducing organisms.
- The points where crossing over occurred are visible as chiasmata during diplotene and diakinesis.
- Multiple cross-overs can occur between a single pair of homologous chromosomes, increasing variation.
- Crossing over, combined with independent assortment of chromosomes at metaphase I, generates enormous genetic diversity.
How to Use This Guide (continued)
This guide is structured for layered revision:
- First pass: Read Chapter Introduction and Key Definitions to orient yourself.
- Second pass: Study Detailed Concepts section by section, matching with your NCERT textbook.
- Third pass: Use the tables and Common Mistakes for quick comparison and self-testing.
- Final pass: Use Quick Revision and Exam-Day Recall for last-minute review.
Additional Detail: Meiosis I vs Meiosis II
| Feature | Meiosis I | Meiosis II |
|---|---|---|
| Type | Reductional division | Equational division |
| Chromosome number | 2n → n | n → n (stays the same) |
| DNA replication before | Yes (in interphase before Meiosis I) | No (no interphase between I and II) |
| Homologous pairing | Yes (bivalents form) | No |
| Crossing over | Yes (prophase I, pachytene) | No |
| Separation in anaphase | Homologous chromosomes separate | Sister chromatids separate |
| Genetic variation | Yes (crossing over + independent assortment) | No new variation |
| Number of cells produced | 2 (from 1) | 4 (from the 2 produced by Meiosis I) |
Additional Detail: Stages of Mitosis in Depth
Prophase — key points to remember for exams:
- Chromosomes first become visible as thin threads, then condense and thicken.
- Each chromosome consists of two sister chromatids held at the centromere.
- The nucleolus and nuclear envelope disappear — this marks the transition to metaphase.
- In animal cells, centrioles migrate to opposite poles and organise the spindle fibres.
Metaphase — key points to remember for exams:
- This is the stage where chromosomes are most condensed and easiest to observe under a microscope.
- The metaphase plate is an imaginary plane equidistant from both poles — chromosomes do not physically line up on a plate.
- The kinetochore (a protein structure on the centromere) attaches to spindle microtubules.
Anaphase — key points to remember for exams:
- The centromere splits first, then sister chromatids are pulled to opposite poles.
- The chromosomes appear V-shaped as they are pulled by the centromere with arms trailing.
- Anaphase is the shortest stage of mitosis.
Telophase — key points to remember for exams:
- Essentially the reverse of prophase — chromosomes decondense, nuclear envelope reforms.
- The nucleolus reappears in each new nucleus.
- Cytokinesis may begin during telophase or slightly after.
NCERT Reading Method
1. Read the chapter once to understand the full cell cycle flow and both types of division. 2. Mark all definitions (synapsis, crossing over, chiasmata, bivalent, tetrad, G₀ phase) and the prophase I sub-stage sequence. 3. Pay close attention to the difference between Anaphase I (homologous chromosomes separate) and Anaphase II (sister chromatids separate) — this is the most tested distinction. 4. Solve in-text and exercise questions without copying — especially the comparison of mitosis and meiosis. 5. Compare your answers with NCERT language and correct missing keywords like "reductional division" and "equational division." 6. Reattempt wrong questions after one day.
Answer-Writing Tips
- Use NCERT keywords in definitions — "equational division" for mitosis, "reductional division" for Meiosis I.
- Draw labelled diagrams wherever relevant — mitosis stages, prophase I sub-stages, and the cell cycle are all high-yield diagrams.
- For comparison questions, always use a table format — "Compare mitosis and meiosis" is the single most frequently asked 5-mark question.
- Write prophase I sub-stages in the correct order: Leptotene, Zygotene, Pachytene, Diplotene, Diakinesis.
- In 5-mark answers on mitosis, describe each stage with 2–3 key events and a diagram.
- Mention the mnemonic for prophase I: "Let Zoologists Paint Different Diagrams."
Practice Checklist
- Solve all NCERT exercise questions from this chapter — especially the comparison questions.
- Prepare five one-mark facts: prophase I sub-stage order, G₀ phase definition, metaphase plate, synapsis stage (zygotene), crossing over stage (pachytene).
- Write one three-mark answer on "Explain the significance of meiosis."
- Write one five-mark answer on "Compare mitosis and meiosis" using a table.
- Draw and label the five stages of prophase I from memory.
- Update your mistake notebook before moving to the next chapter.
Quick Revision Plan
| Time | Task |
|---|---|
| 15 minutes | Read marked NCERT lines — focus on prophase I sub-stages and mitosis stages |
| 20 minutes | Revise this guide, redrawing mitosis/meiosis diagrams and comparison table |
| 25 minutes | Solve textbook or sample questions — especially comparison and significance questions |
| 10 minutes | Correct mistakes and note weak areas |
Quick Revision
| Topic | One-Line Recall |
|---|---|
| Cell cycle phases | G₁ (growth) → S (DNA replication) → G₂ (preparation) → M (division); G₀ = non-dividing |
| Mitosis | One division → 2 identical diploid cells; for growth, repair, asexual reproduction |
| Meiosis | Two divisions → 4 different haploid cells; for gamete formation, genetic variation |
| Prophase I sub-stages | Leptotene → Zygotene (synapsis) → Pachytene (crossing over) → Diplotene (chiasmata) → Diakinesis |
| Key difference: Anaphase | Mitosis: sister chromatids separate; Meiosis I: homologous chromosomes separate |
| Cytokinesis | Animal: cleavage furrow (centripetal); Plant: cell plate (centrifugal) |
| Significance of meiosis | Maintains chromosome number + introduces genetic variation |
| Meiosis I vs Meiosis II | Meiosis I = reductional (2n→n); Meiosis II = equational (similar to mitosis) |
Exam-Day Recall
Before the exam, revise Cell Cycle and Cell Division using your marked NCERT pages, the prophase I mnemonic, and the mitosis-vs-meiosis comparison table. If you can explain the five prophase I sub-stages, the difference between Anaphase I and Anaphase II, and the significance of meiosis aloud in five minutes, your basics are ready.