Cell Division - Mitosis and Meiosis Complete NEET Notes 2026
Master Cell Division for NEET 2026 with comprehensive notes on cell cycle, mitosis, meiosis, and their significance. NCERT-aligned content with diagrams and previous year questions.
Key Takeaways
- 1Cell cycle consists of Interphase (G1, S, G2) and M phase (Mitosis + Cytokinesis)
- 2Mitosis produces 2 identical diploid cells; Meiosis produces 4 haploid cells
- 3Crossing over occurs during Prophase I of meiosis - source of genetic variation
- 4Checkpoints at G1/S and G2/M regulate cell division
- 5Meiosis I is reductional; Meiosis II is equational division
Remember these points for your NEET preparation
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Cell Division - Mitosis and Meiosis Complete NEET Notes
Cell Division contributes 3-5 questions in NEET and is crucial for understanding genetics and reproduction. This guide covers all NCERT concepts for NEET 2026.
Cell Cycle
The cell cycle is the series of events that take place in a cell leading to its division and duplication.
Duration of Cell Cycle
| Cell Type | Duration | Example |
|---|---|---|
| Fast-dividing cells | 8-10 hours | Bone marrow cells |
| Average cells | 24 hours | Human cells in culture |
| Non-dividing cells | Arrested in G0 | Neurons, muscle cells |
Phases of Cell Cycle
1. Interphase (90-95% of cell cycle)
| Phase | Events | Duration (24h cycle) |
|---|---|---|
| G1 (Gap 1) | Cell growth, protein synthesis, organelle duplication | ~11 hours |
| S (Synthesis) | DNA replication, histone synthesis | ~8 hours |
| G2 (Gap 2) | Continued growth, preparation for mitosis | ~4 hours |
G0 Phase: Quiescent stage where cells exit the cell cycle (neurons, cardiac muscle cells).
NEET Important: DNA content doubles during S phase (2C → 4C), but chromosome number remains same.
2. M Phase (Mitotic Phase)
Comprises:
- Karyokinesis: Nuclear division
- Cytokinesis: Cytoplasmic division
Mitosis
Mitosis is equational division producing two identical daughter cells with the same chromosome number as the parent cell.
Where Does Mitosis Occur?
- Somatic cells
- Growth and repair
- Asexual reproduction
Stages of Mitosis
Prophase (Longest stage)
- Chromatin condenses into chromosomes
- Each chromosome has 2 sister chromatids joined at centromere
- Nucleolus disappears
- Nuclear envelope begins to break down
- Centrioles move to opposite poles (animal cells)
- Spindle formation begins
Prometaphase
- Nuclear envelope completely breaks down
- Spindle fibers attach to kinetochores
- Chromosomes move toward cell equator
Metaphase (Best stage to study chromosomes)
- Chromosomes align at metaphase plate (equator)
- Each chromosome attached to spindle fibers from both poles
- Maximum condensation of chromosomes
- Metaphase arrest - used for karyotyping
Anaphase (Shortest stage)
- Sister chromatids separate at centromere
- Chromatids (now chromosomes) move to opposite poles
- Anaphase A: Chromosome movement
- Anaphase B: Pole separation
- Cell elongates
Telophase
- Chromosomes reach poles and decondense
- Nuclear envelope reforms
- Nucleolus reappears
- Spindle fibers disappear
Cytokinesis
| Feature | Animal Cells | Plant Cells |
|---|---|---|
| Mechanism | Cleavage furrow (contractile ring) | Cell plate formation |
| Direction | Centripetal (outside → inside) | Centrifugal (inside → outside) |
| Structure | Actin-myosin ring | Vesicles from Golgi |
Meiosis
Meiosis is reductional division producing four haploid daughter cells from one diploid parent cell.
Where Does Meiosis Occur?
- Germ cells in gonads
- Produces gametes (eggs, sperm)
- Sporocytes in plants
Meiosis I (Reductional Division)
Prophase I (Longest and most complex)
Five substages (Mnemonic: LZPDD)
| Substage | Events |
|---|---|
| Leptotene | Chromosomes become visible, attached to nuclear envelope |
| Zygotene | Homologous chromosomes pair up (synapsis), form bivalents |
| Pachytene | Crossing over occurs, recombination nodules form |
| Diplotene | Chiasmata visible, bivalents begin to separate |
| Diakinesis | Terminalization of chiasmata, nuclear envelope breaks down |
Key Terms:
- Synapsis: Pairing of homologous chromosomes
- Bivalent/Tetrad: Paired homologs (4 chromatids)
- Synaptonemal complex: Protein structure holding homologs
- Crossing over: Exchange of genetic material between non-sister chromatids
- Chiasmata: X-shaped structures where crossing over occurred
Metaphase I
- Bivalents align at metaphase plate
- Random arrangement (independent assortment)
- Homologs face opposite poles
Anaphase I
- Homologous chromosomes separate (NOT sister chromatids)
- Move to opposite poles
- Chromosome number halved (2n → n)
Telophase I & Cytokinesis I
- Chromosomes reach poles
- Nuclear envelope may reform (species-dependent)
- Two haploid cells formed
- Each chromosome still has 2 chromatids
Interkinesis
- Brief rest period between Meiosis I and II
- NO DNA replication occurs
Meiosis II (Equational Division)
Similar to mitosis but in haploid cells.
| Phase | Events |
|---|---|
| Prophase II | Chromosomes condense, spindle forms |
| Metaphase II | Chromosomes align at equator |
| Anaphase II | Sister chromatids separate |
| Telophase II | 4 haploid cells formed |
Mitosis vs Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Type | Equational | Reductional (I) + Equational (II) |
| Divisions | One | Two |
| Daughter cells | 2 diploid | 4 haploid |
| Genetic identity | Identical to parent | Genetically different |
| Crossing over | No | Yes (Prophase I) |
| Synapsis | No | Yes |
| Chiasmata | No | Yes |
| Occurs in | Somatic cells | Germ cells |
| Purpose | Growth, repair | Gamete formation |
Significance of Cell Division
Significance of Mitosis
- Growth: Increases cell number
- Repair: Replaces damaged cells
- Regeneration: Wound healing
- Asexual reproduction: Binary fission, budding
- Genetic stability: Maintains chromosome number
Significance of Meiosis
- Gamete formation: Produces haploid cells
- Maintains chromosome number: 2n → n → 2n (after fertilization)
- Genetic variation:
- Crossing over
- Independent assortment
- Random fertilization
- Evolution: Provides raw material for natural selection
Cell Cycle Regulation
Checkpoints
| Checkpoint | Location | Function |
|---|---|---|
| G1 (Restriction point) | End of G1 | Checks cell size, nutrients, DNA damage |
| G2/M | End of G2 | Checks DNA replication completion |
| Metaphase (Spindle) | During M | Checks spindle attachment |
Cyclin-CDK Complex
- Cyclins: Regulatory proteins (levels fluctuate)
- CDKs (Cyclin-dependent kinases): Enzyme proteins (constant)
- Different Cyclin-CDK combinations drive cell cycle progression
Cancer Connection: Mutations in cell cycle regulators (like p53) can lead to uncontrolled cell division.
Important Diagrams for NEET
- Cell cycle diagram showing all phases
- Mitosis stages with chromosome behavior
- Meiosis I and II showing chromosome movement
- Crossing over at molecular level
- Comparison of mitosis and meiosis
Previous Year NEET Questions
Q1 (NEET 2023): During which stage of meiosis does crossing over occur?
- (a) Leptotene
- (b) Zygotene
- (c) Pachytene ✓
- (d) Diplotene
Q2 (NEET 2022): The shortest phase of mitosis is:
- (a) Prophase
- (b) Metaphase
- (c) Anaphase ✓
- (d) Telophase
Q3 (NEET 2021): Synapsis occurs between:
- (a) Sister chromatids
- (b) Homologous chromosomes ✓
- (c) Non-homologous chromosomes
- (d) A chromosome and spindle
Q4 (NEET 2020): DNA replication occurs in which phase of cell cycle?
- (a) G1 phase
- (b) S phase ✓
- (c) G2 phase
- (d) M phase
Q5 (NEET 2019): Chiasmata are visible during:
- (a) Leptotene
- (b) Zygotene
- (c) Pachytene
- (d) Diplotene ✓
Quick Revision Points
- Cell cycle phases: G1 → S → G2 → M
- DNA replication: S phase only
- Longest phase of cell cycle: G1
- Longest phase of mitosis: Prophase
- Shortest phase of mitosis: Anaphase
- Best stage for karyotyping: Metaphase
- Crossing over location: Pachytene (Prophase I)
- Chiasmata visible: Diplotene
- Reductional division: Meiosis I
- Equational division: Mitosis and Meiosis II
- Number of cells from meiosis: 4 haploid
- Number of cells from mitosis: 2 diploid
FAQs
Q: Why is meiosis I called reductional division? A: Because chromosome number is halved (2n → n) as homologous chromosomes separate. Sister chromatids remain together until Meiosis II.
Q: What is the significance of crossing over? A: Crossing over exchanges genetic material between homologous chromosomes, creating new combinations of alleles. This increases genetic diversity in offspring.
Q: Why doesn't DNA replication occur between Meiosis I and II? A: After Meiosis I, each chromosome still consists of two sister chromatids (from S phase before Meiosis I). These need to be separated in Meiosis II to produce haploid gametes.
Q: What happens if cell cycle checkpoints fail? A: Checkpoint failure can lead to uncontrolled cell division, accumulation of DNA damage, and potentially cancer. The p53 protein (tumor suppressor) is crucial for checkpoint control.
Q: Why is metaphase the best stage for studying chromosomes? A: During metaphase, chromosomes are maximally condensed, aligned at the cell equator, and clearly visible under a microscope. This makes counting and identifying chromosomes easier (karyotyping).