Quick Summary
The cell cycle is the series of events leading to cell division and production of daughter cells. It has two main phases: Interphase (preparation) and M phase (division). Mitosis produces two identical daughter cells (for growth/repair); meiosis produces four haploid cells (for sexual reproduction). Both are extensively tested in NEET.
Cell Cycle Phases
Interphase (90% of cell cycle time)
| Phase | Events |
|---|---|
| G₁ (Gap 1) | Cell growth; synthesis of proteins, RNA; preparation for DNA replication |
| S (Synthesis) | DNA replication; chromosome number same but DNA amount doubles |
| G₂ (Gap 2) | Further cell growth; synthesis of proteins needed for division |
| G₀ | Resting/quiescent phase; cells exit cycle (nerve cells, muscle cells) |
M Phase (Mitotic Phase)
Actual cell division — mitosis or meiosis.
Mitosis (Equational Division)
Mnemonic: PMATI (Prophase, Metaphase, Anaphase, Telophase, Interkinesis)
Stages of Mitosis
| Stage | Key Events |
|---|---|
| Prophase | Chromatin condenses into chromosomes; spindle fibres form; nucleolus and nuclear envelope disappear |
| Metaphase | Chromosomes align at the equatorial plate; spindle fibres attach to centromeres (kinetochores) |
| Anaphase | Centromeres split; sister chromatids pulled to opposite poles; cell elongates |
| Telophase | Nuclear envelope reforms around each set; chromosomes decondense; nucleolus reappears |
| Cytokinesis | Cytoplasm divides — cell plate (plants) or cleavage furrow (animals) |
Significance of Mitosis: Growth, repair and regeneration, asexual reproduction, maintains chromosome number.
Meiosis (Reductional Division)
Occurs in: Reproductive organs (testes, ovaries, anthers, ovules)
Meiosis I (Reductional)
| Stage | Key Events |
|---|---|
| Prophase I | Longest phase; 5 sub-stages (ZLPDD): Leptotene (chromosomes condense), Zygotene (homologue pairing = synapsis, forming bivalents), Pachytene (crossing over occurs at chiasmata), Diplotene (chromosomes separate slightly, chiasmata visible), Diakinesis (chromosomes maximally condensed, nuclear envelope breaks) |
| Metaphase I | Bivalents (not individual chromosomes) align at equatorial plate |
| Anaphase I | Homologous chromosomes (not chromatids) separate to poles |
| Telophase I | Two haploid nuclei formed (each with 2 chromatids per chromosome) |
Meiosis II (Similar to Mitosis)
Separates sister chromatids → 4 haploid cells
Comparison: Mitosis vs Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Divisions | 1 | 2 |
| Daughter cells | 2 | 4 |
| Chromosome number | 2n → 2n | 2n → n |
| Crossing over | Absent | Present (Prophase I) |
| Occurs in | Somatic cells | Germ cells |
| Purpose | Growth, repair | Sexual reproduction |
| Genetic outcome | Identical | Varied (recombination) |
Important Questions
One Mark
- In which phase of meiosis does crossing over occur?
- What is synapsis?
- How many daughter cells are produced by meiosis?
- Name the organelle that forms the spindle fibres in animal cells.
Short Answer (3 marks)
- Describe the events of Prophase I of meiosis. Name its five sub-stages.
- What is the significance of crossing over in meiosis?
- Compare the metaphase of mitosis with metaphase I of meiosis.
Long Answer (5 marks)
- Describe the stages of mitosis with diagrams. What is the significance of mitosis?
- Compare mitosis and meiosis on the basis of seven characters. What is the biological significance of meiosis?
Common Mistakes
- S phase: DNA doubles but chromosome NUMBER stays the same — confusion between DNA content (2C → 4C) and chromosome number.
- Anaphase (mitosis): sister chromatids separate; Anaphase I (meiosis): homologous chromosomes separate — completely different.
- Crossing over occurs in Prophase I, specifically at pachytene stage — not at metaphase.
- Meiosis II is like mitosis but starts with haploid cells (n), not diploid.
Board Exam Tips
- Mitosis stages with diagrams (4 phases) is a standard 5-mark question — draw clearly and label.
- Prophase I sub-stages (ZLPDD) are frequently asked as a 3-mark question.
- Mitosis vs meiosis comparison table (5–7 characters) is a reliable 5-mark question.
- NEET: Meiosis I stages, crossing over, and the result of cell division are tested in almost every paper.
Frequently Asked Questions
Interphase (G1 → S → G2) and M phase (mitosis/meiosis). G1: cell growth, protein synthesis. S: DNA replication (chromosome number stays same but DNA doubles). G2: preparation for division. M phase: actual cell division.