Cell Cycle and Cell Division
Introduction
The cell cycle is the series of events through which a cell grows, replicates its DNA, and divides to produce daughter cells. In ICSE Class 10 Biology, you study the two main types of cell division — mitosis and meiosis — and their significance.
The Cell Cycle
| Phase | Description |
|---|---|
| Interphase | Cell grows and DNA replicates (G₁, S, G₂ phases) |
| M Phase | Mitosis (division of nucleus) + Cytokinesis (division of cytoplasm) |
Interphase Sub-phases
- G₁ phase: Cell growth, protein synthesis, organelle duplication.
- S phase: DNA replication (each chromosome becomes two chromatids).
- G₂ phase: Further growth and preparation for mitosis.
Mitosis
Mitosis produces two genetically identical daughter cells (diploid, 2n).
Stages of Mitosis
| Stage | Key events |
|---|---|
| Prophase | Chromosomes condense, nuclear membrane breaks down, spindle fibres form, centrioles move to poles |
| Metaphase | Chromosomes align at the equator, spindle fibres attach to centromeres |
| Anaphase | Sister chromatids separate and move to opposite poles |
| Telophase | Chromosomes decondense, nuclear membrane reforms, spindle fibres disappear |
| Cytokinesis | Cytoplasm divides (cell plate in plants, cleavage furrow in animals) |
Significance of Mitosis
- Growth and repair of tissues.
- Asexual reproduction (in unicellular organisms).
- Replacement of worn-out cells.
Meiosis
Meiosis produces four genetically different daughter cells (haploid, n). It involves two successive divisions.
Meiosis I (Reduction Division)
| Stage | Key events |
|---|---|
| Prophase I | Homologous chromosomes pair (synapsis), crossing over occurs (exchange of genetic material) |
| Metaphase I | Homologous pairs align at the equator |
| Anaphase I | Homologous chromosomes separate (not chromatids) |
| Telophase I | Two haploid cells form |
Meiosis II (Equational Division)
Similar to mitosis: sister chromatids separate to produce four haploid cells.
Significance of Meiosis
- Maintains chromosome number across generations (halves it during gamete formation).
- Introduces genetic variation through crossing over and independent assortment.
Comparison: Mitosis vs Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Number of divisions | 1 | 2 |
| Daughter cells | 2 (identical) | 4 (genetically different) |
| Chromosome number | Same as parent (2n) | Half of parent (n) |
| Crossing over | No | Yes (in Prophase I) |
| Occurs in | Somatic (body) cells | Germ cells (gamete formation) |
| Purpose | Growth, repair | Sexual reproduction |
Chromosomes
- Structure: DNA + histone proteins.
- Centromere: Point where sister chromatids are joined.
- Karyotype: The organised arrangement of chromosomes.
| Term | Definition |
|---|---|
| Chromatid | One half of a replicated chromosome |
| Centromere | Constriction point holding chromatids together |
| Homologous chromosomes | Pair of chromosomes (one from each parent) |
| Diploid (2n) | Two sets of chromosomes |
| Haploid (n) | One set of chromosomes |
Common Mistakes and Fixes
| Mistake | Fix |
|---|---|
| Confusing sister chromatids with homologous chromosomes | Chromatids = identical copies; Homologous = similar but from different parents |
| Thinking crossing over occurs in mitosis | Crossing over occurs only in Prophase I of meiosis |
| Mixing anaphase events in mitosis vs meiosis I | Anaphase: chromatids separate; Anaphase I: homologous chromosomes separate |
| Calling cytokinesis part of mitosis | Cytokinesis is a separate process following nuclear division |
ICSE Exam Focus
This chapter carries 4–6 marks. Key topics: stages of mitosis and meiosis, differences between the two, significance of each, chromosome terminology.
Marks Blueprint: Mitosis stages — 2 marks, Meiosis — 2 marks, Differences/significance — 2 marks.
Self-Test Questions
-
List the stages of mitosis in order and describe what happens in each.
-
What is crossing over? In which stage of cell division does it occur?
-
Differentiate between mitosis and meiosis (at least four differences).
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What is the significance of meiosis in sexual reproduction?
-
Define: (a) chromatid, (b) centromere, (c) homologous chromosomes, (d) diploid.
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Why is mitosis important for the growth of an organism?
