Cell Division
Cell division is the process by which one parent cell divides to produce daughter cells. It underlies growth, tissue repair, and reproduction.
A. The Cell Cycle
What is the Cell Cycle?
- The cell cycle is the series of events a cell goes through from its formation to its division into two daughter cells
- It has two main phases: Interphase (preparation) and Mitotic phase / M-phase (division)
- Interphase is the longest phase of the cell cycle — the cell grows and prepares for division
- Interphase is sometimes incorrectly called a resting phase — in reality it is a period of intense metabolic activity
Cell Cycle
Cell Cycle = Interphase + Mitotic Phase (M-phase)
Interphase is the longest phase. M-phase includes mitosis + cytokinesis.
Phases of Interphase
| Sub-phase | Full Name | Key Events |
|---|---|---|
| G₁ | First Growth Phase | Cell grows in size; proteins and organelles are synthesised; cell carries out normal metabolic activities |
| S | Synthesis Phase | DNA replication occurs — DNA amount doubles so each daughter cell receives a complete set of chromosomes |
| G₂ | Second Growth Phase | Cell continues to grow; organelles duplicate; cell prepares enzymes and proteins needed for division |
⚡ MCQ Tip DNA replication occurs in the S phase of interphase — not during mitosis itself. After S phase, each chromosome consists of two identical sister chromatids.
B. Mitosis
Mitosis Overview
- Produces 2 genetically identical daughter cells — same chromosome number as parent
- Daughter cells are diploid (2n) — in humans: 46 chromosomes each
- Occurs in somatic (body) cells — skin, muscle, liver, bone marrow
- Purpose: growth, tissue repair, and asexual reproduction
- One division cycle — PMAT + Cytokinesis: Prophase → Metaphase → Anaphase → Telophase → Cytokinesis
- Prophase is the longest phase of mitosis
Mitosis Result
1 parent cell (2n) → 2 identical daughter cells (2n)
No reduction in chromosome number. Genetically identical.
Phases of Mitosis — PMATC
| Phase | Key Events |
|---|---|
| Prophase (longest phase) |
Chromosomes condense and become visible under the microscope; nuclear envelope breaks down; spindle fibres begin to form from centrioles |
| Metaphase | Chromosomes line up at the cell's equator (metaphase plate); spindle fibres attach to centromeres of each chromosome |
| Anaphase (shortest phase) |
Sister chromatids are pulled apart to opposite poles by shortening spindle fibres; cell elongates |
| Telophase | Nuclear envelopes reform around each set of chromosomes at the poles; chromosomes decondense; two nuclei form |
| Cytokinesis | Cytoplasm divides — in animals: cleavage furrow pinches the cell; in plants: a cell plate forms between the two nuclei. Two daughter cells are produced. |
⚡ Mnemonic Please Make A Tasty Cake — Prophase, Metaphase, Anaphase, Telophase, Cytokinesis
Significance of Mitosis
- Growth: Increase in number of cells in multicellular organisms — body grows from a single fertilised egg by repeated mitosis
- Repair & Replacement: Worn-out or damaged cells are replaced — e.g. skin cells, red blood cells
- Asexual Reproduction: Unicellular organisms such as Amoeba reproduce by mitosis (binary fission)
- Genetic Stability: Daughter cells are genetically identical to parent — maintains chromosome number across generations of cells
C. Meiosis
Meiosis Overview
- Produces 4 genetically different daughter cells — half the chromosome number of the parent
- Daughter cells are haploid (n) — in humans: 23 chromosomes each (gametes)
- Occurs in reproductive organs (gonads) — ovaries and testes
- Purpose: formation of gametes (sperm and egg cells) for sexual reproduction
- Consists of two successive divisions: Meiosis I (reduction division) + Meiosis II (equatorial division)
- Crossing over occurs during Prophase I — homologous chromosomes exchange genetic segments, creating genetic variation
Meiosis Result
1 parent cell (2n) → 4 unique daughter cells (n)
Chromosome number halved. Each cell is genetically unique.
⚡ MCQ Tip Human body cells: 46 chromosomes (2n). After meiosis → gametes with 23 chromosomes (n). Fertilisation restores 46.
Meiosis I — Reduction Division
Meiosis I separates homologous chromosomes. Chromosome number is reduced from diploid (2n) to haploid (n).
| Phase | Key Events |
|---|---|
| Prophase I (longest & most complex) |
Chromosomes condense; homologous chromosomes pair up (synapsis) to form bivalents (tetrads); crossing over occurs — homologs exchange DNA segments creating genetic variation; nuclear envelope breaks down |
| Metaphase I | Bivalents (pairs of homologous chromosomes) line up at the equator; spindle fibres attach to centromeres |
| Anaphase I | Homologous chromosomes (not sister chromatids) separate and move to opposite poles — this is the reduction step |
| Telophase I | Two haploid nuclei form; nuclear envelopes may reform; cytokinesis may occur — two haploid cells result, each with duplicated chromosomes |
⚡ Key Distinction In Anaphase I → homologous chromosomes separate. In Anaphase II (and Anaphase of mitosis) → sister chromatids separate.
Meiosis II — Equatorial Division
Meiosis II is similar to mitosis — it separates sister chromatids. No further chromosome reduction occurs.
| Phase | Key Events |
|---|---|
| Prophase II | Chromosomes recondense; nuclear envelope breaks down; spindle fibres form |
| Metaphase II | Chromosomes (single, not in pairs) line up at the equator in each cell |
| Anaphase II | Sister chromatids are pulled apart to opposite poles by spindle fibres |
| Telophase II + Cytokinesis | Four haploid nuclei form; cytokinesis produces 4 haploid daughter cells — each genetically unique |
Significance of Meiosis
- Gamete Formation: Produces haploid gametes (sperm and eggs) needed for sexual reproduction
- Maintains Chromosome Number: Halving chromosome number in gametes ensures that fertilisation restores the diploid number — e.g. 23 + 23 = 46 in humans
- Genetic Variation: Crossing over during Prophase I shuffles genes between homologous chromosomes, producing offspring that are not genetically identical to either parent — raw material for evolution
- Evolution: Genetic variation produced by meiosis and fertilisation drives natural selection and evolution
D. Mitosis vs Meiosis
Comparison Table
| Feature | Mitosis | Meiosis |
|---|---|---|
| Daughter cells produced | 2 | 4 |
| Chromosome number | Diploid (2n) — same as parent | Haploid (n) — half of parent |
| Genetic variation | None — cells are identical | Yes — crossing over and random assortment |
| Number of divisions | One | Two (Meiosis I + II) |
| Where it occurs | Somatic (body) cells | Gonads (ovaries & testes) |
| Purpose | Growth, repair, asexual reproduction | Sexual reproduction (gamete formation) |
| Crossing over | Does not occur | Occurs in Prophase I |
| Synapsis of homologs | Does not occur | Occurs in Prophase I |
| What separates in Anaphase | Sister chromatids | Anaphase I: homologs; Anaphase II: sister chromatids |
| Example (humans) | Skin, liver, blood cells | Sperm and egg cells |
Quick MCQ Revision
| Fact | Answer |
|---|---|
| The two main phases of the cell cycle | Interphase + Mitotic (M) phase |
| Longest phase of the cell cycle | Interphase |
| DNA replication occurs in which sub-phase? | S (Synthesis) phase of Interphase |
| G₁ phase function | Cell growth and protein synthesis |
| G₂ phase function | Further growth; preparation of enzymes for division |
| Mitosis produces | 2 identical diploid (2n) cells |
| Meiosis produces | 4 unique haploid (n) cells |
| Longest phase of mitosis | Prophase |
| Mitosis phases (mnemonic) | PMATC — "Please Make A Tasty Cake" |
| Chromosomes align at the equator (metaphase plate) | Metaphase |
| Sister chromatids pulled to opposite poles | Anaphase |
| Cytokinesis in plant cells | Cell plate forms between two daughter nuclei |
| Cytokinesis in animal cells | Cleavage furrow pinches the cytoplasm |
| Crossing over occurs in | Prophase I of Meiosis I — creates genetic variation |
| Synapsis / bivalent (tetrad) formation occurs in | Prophase I |
| Meiosis I separates | Homologous chromosomes (reduction division) |
| Meiosis II separates | Sister chromatids (equatorial division) |
| Human body cells | 46 chromosomes (23 pairs) — diploid (2n) |
| Human gametes | 23 chromosomes — haploid (n) |
| Mitosis location | Somatic (body) cells — growth and repair |
| Meiosis location | Gonads — ovaries and testes |
| Significance of meiosis for evolution | Crossing over produces genetic variation — raw material for natural selection |