Cell Division: Mitosis, Meiosis, Growth & Repair

TAKEAWAYS:

• Cells divide by mitosis (growth/repair) or meiosis (gamete formation) following strict stages.
• The cell cycle has interphase (G₁, S, G₂) and mitosis/meiosis phases, controlled by checkpoints.
• Mitosis produces genetically identical diploid cells; meiosis halves chromosomes to make haploid gametes.
• Errors in division cause diseases (e.g., cancer) or genetic disorders (e.g., Down syndrome).
• Plant cells form a cell plate; animal cells undergo cytokinesis by cleavage furrow.

### **1. Why Do Cells Divide?**
Cells divide for three main reasons:
1. **Growth**: Multicellular organisms grow by increasing cell number.
2. **Repair**: Damaged cells are replaced (e.g., skin cuts heal).
3. **Reproduction**: Single-celled organisms (e.g., *Amoeba*) divide to reproduce.



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### **2. The Cell Cycle: A Controlled Process**
The cell cycle is divided into **interphase** (preparation) and **mitotic phase** (division).

#### **A. Interphase (90% of the Cycle)**
- **G₁ Phase (Gap 1)**: Cell grows, organelles duplicate.
- **S Phase (Synthesis)**: DNA replicates (chromosomes become sister chromatids).
- **G₂ Phase (Gap 2)**: Cell prepares for mitosis (checks DNA replication).

```figure
{"type":"timeline","events":[{"date":"G₁","label":"Cell grows"},{"date":"S","label":"DNA replicates"},{"date":"G₂","label":"Prepares for mitosis"},{"date":"M","label":"Mitosis"}],"caption":"Stages of the cell cycle"}

B. Mitotic Phase (M Phase)

• Mitosis: Division of the nucleus (4 stages: prophase, metaphase, anaphase, telophase).
• Cytokinesis: Division of the cytoplasm (forms two daughter cells).

Checkpoints: Control points (e.g., G₁/S checkpoint) ensure DNA is error-free before division.

3. Mitosis: Asexual Division

Mitosis produces two genetically identical diploid (2n) cells for growth/repair.

Stages of Mitosis

flowchart TD
    A["Prophase"] --> B["Metaphase"]
    B --> C["Anaphase"]
    C --> D["Telophase"]
    D --> E["Cytokinesis"]

1. Prophase:

   • Chromosomes condense (visible under microscope).
   • Spindle fibers form (from centrosomes).
   • Nuclear membrane breaks down.

2. Metaphase:

   • Chromosomes align at the metaphase plate (equatorial plane).
   • Spindle fibers attach to centromeres.

3. Anaphase:

   • Sister chromatids separate and move to opposite poles.
   • Cell elongates.

4. Telophase:

   • Chromosomes decondense.
   • Nuclear membranes reform.

5. Cytokinesis:

   • Animal cells: Cleavage furrow pinches the cell (actin/myosin ring).
   • Plant cells: Cell plate forms (vesicles from Golgi fuse).

Example: A human skin cell divides by mitosis to replace damaged cells.

4. Meiosis: Sexual Division

Meiosis produces four genetically unique haploid (n) gametes (sperm/egg) for reproduction.

Key Differences from Mitosis

Stages of Meiosis

flowchart TD
    A["Meiosis I"] --> B["Prophase I"]
    B --> C["Metaphase I"]
    C --> D["Anaphase I"]
    D --> E["Telophase I"]
    E --> F["Meiosis II"]
    F --> G["Prophase II"]
    G --> H["Metaphase II"]
    H --> I["Anaphase II"]
    I --> J["Telophase II"]
    J --> K["4 Haploid Cells"]

1. Prophase I:

   • Synapsis: Homologous chromosomes pair (tetrads).
   • Crossing over: Exchange of genetic material (increases diversity).

2. Metaphase I:

   • Homologous pairs align at the plate.

3. Anaphase I:

   • Homologs separate (sister chromatids stay together).

4. Telophase I:

   • Two haploid cells form (each with one chromosome from each pair).

5. Meiosis II:

   • Similar to mitosis (sister chromatids separate).

Example: A human sperm cell is produced after meiosis in testes.

5. Errors in Cell Division

• Mitotic Errors:
  • Cancer: Uncontrolled mitosis (e.g., skin cancer from UV damage).
• Meiotic Errors:
  • Down Syndrome: Nondisjunction in Meiosis I (extra chromosome 21).

6. Practical Applications

• Cloning: Mitosis used in plant propagation (e.g., strawberry runners).
• Genetic Engineering: Meiosis studied to understand inheritance.

Exam Tip

1. Diagrams: Draw mitosis/meiosis stages with labeled chromosomes.
2. Key Terms: Define homologous chromosomes, sister chromatids, cleavage furrow.
3. Comparisons: Compare mitosis vs. meiosis in a table.
4. Applications: Link cell division to diseases (cancer) or disorders (Down syndrome).
5. NEB Questions:
   • "Describe the stages of mitosis with a diagram."
   • "Why is meiosis important for sexual reproduction?"

Solved Example: Q: How many chromosomes are in a human gamete? A: Humans are diploid (2n = 46). Meiosis reduces this to n = 23 chromosomes in gametes.

NEB-Style Question: Q: Differentiate between mitosis and meiosis with respect to chromosome number and genetic variation. *A: