# Cell Division
Cells divide for growth, repair, and reproduction. There are two types of cell division: mitosis (for growth/repair) and meiosis (for gametes). Understanding the cell cycle and stem cells is essential.
1. The Cell Cycle
Three main stages:
- Interphase (longest stage): cell grows, DNA replicates, organelles increase
- Mitosis: nucleus divides into two identical nuclei
- Cytokinesis: cytoplasm divides; two daughter cells formed
2. Mitosis
Produces two genetically identical diploid daughter cells.
Stages (PMAT):
- Prophase: chromosomes condense and become visible; nuclear membrane breaks down
- Metaphase: chromosomes line up in the middle
- Anaphase: chromosomes pulled apart to opposite poles
- Telophase: nuclear membranes form; chromosomes decondense
Purpose: growth, repair, asexual reproduction
3. Meiosis
Produces four genetically different haploid gametes (sex cells).
- Two divisions: Meiosis I and Meiosis II
- Meiosis I: homologous pairs separate → 2 haploid cells
- Meiosis II: sister chromatids separate → 4 haploid cells
Genetic variation from:
- Independent assortment: random orientation of homologous pairs
- Crossing over: exchange of DNA between homologous chromosomes
4. Comparing Mitosis and Meiosis
| Feature | Mitosis | Meiosis |
|---|---|---|
| Daughter cells | 2 | 4 |
| Genetically | Identical | Different |
| Ploidy | Diploid (2n) | Haploid (n) |
| Purpose | Growth/repair | Gamete production |
| Divisions | 1 | 2 |
5. Stem Cells
Undifferentiated cells that can divide and differentiate into specialised cells.
Types
- Embryonic stem cells: can become any cell type (totipotent/pluripotent)
- Adult stem cells: limited differentiation (e.g. bone marrow → blood cells)
- Meristems (plants): found in root/shoot tips; can differentiate throughout life
Medical Uses
- Treat blood diseases (bone marrow transplants)
- Potential to treat paralysis, diabetes, Alzheimer's
Ethical Concerns
- Embryonic stem cells require destruction of embryo
- Some people believe this is morally wrong
- Risk of tumour formation (uncontrolled division)
6. Practice Questions
- List the stages of the cell cycle.
- Compare the number of daughter cells in mitosis and meiosis.
- Why is meiosis important for sexual reproduction?
- What are the advantages of using embryonic stem cells over adult stem cells?
- Explain how crossing over produces genetic variation.
Want to check your answers and get step-by-step solutions?
Summary
- Mitosis: 2 identical diploid cells; growth and repair
- Meiosis: 4 different haploid cells; gamete production
- Cell cycle: interphase → mitosis → cytokinesis
- Stem cells: undifferentiated; potential medical applications
- Genetic variation: independent assortment + crossing over