Overview of Cell Division
All eukaryotic cells undergo cell division to replicate. At the GCSE Higher Tier, you must distinguish between two primary cellular replication pathways: **mitosis** and **meiosis**. Learning where they occur and what they produce is critical for securing maximum marks in short-answer questions.
Mitosis: Growth and Repair
Mitosis is the cell division process that produces **two genetically identical diploid daughter cells** from a single parent cell. It is the process responsible for:
- Organism growth (producing more cells).
- Repair of damaged tissues (replacing dead cells).
- Asexual reproduction in single-celled eukaryotes and plants.
During mitosis, chromosomes duplicate once, align in the center, and pull apart. The cell splits once, resulting in identical clones containing a full set of chromosomes (diploid, $2n$).
Meiosis: Gamete Production
Meiosis is a specialized form of cell division that occurs only in reproductive organs (testes and ovaries in humans) to produce **gametes** (sperm and egg cells). It produces **four genetically varied haploid daughter cells**.
During meiosis, chromosomes duplicate once but the cell undergoes **two rounds of division**. This halves the genetic material, producing cells with only a single set of chromosomes (haploid, $n$). Crossing over (swapping of sections of DNA) occurs, creating unique genetic variations.
Mitosis vs. Meiosis Comparison
This table summarizes the core differences you are expected to recall in exam questions:
| Feature | Mitosis | Meiosis |
|---|---|---|
| Purpose | Growth, repair, asexual reproduction | Gamete production (sexual reproduction) |
| Where it occurs | Body cells (somatic cells) | Reproductive organs (testes/ovaries) |
| Number of divisions | 1 division | 2 divisions |
| Daughter cells produced | 2 identical cells | 4 varied cells |
| Ploidy status | Diploid (46 chromosomes in humans) | Haploid (23 chromosomes in humans) |
Worked Exam Example
Worked GCSE Example
Question: Explain how meiosis creates genetic variation and why this variation is beneficial for a population of organisms.
Solution:
- Mechanism of variation in meiosis: During meiosis, homologous chromosomes pair up and swap sections of DNA in a process called **crossing over**. Additionally, maternal and paternal chromosomes are separated randomly (independent assortment). This ensures all 4 haploid gametes are genetically unique.
- Benefit of variation: Genetic variation ensures that individuals in a population have different characteristics. If environmental conditions change (e.g. a new disease emerges), some individuals may possess alleles that make them resistant, allowing them to survive and reproduce, preventing extinction of the species.
Frequently Asked Questions
Crossing over is the process during the first division of meiosis where homologous (matching) chromosomes line up and swap segments of their DNA. This mixes maternal and paternal alleles, creating entirely new combinations of genes in the resulting gametes.
No, mitosis only occurs in eukaryotic organisms (which have a nucleus). Bacteria are prokaryotes and reproduce using a simpler cell division process called **binary fission**.
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