Reproduction and Inheritance

Cell Division – Revision Notes

Key Definitions and Terminology

• **Mitosis**: A type of cell division that produces **two genetically identical daughter cells** with the same number of chromosomes as the parent cell (diploid).
• **Meiosis**: A type of cell division that produces **four genetically different daughter cells (gametes)**, each with **half** the number of chromosomes of the parent cell (haploid).
• **Diploid**: A cell that contains **two complete sets** of chromosomes (in humans, 2n = 46).
• **Haploid**: A cell that contains **one set** of chromosomes (in humans, n = 23).
• **Chromosome**: A thread-like structure made of **DNA** found in the nucleus, which carries genetic information in the form of genes.
• **Gamete**: A sex cell (sperm or egg in animals; pollen or egg cell in plants) that is haploid, produced by meiosis.

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Main Concepts

1. The Purpose of Mitosis

• Mitosis is used for **growth**, **repair of damaged tissues**, and **asexual reproduction**.
• It maintains the **diploid chromosome number** — every daughter cell is a genetic clone of the parent cell.
• Before mitosis occurs, **DNA replication** takes place so that each chromosome is copied. The cell then divides once, distributing identical copies to each daughter cell.

2. The Purpose of Meiosis

• Meiosis occurs only in the **reproductive organs** (testes and ovaries in animals; anthers and ovules in flowering plants).
• It produces **gametes** for **sexual reproduction**.
• Meiosis involves **two successive divisions**:
• **First division**: Separates pairs of homologous chromosomes → chromosome number is halved.
• **Second division**: Separates the copies of each chromosome (similar to mitosis).
• The result is **four haploid cells** that are all **genetically different** from each other and from the parent cell.

3. How Meiosis Produces Genetic Variation

• During meiosis, chromosomes from each parent pair are **shuffled randomly** into gametes (independent assortment).
• **Crossing over** can occur between homologous chromosomes, creating new combinations of alleles.
• This genetic variation is the raw material for **natural selection** and evolution.

4. Importance of Halving the Chromosome Number

• When two haploid gametes fuse at **fertilisation**, the diploid number is **restored** (e.g., 23 + 23 = 46 in humans).
• Without meiosis halving the chromosome number, the number of chromosomes would **double every generation**, which is unsustainable.

5. Comparing Mitosis and Meiosis

| Feature | Mitosis | Meiosis |

|---|---|---|

| Number of divisions | 1 | 2 |

| Number of daughter cells | 2 | 4 |

| Chromosome number in daughter cells | Diploid (2n) | Haploid (n) |

| Genetic composition | Identical to parent | Genetically different |

| Where it occurs | Body (somatic) cells | Reproductive organs |

| Purpose | Growth, repair, asexual reproduction | Production of gametes |

6. Stem Cells and Mitosis (Brief Link)

• Stem cells divide by **mitosis** and can then **differentiate** into specialised cell types, which is why mitosis is essential for growth and development from a fertilised egg.

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Worked Examples and Real-World Applications

Example 1: Human Chromosome Numbers

• A human body cell has **46 chromosomes** (diploid, 2n = 46).
• Meiosis in the ovaries produces egg cells with **23 chromosomes** (haploid, n = 23).
• Meiosis in the testes produces sperm cells with **23 chromosomes**.
• At fertilisation: 23 (egg) + 23 (sperm) = **46 chromosomes** in the zygote → diploid number restored.

Example 2: Healing a Cut

• When you cut your skin, the damaged cells are replaced by **mitosis**.
• Cells surrounding the wound divide to produce **genetically identical** replacement cells, ensuring the new tissue functions correctly and matches the original.

Example 3: Genetic Variation in Siblings

• Siblings from the same parents look different because meiosis produces gametes that are **genetically unique** each time. Each child receives a **different combination** of the parents' alleles, explaining why brothers and sisters are similar but not identical (unless they are identical twins, formed from the same zygote splitting by mitosis).

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Exam Technique Tips

Tip 1: Use Precise Comparative Language

When comparing mitosis and meiosis, Edexcel mark schemes reward direct comparative statements rather than two separate descriptions. For example, write:

> *"Mitosis produces two daughter cells whereas meiosis produces four."*

Using words like "whereas", "but", or "however" in a single sentence scores the comparison mark. Simply listing facts about each process separately may not earn the mark.

Tip 2: State "Genetically Identical" vs "Genetically Different" Explicitly

A very common lost mark is failing to state the genetic outcome clearly. Edexcel mark schemes specifically look for:

• Mitosis → daughter cells are **"genetically identical to the parent cell"**
• Meiosis → daughter cells are **"genetically different from each other and from the parent cell"**

Do not just say "the same" or "different" — always include "genetically identical" or "genetically different" as a complete phrase. Also specify what they are identical/different to (the parent cell and/or each other).

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*End of revision notes — Cell Division*