Meiosis (AQA GCSE Biology): Revision Notes
Meiosis
What is meiosis?
Meiosis is a special type of cell division that happens in reproductive organs. It creates gametes (sex cells like sperm and egg cells). This process is essential for sexual reproduction.
The main job of meiosis is to reduce the number of chromosomes by half. This ensures that when two gametes join together, the offspring has the correct number of chromosomes.
This chromosome reduction is crucial - without it, the chromosome number would double with each generation! Meiosis maintains the species' characteristic chromosome number across generations.
How meiosis works
The parent cell
- The parent cell is diploid
- This means it has two sets of chromosomes
- In humans, body cells have 46 chromosomes (23 pairs)
The division process
When a cell divides by meiosis, the process involves two consecutive divisions that are fundamentally different from ordinary cell division:
- The parent cell divides twice
- This creates four daughter cells
- Each daughter cell gets only one chromosome from each pair
- All four daughter cells are genetically different from each other
The two divisions in meiosis are called meiosis I and meiosis II. During meiosis I, homologous chromosomes separate, while in meiosis II, sister chromatids separate - similar to what happens in mitosis.
The daughter cells
- Each daughter cell is haploid
- This means it has one set of chromosomes
- In humans, gametes have 23 chromosomes
- These cells become sperm or egg cells
Even though the cells produced are called "daughter cells", they can become sperm cells too! The term "daughter cell" simply refers to any cell produced by division, regardless of whether it becomes a male or female gamete.
Fertilisation
Fertilisation happens when a male gamete (sperm) joins with a female gamete (egg). Here's what occurs during this crucial process:
- The sperm has 23 chromosomes
- The egg has 23 chromosomes
- They combine to form a zygote
- The zygote has 46 chromosomes (23 + 23)
- This restores the normal diploid number
Worked Example: Chromosome Numbers in Fertilisation
Step 1: Male gamete contributes Sperm = 23 chromosomes (haploid)
Step 2: Female gamete contributes
Egg = 23 chromosomes (haploid)
Step 3: Fertilisation occurs Zygote = 23 + 23 = 46 chromosomes (diploid)
Result: The diploid number is restored!
The zygote then divides many times by mitosis to form an embryo. As it develops, the cells specialise for different jobs.
Why meiosis creates variation
Sexual reproduction produces variation in offspring because of several key mechanisms:
- Each gamete contains one chromosome from each pair in the parent
- The fertilised cell gets one set of chromosomes from each parent
- Different combinations of chromosomes create genetic differences
- Each chromosome carries different versions of genes called alleles
This genetic shuffling is why siblings (except identical twins) look different from each other, even though they have the same parents. The random assortment of chromosomes during meiosis ensures genetic diversity.
Key differences between meiosis and mitosis
| Meiosis | Mitosis |
|---|---|
| Produces 4 daughter cells | Produces 2 daughter cells |
| Daughter cells are genetically different | Daughter cells are genetically identical |
| Happens in reproductive organs | Happens in body cells |
| Creates haploid gametes | Creates diploid body cells |
Key Points to Remember:
- Meiosis creates gametes with half the normal number of chromosomes
- Fertilisation restores the full number of chromosomes
- Meiosis produces genetically different cells, which creates variation
- Human body cells have 46 chromosomes, human gametes have 23 chromosomes
- The process involves two divisions to create four haploid daughter cells