What is Meiosis? Revision Notes | GCSE OCR Foundation Biology

What this lesson covers

Understand how meiosis produces gametes with half the number of chromosomes through two divisions, creating genetically different cells in reproductive...

• Explain what meiosis is and why it's essential for sexual reproduction
• Describe how meiosis produces four genetically different gametes from one cell
• Compare the outcomes of meiosis with mitosis in terms of chromosome number
• Apply knowledge of meiosis to explain genetic variation in offspring

Key ideas to keep in view

Use this lesson to stay inside B5.1 Inheritance while connecting the detail back to the wider B5: Genes, inheritance and selection topic.

• B5: Genes, inheritance and selection
• B5.1 Inheritance
• GCSE OCR Foundation Biology

Lesson notes preview

Read the core explanations from What is Meiosis? before testing yourself from memory.

• 1. Why Do We Need Special Sex Cells?
• Your body contains around 37 trillion cells. Nearly all of them have 46 chromosomes - 23 pairs containing all your genetic information. But there's a problem.
• If sperm and egg cells also had 46 chromosomes each, a baby would end up with 92 chromosomes. The next generation would have 184. Within a few generations, cells would be drowning in genetic material.
• Nature solved this with a clever trick called meiosis - a special type of cell division that cuts the chromosome number in half. This creates gametes (sex cells) with just 23 chromosomes each, so when they fuse, the baby gets exactly 46.
• A type of cell division that occurs in reproductive organs to produce gametes. One cell divides twice to form four genetically different cells, each with half the chromosome number of the parent cell.
• Think of meiosis as nature's lottery system. It starts with one cell containing two copies of each chromosome (diploid). Through two rounds of division, it produces four gametes, each with just one copy of each chromosome (haploid).
• Here's the crucial part: before the cell divides, copies of the genetic information are made. This ensures each gamete gets a complete set of instructions, even though it's only half the usual amount.
• But meiosis doesn't just halve the chromosome number - it shuffles the genetic deck. All four gametes end up genetically different from each other and from the parent cell. This genetic mixing is why you look different from your siblings.
• 3. The Meiosis Process - Two Divisions, Four Outcomes
• Meiosis happens in two stages, cleverly called meiosis I and meiosis II. Picture a cell with 4 chromosomes (2 pairs) to keep things simple.

Continue through the topic

Move to the related lessons or back to the topic page when you need the wider sequence before exam-style practice.