Fertilisation and Development Revision Notes | GCSE OCR Foundation Biology

What this lesson covers

Learn how meiosis halves chromosome number in gametes, fertilisation restores the full number, and how the new cell develops through mitosis and differe...

• Explain how meiosis halves the number of chromosomes in gametes
• Describe how fertilisation restores the full chromosome number
• Explain the role of mitosis in embryo development
• Compare the chromosome numbers in gametes and body cells

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 Fertilisation and Development before testing yourself from memory.

• Right now, you have about 37 trillion cells in your body. Every single one started from just two cells meeting - a sperm and an egg. But here's the puzzle: if body cells have 46 chromosomes, and sperm cells have 46 chromosomes, and egg cells have 46 chromosomes, why don't babies end up with 92 chromosomes?
• The answer lies in a clever biological trick. Your body makes special cells - gametes - that have exactly half the normal number of chromosomes. When they join together at fertilisation, they restore the full set. It's like having two half-tickets that make one complete ticket.
• Cells containing one complete set of chromosomes (n), found in gametes like sperm and egg cells
• Think of chromosomes like a recipe book. Your body cells are diploid - they have two copies of every recipe (2n = 46 chromosomes in humans). This gives you a backup copy if one gets damaged.
• But gametes are different. They're haploid - they only carry one copy of each recipe (n = 23 chromosomes). This halving is crucial. When a sperm (23 chromosomes) meets an egg (23 chromosomes), the resulting zygote has the full set again: 23 + 23 = 46 chromosomes.
• A type of cell division that produces four genetically different haploid gametes from one diploid parent cell
• Meiosis is like a special factory that takes one cell with 46 chromosomes and produces four cells with 23 chromosomes each. But it's not just simple division - there's shuffling involved too.
• Here's what happens: the cell goes through two rounds of division. In the first round, the chromosome pairs separate. In the second round, the individual chromosomes split. The result? Four genetically unique gametes, each with exactly half the chromosomes of the parent cell.
• Trap: Many students think meiosis just cuts chromosomes in half randomly. Actually, it carefully separates whole chromosomes, ensuring each gamete gets one complete copy of each chromosome type.
• The fusion of a haploid sperm cell with a haploid egg cell to form a diploid zygote

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.