AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Gametes join at fertilisation to restore the normal number of chromosomes.…
Full specification wording
Meiosis: Gametes join at fertilisation to restore the normal number of chromosomes. The new cell divides by mitosis. The number of cells increases. As the embryo develops cells differentiate.
Fertilisation is the fusion of two gametes.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 7 of 7
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Fertilisation is the fusion of two gametes. Each gamete has half the normal chromosome number, so when they join, the resulting zygote has the full, normal number again. In humans, a sperm with 23 chromosomes fuses with an egg with 23 chromosomes to make a zygote with 46 chromosomes. The zygote then divides by mitosis, a type of cell division that produces genetically identical cells. Each mitosis division copies the chromosomes and splits the cell into two, so the number of cells increases rapidly as the embryo grows. As the embryo develops, cells become specialised for particular jobs, such as nerve cells, muscle cells or root hair cells. This process is called differentiation, and it allows a multicellular organism to carry out many different functions.
What earns the marks
- State that fertilisation is the fusion of two gametes, restoring the full chromosome number in the zygote.
- Explain that each gamete has half the normal chromosome number, so the zygote has the normal number.
- Describe mitosis as cell division that produces genetically identical cells and increases the number of cells in the embryo.
- Explain that as the embryo develops, cells differentiate to become specialised for different functions.
- Use the human example of 23 chromosomes in each gamete and 46 in the zygote to illustrate the restoration of chromosome number.
Common mistakes and how to put them right
Thinking the zygote has double the normal chromosome number.
Put it right by stating that each gamete has half the number, so fusion restores the normal number.
Confusing mitosis with meiosis in embryo growth.
Put it right by stating that the embryo grows by mitosis, which produces identical cells, not by meiosis.
Believing differentiation changes the genes in a cell.
Put it right by explaining that differentiation means different genes are expressed, not that the DNA changes.
Exam tips
- Use the terms 'haploid' and 'diploid' correctly if you use them: gametes are haploid and the zygote is diploid.
- When describing embryo development, link mitosis to an increase in cell number and differentiation to specialisation.
- Give a named example of a specialised cell to show understanding of differentiation, such as a nerve cell or a root hair cell.
Try the lesson
The start of the Meiosis: Gametes join at fertilisation to restore the normal number of chromosomes. The new cell divides by mitosis. The number of cells increases. As the embryo develops cells differentiate. lesson, made from checked teaching notes for this specification point.
Have a go first
What happens to the chromosome number when two gametes join at fertilisation?
Listen
Fertilisation is the fusion of two gametes. Each gamete has half the normal chromosome number, so when they join, the resulting zygote has the full, normal number again. In humans, a sperm with 23 chromosomes fuses with an egg with 23 chromosomes to make a zygote with 46 chromosomes. The zygote then divides by mitosis, a type of cell division that produces genetically identical cells. Each mitosis division copies the chromosomes and splits the cell into two, so the number of cells increases rapidly as the embryo grows. As the embryo develops, cells become specialised for particular jobs, such as nerve cells, muscle cells or root hair cells. This process is called differentiation, and it allows a multicellular organism to carry out many different functions. A strong answer earns marks for these points. State that fertilisation is the fusion of two gametes, restoring the full chromosome number in the zygote. Explain that each gamete has half the normal chromosome number, so the zygote has the normal number. Describe mitosis as cell division that produces genetically identical cells and increases the number of cells in the embryo. Explain that as the embryo develops, cells differentiate to become specialised for different functions. A common slip: Thinking the zygote has double the normal chromosome number. Instead, stating that each gamete has half the number, so fusion restores the normal number. Exam tip: Use the terms 'haploid' and 'diploid' correctly if you use them: gametes are haploid and the zygote is diploid.
See it
Put it in order
Put these steps in the order they happen.
- 1. New cell divides by mitosis
- 2. Normal number of chromosomes restored
- 3. Gametes join at fertilisation
- 4. Cells differentiate
- 5. Embryo develops
- 6. Number of cells increases
Key words
Say what each word means before you turn the card.
Fertilisation
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Chromosome number
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Mitosis and growth
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Cell differentiation
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See every definition
- Fertilisation
- The fusion of two gametes, which restores the normal number of chromosomes in the zygote.
- Chromosome number
- Each gamete has half the normal chromosome number, so when they join the zygote has the full, normal number again.
- Mitosis and growth
- A type of cell division that produces genetically identical cells, so the number of cells increases rapidly as the embryo grows.
- Cell differentiation
- As the embryo develops, cells become specialised for particular jobs, allowing a multicellular organism to carry out many different functions.
Label the diagram
Some boxes have lost their names. Pick the right name for each number.
1Box 1
2Box 2
3Box 3
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