AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
Students should be able to explain how meiosis halves the number of…
Full specification wording
Meiosis: Students should be able to explain how meiosis halves the number of chromosomes in gametes and fertilisation restores the full number of chromosomes.
Meiosis is a type of cell division that produces gametes.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 1 of 7
Study notes
From the checked teaching notes for this specification point.
The idea
Meiosis is a type of cell division that produces gametes. A human body cell has 46 chromosomes, arranged as 23 pairs. Meiosis halves this number, so each gamete has 23 single chromosomes. This happens because the cell divides twice: the first division separates homologous chromosomes, and the second division separates the two copies of each chromosome. The gametes are therefore haploid. At fertilisation, a male gamete and a female gamete fuse. Their nuclei join, so the chromosome numbers add together: 23 plus 23 gives 46. The resulting zygote is diploid and has the full number of chromosomes, with one set from each parent. This restoration is essential so that each new generation has the correct chromosome number and genetic information from both parents.
What earns the marks
- Meiosis halves the chromosome number, producing haploid gametes.
- In humans, body cells are diploid with 46 chromosomes, while gametes are haploid with 23 chromosomes.
- Meiosis involves two divisions, which reduces the chromosome number from diploid to haploid.
- Fertilisation is the fusion of a male and a female gamete, and their nuclei join.
- Fertilisation restores the diploid number because the haploid chromosome sets from the two gametes combine, for example 23 plus 23 gives 46.
Common mistakes and how to put them right
Saying that meiosis produces diploid gametes.
Put it right: Gametes are haploid.
Thinking that fertilisation doubles the chromosome number beyond the normal diploid number.
Put it right: It restores the diploid number.
Confusing meiosis with mitosis.
Put it right: Mitosis produces genetically identical diploid cells, while meiosis produces haploid gametes.
Exam tips
- Use the terms haploid and diploid correctly, and quote the human numbers 23 and 46 as a clear example.
- Explain the two divisions of meiosis when describing how the chromosome number is halved.
- Link fertilisation to the fusion of gamete nuclei and state that the diploid number is restored.
Try the lesson
The start of the Meiosis: Students should be able to explain how meiosis halves the number of chromosomes in gametes and fertilisation restores the full number of chromosomes. lesson, made from checked teaching notes for this specification point.
Have a go first
A human body cell contains 46 chromosomes. How many chromosomes are in a human gamete produced by meiosis?
Listen
Meiosis is a type of cell division that produces gametes. A human body cell has 46 chromosomes, arranged as 23 pairs. Meiosis halves this number, so each gamete has 23 single chromosomes. This happens because the cell divides twice: the first division separates homologous chromosomes, and the second division separates the two copies of each chromosome. The gametes are therefore haploid. At fertilisation, a male gamete and a female gamete fuse. Their nuclei join, so the chromosome numbers add together: 23 plus 23 gives 46. The resulting zygote is diploid and has the full number of chromosomes, with one set from each parent. This restoration is essential so that each new generation has the correct chromosome number and genetic information from both parents. A strong answer earns marks for these points. Meiosis halves the chromosome number, producing haploid gametes. In humans, body cells are diploid with 46 chromosomes, while gametes are haploid with 23 chromosomes. Meiosis involves two divisions, which reduces the chromosome number from diploid to haploid. Fertilisation is the fusion of a male and a female gamete, and their nuclei join. A common slip: Saying that meiosis produces diploid gametes. Instead, gametes are haploid. Exam tip: Use the terms haploid and diploid correctly, and quote the human numbers 23 and 46 as a clear example.
See it
Put it in order
Put these steps in the order they happen.
- 1. Second division separates the two copies of each chromosome
- 2. Meiosis: first division separates homologous chromosomes
- 3. Diploid body cell: 46 chromosomes (23 pairs)
- 4. Zygote: diploid, 23 plus 23 gives 46 chromosomes
- 5. Fertilisation: male and female gamete fuse and their nuclei join
- 6. Haploid gametes: 23 single chromosomes
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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