Meiosis — AQA GCSE Combined Science
Test yourself on Meiosis with AQA GCSE practice questions.
7 days Premium · Then free forever · No card, no charge
Meiosis explained
Meiosis is a type of cell division that produces gametes with half the normal chromosome number.
Read the full explanation
A human body cell has 46 chromosomes (23 pairs), but a gamete has 23 single chromosomes. During meiosis, the cell divides twice: the first division separates homologous chromosomes, and the second separates sister chromatids, so four haploid cells form. Because the chromosomes are shuffled and recombined, each gamete is genetically unique. At fertilisation, a male gamete nucleus fuses with a female gamete nucleus, forming a zygote. The two haploid sets combine, restoring the diploid number, so the zygote has 46 chromosomes. This keeps the chromosome number constant across generations and creates genetic variation in the offspring.
Cells in reproductive organs divide by meiosis to form gametes.
Meiosis is a specialised type of cell division that occurs exclusively in reproductive organs, such as the testes and ovaries in humans, or the anthers and ovaries in flowering plants. The purpose of meiosis is to form gametes, which are sex cells like sperm, eggs, and pollen. During meiosis, the original cell divides twice to produce four gametes. Each gamete receives a single set of chromosomes, halving the normal genetic amount. For example, human body cells have 46 chromosomes, but human gametes have 23. This ensures that when a male and female gamete fuse during fertilisation, the resulting cell has the correct, restored number of chromosomes.
When a cell divides to form gametes:
When a cell in a reproductive organ divides to form gametes by meiosis, a specific sequence of events occurs. First, copies of the genetic information are made. Following this replication, the cell divides twice to form four gametes. Each of these four gametes contains a single set of chromosomes, which is half the number found in a normal body cell. Crucially, all the gametes produced are genetically different from each other and from the original parent cell. For example, a human cell with 46 chromosomes will copy its genetic material and divide twice to produce four sperm or egg cells, each containing 23 chromosomes and a unique genetic combination.
copies of the genetic information are made
Before a cell divides by meiosis, its genetic information must be copied so that each new cell receives a complete set. The genetic information is the DNA packaged into chromosomes, found in the nucleus of the cell. Before division, the DNA is replicated, so each chromosome forms two identical copies. This copying is essential because meiosis involves two divisions, producing four gametes. Without replication, the four gametes could not each receive a single set of chromosomes. For example, a human body cell has 46 chromosomes; after copying, there are still 46 chromosomes but each consists of two identical copies, ready for the first meiotic division.
the cell divides twice to form four gametes, each with a single set of chromosomes
Meiosis is a type of cell division that produces gametes, such as sperm and egg cells. During this process, a cell in a reproductive organ divides twice to form four gametes. Each of these four gametes contains a single set of chromosomes, meaning the chromosome number has been halved. For example, a human body cell contains 46 chromosomes, but each gamete produced by meiosis contains only 23. This halving is essential so that when two gametes fuse at fertilisation, the normal chromosome number is restored. Furthermore, every gamete produced is genetically different from the others, which introduces genetic variation into the offspring.
all gametes are genetically different from each other.
Gametes are sex cells, such as sperm and eggs in animals, or pollen and eggs in flowering plants. They are produced by meiosis in reproductive organs. During this process, genetic information is copied, and the cell then divides twice to form four gametes. Each gamete receives a single set of chromosomes, halving the normal chromosome number. Because the chromosomes are randomly distributed during these two divisions, every resulting cell carries a unique combination of alleles. Consequently, no two sex cells produced by the same parent are identical. For example, a human parent produces sperm or eggs containing 23 chromosomes, but the specific alleles in each gamete vary, explaining why siblings inherit distinct characteristics.
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. 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.
Your focus
- Describe how meiosis halves the chromosome number to form haploid gametes.
- Explain how fertilisation restores the diploid chromosome number in the zygote.
- Relate haploid and diploid numbers to human cells and explain why chromosome number stays constant across generations.
Show all 21 objectives
- Identify the reproductive organs where meiosis takes place.
- State that cells in reproductive organs divide by meiosis to form gametes.
- Recognise examples of gametes in animals and plants.
- State that copies of genetic information are made before cell division in meiosis.
- Describe how the cell divides twice to form four gametes with a single set of chromosomes.
- Explain that all gametes produced by meiosis are genetically different.
- State that genetic information is copied before meiosis.
- Describe the structure of a replicated chromosome as having two identical copies of DNA.
- Explain why copying is necessary for four gametes to each receive a single set of chromosomes.
- Describe that a cell divides twice during meiosis to form four gametes.
- State that each gamete contains a single set of chromosomes.
- Explain that all gametes produced by meiosis are genetically different.
- Describe how meiosis produces gametes with half the normal chromosome number.
- Explain how the random distribution of chromosomes leads to genetic variation in gametes.
- Apply the concept of genetic variation in gametes to examples such as human sperm and egg cells.
- Describe how fertilisation restores the normal chromosome number in the zygote.
- Explain how mitosis increases the number of cells during embryo development.
- Describe how cells differentiate as the embryo develops to become specialised.
Meiosis exam tips
Marking 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, producing four genetically different haploid cells.
- Fertilisation is the fusion of two gamete nuclei, restoring the diploid chromosome number in the zygote.
- The restoration of the diploid number keeps the chromosome number constant between generations.
- Meiosis occurs in reproductive organs, such as testes and ovaries.
- The process produces gametes, which are sex cells like sperm and egg cells.
- The cell divides twice to produce four gametes.
- Each gamete contains a single set of chromosomes.
- Gametes are genetically different from each other.
- Copies of the genetic information are made before the cell divides.
- The cell divides twice during the process of meiosis.
- Four gametes are formed from the original cell.
- Each gamete contains a single set of chromosomes.
- All gametes produced are genetically different from each other.
- Genetic information is carried as DNA in chromosomes in the nucleus.
- The DNA is copied before meiosis begins.
- After copying, each chromosome consists of two identical copies of the DNA.
- The number of chromosomes is unchanged by copying, but the amount of DNA is doubled.
- Copying ensures each of the four gametes can receive one full set of chromosomes.
- The cell divides twice during meiosis.
- Four gametes are produced from the original cell.
- Each gamete contains a single set of chromosomes, which is half the original number.
- All four gametes are genetically different from each other.
- In humans, gametes contain 23 chromosomes, whereas body cells contain 46.
- State that gametes are produced by meiosis in reproductive organs.
- Describe how the parent cell divides twice to form four gametes.
- Explain that meiosis halves the chromosome number, leaving each gamete with a single set of chromosomes.
- Identify that the random distribution of chromosomes during division results in unique combinations of alleles in each gamete.
- Use the example of human gametes containing 23 chromosomes to illustrate the halving of the genetic material.
- 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.
Examiner Tips
- 💡Use the terms haploid and diploid correctly and give the human chromosome numbers as an example.
- 💡Describe meiosis as two divisions producing four gametes, and fertilisation as the fusion of two gamete nuclei.
- 💡Explain the consequence: the diploid number is restored, so chromosome number stays constant across generations.
- 💡Always specify that meiosis takes place in reproductive organs when asked where gametes are formed.
- 💡Remember to state that the gametes produced are genetically different from each other.
- 💡Learn the three key bullet points for meiosis: genetic information is copied, the cell divides twice to form four gametes, and all gametes are genetically different.
- 💡Do not describe the detailed stages of meiosis, as this is not required; focus on the overall outcomes.
- 💡Link copying to the need for four gametes each with a single set of chromosomes.
- 💡Clearly state that the genetic material is copied before the cell divides.
- 💡State that copying occurs before meiosis, not during it.
- 💡State clearly that the cell divides twice to produce four gametes.
- 💡Emphasise that each gamete has a single set of chromosomes, which is half the normal number.
- 💡When comparing cell divisions, explicitly state that meiosis leads to variation because the resulting cells are non-identical, unlike mitosis.
- 💡Remember that you do not need to know the complex stages of meiosis; focus on the copying of genetic material followed by two divisions.
- 💡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.
Common Mistakes
- Saying meiosis produces identical cells: correct this by stating that meiosis produces genetically different gametes due to recombination and independent assortment.
- Confusing haploid and diploid numbers: correct this by stating that gametes are haploid (23 in humans) and the zygote is diploid (46 in humans).
- Stating that fertilisation doubles the chromosome number permanently: correct this by explaining that it restores the diploid number, which stays constant in body cells.
- Confusing meiosis with mitosis; correction: mitosis produces genetically identical cells for growth, whereas meiosis produces genetically different gametes.
- Believing meiosis occurs in all body cells; correction: meiosis is restricted to reproductive organs like testes and ovaries.
- Thinking gametes have the full number of chromosomes; correction: gametes have a single set of chromosomes so fertilisation can restore the normal number.
- Forgetting that genetic material is copied first; correction: copies of the genetic information are made before the cell divides.
- Stating that only one division occurs; correction: the cell must divide twice to produce four gametes.
- Claiming the resulting gametes are identical; correction: all four gametes are genetically different from each other.
- Thinking the chromosome number doubles during copying; correction: the number of chromosomes stays the same, but each chromosome now has two identical copies of DNA.
- Confusing copying with the first meiotic division; correction: copying happens before division begins.
- Believing gametes are produced directly from the original cell without replication; correction: replication must occur first so that four gametes each receive a complete set.
- Stating that meiosis produces two cells; correction: it involves two divisions that produce four gametes.
- Thinking the chromosome number stays the same; correction: the chromosome number is halved so each gamete has a single set.
- Believing all gametes produced are identical; correction: all gametes formed by meiosis are genetically different from each other.
- Confusing meiosis with mitosis: correct this by stating that meiosis produces four non-identical gametes with half the chromosome number, whereas mitosis produces two genetically identical body cells.
- Thinking all gametes from one parent carry the exact same alleles: correct this by explaining that while they carry the same genes, the random distribution of chromosomes means they carry different combinations of alleles.
- Believing that the chromosome number halves because the DNA is not copied before division: correct this by clarifying that genetic information is copied first, but the cell then divides twice, resulting in half the original chromosome number.
- Thinking the zygote has double the normal chromosome number: correct this by stating that each gamete has half the number, so fusion restores the normal number.
- Confusing mitosis with meiosis in embryo growth: correct this by stating that the embryo grows by mitosis, which produces identical cells, not by meiosis.
- Believing differentiation changes the genes in a cell: correct this by explaining that differentiation means different genes are expressed, not that the DNA changes.