Meiosis — AQA GCSE Biology
Test yourself on Meiosis with AQA GCSE practice questions.
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Meiosis explained
Meiosis is a type of cell division that produces gametes.
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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.
Cells in reproductive organs divide by meiosis to form gametes.
Reproductive organs contain cells that divide by meiosis to produce gametes. In humans, the testes produce sperm and the ovaries produce egg cells. In flowering plants, the anthers produce pollen and the ovaries produce egg cells. These reproductive organs contain diploid cells, which have two sets of chromosomes. During meiosis, a diploid cell divides twice to form four haploid gametes. Each gamete has half the chromosome number of the parent cell, so it can fuse with another gamete at fertilisation. Meiosis also creates genetic variation because the chromosomes separate independently and crossing over can occur. This means each gamete is genetically different from the others. The gametes are then released or transferred so that fertilisation can take place, restoring the diploid number in the zygote.
When a cell divides to form gametes: • copies of the genetic information are made • the cell divides twice to form four gametes, each with a single set of chromosomes • all gametes are genetically different from each other.
Meiosis is the two-stage division that makes gametes. Before division begins, the DNA is replicated so each chromosome becomes two identical chromatids, giving copies of the genetic information. The cell then divides twice: the first division separates homologous chromosomes, and the second separates the sister chromatids. One starting cell therefore yields four gametes, each with a single set of chromosomes (haploid). Because homologous chromosomes are shuffled independently and crossing over exchanges sections between them, each gamete receives a different combination of alleles, so all four gametes are genetically different. In humans the diploid number is 46, so each gamete has 23 chromosomes; at fertilisation two haploid gametes restore the diploid number.
Gametes join at fertilisation to restore the normal number of chromosomes.
Fertilisation is the fusion of two gametes, usually one from each parent. Because each gamete is haploid and carries a single set of chromosomes, their fusion produces a cell with two sets of chromosomes. This restores the normal diploid number for the species. In humans, a sperm cell with 23 chromosomes fuses with an egg cell with 23 chromosomes, producing a zygote with 46 chromosomes. The zygote therefore contains genetic information from both parents, which is why offspring inherit features from each parent. The restoration of the diploid number is essential because it allows the new cell to grow and divide by mitosis while maintaining the correct chromosome number in the organism.
The new cell divides by mitosis.
After fertilisation, the zygote is a single cell with the normal diploid number of chromosomes. To grow into a multicellular organism, this cell must divide repeatedly. It does so by mitosis, a type of cell division that produces two genetically identical daughter cells. Before each division, the DNA is replicated so that each daughter cell receives a complete set of chromosomes. The chromosome number stays the same, so the diploid number is maintained throughout growth. For example, a human zygote with 46 chromosomes divides by mitosis to produce cells that also contain 46 chromosomes, allowing the embryo to develop and the organism to grow and repair tissues.
The number of cells increases.
In the context of reproduction, this statement refers to the events immediately following fertilisation. When a male and female gamete fuse, they form a single diploid cell called a zygote. This new cell then divides by mitosis, producing identical diploid cells. As this process repeats, the overall number of cells increases rapidly, forming an embryo. For example, a single fertilised human egg divides to form a ball of cells that implants in the uterus. As the embryo develops, these cells begin to differentiate into specialised types, such as muscle or nerve cells, to form the tissues and organs of the new organism. Students must distinguish this post-fertilisation mitotic growth from the meiosis that originally produced the gametes.
As the embryo develops cells differentiate.
After fertilisation, the zygote divides by mitosis to form an embryo. As the embryo develops, its cells become specialised for particular functions, a process called differentiation. Differentiation occurs because different genes are switched on or off in different cells, so each cell produces the proteins needed for its role. For example, a cell that becomes a muscle cell produces muscle proteins, while a cell that becomes a nerve cell develops the structures needed to carry electrical impulses. In animals, most differentiation happens early in development, whereas in plants many cells retain the ability to differentiate throughout life. Students should link differentiation to the expression of genes and to the formation of specialised tissues and organs.
Your focus
- Describe how meiosis halves the chromosome number to form haploid gametes.
- Explain that fertilisation restores the diploid chromosome number.
- Use the human chromosome numbers 23 and 46 to illustrate haploid and diploid cells.
Show all 21 objectives
- Identify reproductive organs that produce gametes by meiosis.
- Describe meiosis as producing four haploid gametes from one diploid cell.
- Name examples of gametes in humans and flowering plants.
- Describe the sequence of events in meiosis, including replication and two divisions.
- State the chromosome number of gametes compared with body cells in humans.
- Explain how meiosis produces four genetically different gametes.
- Describe fertilisation as the fusion of two haploid gametes.
- Explain how fertilisation restores the diploid chromosome number.
- Apply chromosome numbers to a named example such as human sperm, egg and zygote.
- Describe mitosis as division producing two genetically identical diploid cells.
- Explain how mitosis after fertilisation allows growth and development.
- Compare mitosis with meiosis in terms of chromosome number and number of cells produced.
- Describe how a fertilised egg divides by mitosis to form an embryo.
- Explain that the number of cells increases through repeated mitotic divisions.
- State that cells differentiate as the embryo develops to form specialised tissues.
- Define differentiation as the process by which cells become specialised.
- Explain how gene switching leads to different cell types in an embryo.
- Describe the difference in differentiation between animal and plant cells.
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, 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.
- Reproductive organs contain cells that divide by meiosis.
- Meiosis produces gametes such as sperm and egg cells in animals, and pollen and egg cells in flowering plants.
- The cells in reproductive organs are diploid before meiosis, and the gametes produced are haploid.
- One diploid cell divides by meiosis to form four haploid gametes.
- Meiosis introduces genetic variation, so the gametes produced are genetically different from one another.
- DNA replication occurs before division, producing two identical copies (chromatids) of each chromosome.
- Two divisions occur in sequence, so one cell produces four gametes rather than two.
- Each gamete contains one chromosome from each homologous pair, described as a single (haploid) set.
- Gametes differ genetically because of independent assortment of homologous chromosomes and crossing over between them.
- In humans, body cells have 46 chromosomes and gametes have 23, so fertilisation restores 46.
- Fertilisation involves the fusion of two gametes, typically one from each parent.
- Each gamete is haploid and contributes a single set of chromosomes.
- Fusion of the gametes produces a cell with two sets of chromosomes, restoring the diploid number.
- In humans, the fusion of a 23-chromosome sperm and a 23-chromosome egg produces a zygote with 46 chromosomes.
- The zygote contains genetic information from both parents and can divide by mitosis to form the new organism.
- The zygote formed at fertilisation divides by mitosis.
- Mitosis produces two genetically identical daughter cells from one parent cell.
- DNA is replicated before mitosis so each daughter cell receives a complete set of chromosomes.
- The chromosome number remains constant during mitosis, so the diploid number is maintained.
- Mitosis enables growth of the organism and replacement or repair of cells.
- Fertilisation involves the fusion of two haploid gametes to form a single diploid cell known as a zygote.
- The newly formed zygote divides by mitosis to produce genetically identical diploid cells.
- Repeated mitotic divisions cause the overall number of cells to increase, forming an embryo.
- As the embryo develops, the cells differentiate into specialised cells to form different tissues and organs.
- The embryo forms from a zygote that divides by mitosis.
- Differentiation is the process by which cells become specialised for a particular function.
- Different genes are switched on or off in different cells during differentiation.
- Specialised cells produce specific proteins and develop structures suited to their role.
- In animals, most differentiation occurs early in development, while plant cells often retain the ability to differentiate.
Examiner 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.
- 💡Name the reproductive organs and the gametes they produce, for example testes and sperm, or ovaries and egg cells.
- 💡State that meiosis produces four haploid gametes from one diploid cell.
- 💡Mention that meiosis creates genetic variation, which is important for natural selection and evolution.
- 💡Use the terms haploid and diploid precisely, linking haploid to gametes and diploid to the fertilised cell.
- 💡When asked to explain variation, name both independent assortment and crossing over rather than writing only 'mixing of genes'.
- 💡Check the number of divisions and the number of products in your answer, because these are the easiest facts to state incorrectly.
- 💡State the chromosome numbers of the gametes and the zygote when using humans as an example.
- 💡Use the terms haploid and diploid to show that you understand the change in chromosome number.
- 💡Link fertilisation to the inheritance of genetic information from both parents.
- 💡State that the daughter cells are genetically identical and diploid.
- 💡Use the human zygote with 46 chromosomes as a concrete example of the chromosome number staying constant.
- 💡Link mitosis after fertilisation to growth and development of the organism.
- 💡Clearly state that the division after fertilisation is mitosis, not meiosis.
- 💡Link the increase in cell number directly to the formation of an embryo and subsequent cell differentiation.
- 💡Define differentiation as cells becoming specialised for a particular function.
- 💡Refer to genes being switched on or off rather than to genes being lost or changed.
- 💡Give a named example of a specialised cell, such as a muscle cell or nerve cell, to support the explanation.
Common Mistakes
- Saying that meiosis produces diploid gametes; the correction is that gametes are haploid.
- Thinking that fertilisation doubles the chromosome number beyond the normal diploid number; the correction is that it restores the diploid number.
- Confusing meiosis with mitosis; the correction is that mitosis produces genetically identical diploid cells, while meiosis produces haploid gametes.
- Saying that gametes are produced by mitosis; the correction is that gametes are produced by meiosis.
- Thinking that meiosis produces two gametes; the correction is that one diploid cell produces four haploid gametes.
- Believing that gametes are diploid; the correction is that gametes are haploid, with half the chromosome number of the parent cell.
- Saying the cell divides once and produces two gametes; the correction is that meiosis involves two divisions and produces four gametes.
- Confusing meiosis with mitosis; the correction is that mitosis produces genetically identical diploid cells for growth, whereas meiosis produces genetically varied haploid gametes.
- Stating that gametes are identical clones; the correction is that independent assortment and crossing over make every gamete genetically different.
- Thinking fertilisation doubles the chromosome number permanently; correct this by stating that it restores the normal diploid number.
- Believing gametes are diploid; correct this by stating that gametes are haploid and contain a single set of chromosomes.
- Confusing fertilisation with cell division; correct this by describing fertilisation as the fusion of two gametes.
- Confusing mitosis with meiosis; correct this by stating that mitosis produces two identical diploid cells, whereas meiosis produces four genetically different haploid gametes.
- Thinking the chromosome number halves during mitosis; correct this by stating that the chromosome number stays the same.
- Believing mitosis produces gametes; correct this by stating that mitosis produces body cells for growth and repair.
- Stating that the fertilised egg divides by meiosis; the correction is that the zygote divides by mitosis for growth, whereas meiosis is only for making gametes.
- Confusing the increase in cell number with an increase in chromosome number; the correction is that the chromosome number remains constant (diploid) during these mitotic divisions.
- Forgetting to mention differentiation; the correction is to state that as the cell number increases, the cells also differentiate to form specialised tissues.
- Stating that differentiation changes the genes present in a cell; the correction is that the genes stay the same but different genes are switched on or off.
- Confusing differentiation with cell division; the correction is that division increases cell number, while differentiation makes cells specialised.
- Saying that all cells in an embryo differentiate at the same time; the correction is that differentiation happens as the embryo develops and different cells become specialised at different stages.