Meiosis
Laura Armstrong & Joe Wolfensohn
Teachers
Recall Questions
This topic requires prior knowledge of mitosis and meiosis from GCSE. You can test your knowledge on these below.
What is the main difference between mitosis and meiosis?
Mitosis produces two genetically identical diploid cells, while meiosis produces four genetically unique haploid cells.
Why is meiosis important in sexual reproduction?
Meiosis creates genetic variation by producing gametes with different genetic combinations and ensures the correct chromosome number is maintained across generations.
What is the diploid and haploid number in human cells?
The diploid number is 46 (2n), and the haploid number is 23 (n).
Topic Explainer Video
Check out this @LauraDoesBiology video that explains meiosis or read the full notes below. Once you've gone through the whole note, try out the practice questions!
Meiosis
What is Meiosis?
Meiosis is a type of cell division that produces gametes (sperm and egg cells) in sexually reproducing organisms. It reduces the chromosome number by half, ensuring offspring have the correct diploid number when gametes fuse.
- Occurs in the gonads (testes and ovaries in animals, anthers and ovules in plants).
- Involves two divisions (Meiosis I and Meiosis II).
- Produces four genetically different haploid cells.
- Ensures the correct chromosome number is maintained at fertilisation when two gametes fuse.
Stages of Meiosis
Meiosis I: Reduction Division
Prophase I
- Chromosomes condense and become visible.
- Homologous chromosomes pair up to form bivalents.
- Crossing over occurs at chiasmata, exchanging genetic material between chromatids.
- Nuclear envelope breaks down.
Metaphase I
- Bivalents align randomly along the equator (independent assortment).
- Spindle fibres attach to centromeres.
Anaphase I
- Homologous chromosomes are pulled apart to opposite poles.
- Sister chromatids remain joined.
Telophase I & Cytokinesis
- Nuclear envelope reforms around each set of chromosomes.
- The cytoplasm divides, forming two haploid daughter cells.
Meiosis II: Similar to Mitosis
Prophase II
- Chromosomes re-condense and spindle fibres reform.
Metaphase II
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Chromosomes align at the equator.
Anaphase II
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Centromeres split and sister chromatids separate and move to opposite poles.
Telophase II & Cytokinesis
- Four new nuceli form. The cytoplasm and cell membranes divide.
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Four genetically unique haploid cells are formed.
Overview of Meiosis
- Meiosis 1 separates homologous pairs of chromosomes, resulting in haploid cells with single chromosomes.
- Meiosis II separates sister chromatids (like mitosis).
- The result is 4 genetically different, haploid cells from a single, diploid parent cell.
Key Terms
- Meiosis: A type of cell division that produces four haploid gametes.
- Haploid (n): A cell with single chromosomes (e.g., gametes).
- Diploid (2n): A cell with homologous paris of chromosomes (e.g., somatic cells / body cells).
- Homologous Chromosomes: A pair of chromosomes (one from each parent) that carry the same genes in the same loci but may have different alleles.
- Crossing Over: The exchange of genetic material between chromatids in Prophase I.
- Independent Assortment: The random alignment and separation of homologous chromosomes in Metaphase I, creating new chromosome combinations.
- Bivalent: A pair of homologous chromosomes joined together during Prophase I.
- Chiasma (plural: Chiasmata): The site of crossing over between non-sister chromatids.
No answer provided.
Exam Tips
Details of all the stages of meiosis are not required! This is what your specification says:
The process of meiosis only in sufficient detail to show how:
• Two nuclear divisions result usually in the formation of four haploid daughter cells from a single diploid parent cell.
• Genetically different daughter cells result from the independent segregation of homologous chromosomes.
• Crossing over between homologous chromosomes results in further genetic variation among daughter cells.
No answer provided.
The diagram shows the chromosomes from a cell with a diploid chromosome number of six.
Draw a diagram to show the chromosomes from one of the resulting cells if
The cell divides by mitosis (2 marks)
The cell divides by meiosis (2 marks)
- The Cell Divides by Mitosis (2)
(This should be exactly the same as the parent cell, as mitosis produces genetically identical, diploid cells.)
- The Cell Divides by Meiosis (2)
(There will only be one chromosome from each homologous pair, only three chromosomes should be drawn as the cell is haploid.)
Practice Question
Try to answer the practice question from the TikTok on your own, then watch the video to see how well you did!