Meiosis
Meiosis is a reduction division that occurs in two stages, meiosis I and meiosis II, producing four genetically varied haploid gametes from one diploid cell, which is essential for sexual reproduction.
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Why meiosis is needed
Content standard 6.3 covers meiosis, the type of cell division that occurs in reproductive organs to produce gametes (sperm and egg cells or pollen and ovules). Because fertilisation combines a gamete from each parent, gametes must be haploid (n); if they were diploid like ordinary body cells, the chromosome number would double every generation.
Meiosis is therefore a reduction division that halves the chromosome number.
Meiosis I and meiosis II
- Meiosis I (reduction division): homologous chromosomes pair up, exchange genetic material through crossing over, then separate from each other into two haploid cells, this is the stage that halves the chromosome number.
- Meiosis II (similar to mitosis): in each of the two cells from meiosis I, the sister chromatids of each chromosome separate, producing a total of four haploid cells from the original one diploid cell.
Sources of genetic variation
Meiosis produces gametes that are genetically different from one another through two mechanisms. Crossing over occurs during prophase I, when homologous chromosomes pair closely and exchange segments of genetic material, creating new combinations of alleles on a chromosome.
Independent assortment occurs when homologous chromosome pairs line up randomly at the equator during metaphase I, so each gamete receives a random mix of maternal and paternal chromosomes. Together these mechanisms make every gamete genetically unique.
Significance of meiosis and how it is examined
Meiosis maintains a constant chromosome number across generations, since fertilisation of two haploid gametes restores the diploid number in the offspring, and it introduces genetic variation, which is the raw material for evolution and helps a species adapt to a changing environment. Exam questions commonly ask you to state the number of cells and their chromosome number (haploid or diploid) produced at each stage of meiosis, to describe what happens during crossing over, or to explain why meiosis, rather than mitosis, is needed to form gametes.
Worked exam-style question
Question. An animal cell has a diploid chromosome number of 8. The cell undergoes meiosis to form gametes.
(a) State the number of chromosomes present in each cell immediately after meiosis I is complete. (b) State the number of cells, and the number of chromosomes in each, produced by the end of meiosis II.
(c) Name the two processes that make the four gametes genetically different from one another, and state the stage of meiosis at which each occurs. (d) Explain why it would be harmful to the offspring if gametes were produced by mitosis instead of meiosis.
Model answer. (a) Each cell has 4 chromosomes (haploid, n) immediately after meiosis I, since meiosis I is the reduction division that halves the chromosome number from the original 8. (b) Meiosis produces 4 cells in total, each with 4 chromosomes (haploid, n).
(c) Crossing over occurs during prophase I, and independent assortment occurs during metaphase I, when homologous chromosome pairs line up randomly at the equator. (d) If gametes were produced by mitosis, each gamete would remain diploid (8 chromosomes) instead of haploid; fertilisation of two such gametes would produce a zygote with double the normal chromosome number, and the chromosome number would keep doubling every generation, which would disrupt normal development.
Practice question
Try this. A plant cell has a diploid chromosome number of 14. State the number of chromosomes in a pollen grain produced by this plant, and explain, using the term homologous chromosomes, why this number is correct.
Exam tip
Key terms
Revise these linked terms to answer meiosis questions precisely:
- Mitosis, the other type of cell division, which produces genetically identical diploid cells for growth.
- Chromosome, the structure carrying genetic material that is copied and separated during meiosis.
- Diploid, having two complete sets of chromosomes, the state of the original cell before meiosis.
- Haploid, having one complete set of chromosomes, the state of every gamete produced by meiosis.
Source:SRC-DSKP-EN
Frequently asked questions
How many cells does meiosis produce, and what is their chromosome number?
What is crossing over and why is it important?
How is meiosis different from mitosis?
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