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Meiosis
- It is a reduction cell division.
- The resulting daughter cells have half the original number of chromosomes, i.e., they are haploid (n) and can be used for sexual reproduction.
- To maintain the original chromosome number, the chromosome number in gametes needs to be halved.
- When two haploid gametes join at fertilisation, the resulting zygote is diploid (2n).
Homologous Chromosomes
- Chromosomes are the same size.
- Chromosomes have the same position of centromere.
- Chromosomes carry the same genes at the same loci, but different alleles – one maternal and one paternal.
Stages of Meiosis
- Do you still remember the content of A2/A-level Biology regarding the 8 stages of Meiosis? 👀
1. Prophase I (Meiosis I)
- Chromosomes condense and undergo supercoiling – they shorten and thicken.
- Homologous chromosomes pair up (synapsis) to form a bivalent.
- Non-sister chromatids attach at chiasmata – they may swap sections of chromatids called crossing over.
- Nucleolus disappears and the nuclear envelope disintegrates.
- A spindle forms made of protein microtubules.
2. Metaphase I
- Chromosomes arrange themselves randomly (independent assortment of chromosomes) on the equator of the spindle and attach to the spindle fibres.
3. Anaphase I
- The homologous chromosomes in each bivalent are pulled by the spindle fibres to opposite poles.
4. Telophase I
- In animal cells, new nuclear envelopes form and the cell divides by cytokinesis. There is a brief interphase and the chromosomes uncoil.
- In plant cells, the cell goes straight from anaphase I into meiosis II – no telophase I.
5. Prophase II (Meiosis II)
- Chromosomes condense and undergo supercoiling – they shorten and thicken.
- Nucleolus disappears and the nuclear envelope disintegrates again.
- The spindle reforms at right angles to the previous spindle axis.
6. Metaphase II
- Chromosomes arrange themselves randomly (independent assortment of chromatids) on the equator of the spindle and attach to the spindle fibres.
7. Anaphase II
- Centromeres divide
- Chromatids are pulled by the spindle fibres to opposite poles.
8. Telophase II
- Nuclear envelopes reform around the haploid daughter nuclei.
- In animals, the two cells now divide to give four haploid cells.
- In plants, a tetrad of four haploid cells is formed.
References:
1. https://getrevising.co.uk/resources/ocr-f215-meiosis-and-variation
2. https://www.pinterest.com/pin/750904937854824048/
3. https://en.dopl3r.com/memes/dank/messages-edit-can-you-explain-me-the-cell-division-very-fast-0-8-oo-thanks-you-know-im-something-of-a-scientist-myself/796376
4. https://sites.google.com/site/gampag8/home/3-haploid-and-diploid-cells
5. https://www.thoughtco.com/homologous-chromosomes-definition-373469
That's the end of the topic!
Drafted by Bonnie (Biology)