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

1. 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!

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