Meiosis

 MEIOSIS


Intro. 
In most of the cells in your body, new cells are made through mitosis. This process creates exact copies of cells for growth and repair. But when it comes to reproduction, things work differently.Organisms that reproduce sexually—like humans, dogs, trees, and even insects—use a special type of cell division called meiosis


Meiosis I 

  1. Prophase I

    • Chromosomes condense

    • Homologous chromosomes pair up to form tetrads

    • Crossing over occurs (exchange of genetic material between chromatids)

    • Spindle fibers form

  2. Metaphase I

    • Tetrads align randomly at the middle of the cell

    • This random alignment is called independent assortment

  3. Anaphase I

    • Homologous chromosomes are pulled to opposite poles

    • Sister chromatids remain attached

  4. Telophase I & Cytokinesis

    • Cell divides into two haploid cells

    • Each chromosome still consists of two sister chromatids


Meiosis II

1. Prophase II
  • Chromosomes (still made of sister chromatids) condense again and new spindle fibers form in each haploid cell
2. Metaphase II
  • Chromosomes line up in the middle of each haploid cell
3. Anaphase II
  • Sister chromatids separate and move to opposite poles
4. Telophase II & Cytokinesis
  • Each haploid cell divides forming 4 haploid daughter cells 

In Males (Spermatogenesis):

  • Meiosis I → Two secondary spermatocytes

  • Meiosis II → Four spermatids, which become sperm

In Females (Oogenesis):

  • Meiosis I → One secondary oocyte + one polar body

  • Meiosis II → One ovum + second polar body

  • Only one ovum gets most of the cytoplasm and organelles


Fertilization and the Zygote

  • Sperm (n) + Ovum (n) → Zygote (2n)

  • Zygote has homologous chromosome pairs, one from each parent

  • If sperm and egg were made by mitosis, the chromosome number would double each generation


How Meiosis Creates Genetic Variation

  1. Crossing Over 

    • Chromosomes swap sections, creating new gene combinations

  2. Independent Assortment 

    • Chromosomes line up randomly, leading to many combinations

  3. Random Fertilization

    • Any sperm can fertilize any egg

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