MYP Biology Meiosis Notes

How Does Meiosis Create Gametes With Half the Usual DNA?

1. Meiosis and mitosis both involve cell division, but they solve different biological problems.
2. Mitosis is used for growth and repair, so it makes identical copies of cells with the same chromosome number.
3. Meiosis is used for sexual reproduction, so it must:
   1. halve the chromosome number (haploid gametes)
   2. generate genetic variation
4. Because of this, meiosis includes two divisions and has extra steps such as homologous pairing and crossing over that never occur in mitosis.

Why Does Meiosis Need Two Divisions?

1. Human body cells are diploid.
2. Gametes must be haploid so that fertilisation restores the original diploid number.
3. One division cannot achieve this reduction accurately, which is why meiosis splits chromosomes in two steps:
   1. Meiosis I halves the chromosome number
   2. Meiosis II separates chromatids to avoid duplication
4. This structure ensures every gamete carries one complete set of chromosomes, but in a new combination.

Meiosis I: Reducing Chromosome Number

1. Meiosis I is where the main difference from mitosis happens.
2. Instead of splitting sister chromatids, the cell splits homologous chromosome pairs.
3. Meiosis I also includes the two events that create most genetic variation: crossing over and independent assortment.
4. Once Meiosis I is complete, the cell is haploid, even though chromatids are still attached.

Prophase I

1. Chromosomes condense so they can be moved without breaking.
2. Homologous chromosomes pair up, which is essential because meiosis separates parental versions into different gametes.
3. Crossing over occurs.
4. Sections of DNA swap between homologues.
5. This produces new genetic combinations, increasing variation before the cell even divides.