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.
6. Nuclear membrane breaks down and spindle fibres form.

Metaphase I

1. Homologous pairs randomly line up side-by-side along the centre.
2. This randomness generates independent assortment, which increases variation.
3. Spindle fibres attach to each homolog.

Anaphase I

1. Homologous chromosomes are pulled apart to opposite poles.
2. Sister chromatids remain together.
3. This is the exact moment the cell becomes haploid.

Telophase I and Cytokinesis

1. Chromosomes will reach opposite ends, and the nuclear membranes may briefly reform.
2. The cell divides into two haploid cells, with each chromosome still containing two chromatids, which will separate in Meiosis II.

Meiosis II: Separating Sister Chromatids

1. Meiosis II works like a mitosis-style division, but it starts from haploid cells, not diploid ones.
2. Because the chromatids stayed together during Meiosis I, they must now be separated so each gamete ends up with one chromatid per chromosome.
3. This step does not change chromosome number. It simply refines the distribution to prevent duplication errors.
4. Once Meiosis II finishes, the cell has completed the process of forming four unique haploid gametes.

Prophase II

1. Chromosomes re-condense if they had loosened.
2. Nuclear membranes break down again.
3. New spindle fibres form.
4. This division ensures chromatids are split evenly.

Metaphase II

1. Chromosomes line up single file at the centre.
2. Each still consists of two sister chromatids.
3. This stage looks similar to mitosis, but the starting cells are haploid, not diploid.

Anaphase II

1. Sister chromatids finally separate.
2. They move to opposite poles.
3. Each chromatid becomes an independent chromosome.
4. This ensures each gamete receives one copy of each chromosome.

Telophase II and Cytokinesis

1. Nuclear membranes reform around each set of chromatids.
2. The cells split.
3. Four genetically different haploid gametes are produced.
4. These differences arise from crossing over and independent assortment.

Why Does Meiosis Creates Variation?

1. Crossing over shuffles alleles within chromosomes.
2. Independent assortment shuffles whole chromosomes.
3. These processes operate before fertilisation even happens.
4. When a gamete meets another gamete, variation increases further through random fertilisation.
5. This continuous reshuffling helps populations adapt.