D2.1.9 Meiosis as a reduction division Notes

Meiosis Produces Four Haploid Nuclei from One Diploid Nucleus

1. Diploid cells contain two complete sets of chromosomes (one from each parent).
2. Haploid cells contain one complete set of chromosomes.
3. This reduction prevents chromosome doubling when gametes fuse during fertilization.
4. There are two stages to meiosis.

Meiosis I: Reduction Division

Meiosis I separates homologous chromosomes, pairs of chromosomes with the same genes in the same order but potentially different alleles.

1. Prophase I:
   1. Homologous chromosomes pair up during synapsis, forming structures called bivalents.
   2. Crossing over occurs when non-sister chromatids exchange genetic material, creating new allele combinations.
2. Metaphase I:
   1. Bivalents align at the cell's equator.
   2. Each bivalent's orientation is random—a process called independent assortment that increases genetic diversity.
   3. In humans with 23 chromosome pairs, independent assortment alone can produce over 8 million possible combinations (2²³).
3. Anaphase I:
   1. Homologous chromosomes separate and move to opposite poles.
   2. This halves the chromosome number from diploid to haploid.
   3. Sister chromatids remain attached (unlike in mitosis).
4. Telophase I and Cytokinesis:
   1. The cell divides into two haploid cells.
   2. Each cell contains one chromosome from each homologous pair.

Meiosis II: Separation of Sister Chromatids

Meiosis II resembles mitosis but starts with haploid cells.

1. Prophase II: Chromosomes condense and spindle fibers form.
2. Metaphase II: Chromosomes align at the equator. Spindle fibers attach to kinetochores on each chromatid.
3. Anaphase II: Sister chromatids separate and move to opposite poles.
4. Telophase II and Cytokinesis: The two cells divide, producing four haploid cells with unique genetic compositions.

Why Meiosis Is Essential for Sexual Reproduction

1. Prevents chromosome doubling: Haploid gametes ensure that fertilization restores the diploid number rather than doubling it each generation.
2. Generates genetic variation: Crossing over during Prophase I and independent assortment during Metaphase I create unique combinations of alleles in each gamete.
3. This variation is the raw material for evolution and adaptation.