Meiosis and Genetic Diversity: Summary Notes

Heredity and Genetics

• Heredity: Transmission of traits from one generation to the next.
• Traits are passed through genes (segments of DNA).
• Offspring inherit chromosomes, acquiring genes from parents.

Asexual vs. Sexual Reproduction

• Asexual: Single parent, offspring are clones, variation only through mutations.
• Sexual: Two parents, offspring have unique gene combinations, genetic variation.

Homologous Chromosomes

• Pairs of chromosomes with same size, length, and centromere position.
• One chromosome inherited from each parent.

Karyotypes

• Display of chromosome pairs ordered by size and length.

Cells and Chromosomes

• Somatic (body) cells: Diploid (2n), two sets of chromosomes (e.g., humans 2n = 46).
• Gametic (sex) cells: Haploid (n), one set of chromosomes (e.g., humans n = 23).
• Autosomes: Non-sex-determining chromosomes (22 pairs in humans).
• Sex chromosomes: X and Y (Eggs: X, Sperm: X or Y).

Life Cycles

• Sequence of reproductive stages from conception to reproduction.
• Fertilization (haploid sperm + haploid egg = diploid zygote) alternates with meiosis.

Meiosis

• Process creating haploid gamete cells in diploid organisms.
• Results in daughter cells with half the chromosomes (e.g., human diploid 2n=46 -> haploid n=23).
• Involves two rounds of division: Meiosis I and Meiosis II.

Mitosis vs. Meiosis

• Mitosis: Somatic cells, 1 division, 2 diploid cells, identical daughter cells.
• Meiosis: Gametes, 2 divisions, 4 haploid cells, genetically unique daughter cells.

Key Events in Meiosis

1. Prophase I: Synapsis and crossing over.
2. Metaphase I: Tetrads line up at metaphase plate.
3. Anaphase I: Homologous pairs separate.

Meiosis I

• Prophase I: Synapsis (homologous chromosomes pair) and crossing over (DNA exchange).
• Metaphase I: Tetrads align at metaphase plate with independent orientation.
• Anaphase I: Homologous chromosomes separate; sister chromatids remain attached.
• Telophase I & Cytokinesis: Nuclei and cytoplasm divide, resulting in haploid cells.

Meiosis II

• Prophase II: Spindle forms; no crossing over.
• Metaphase II: Chromosomes line up at metaphase plate.
• Anaphase II: Sister chromatids separate.
• Telophase II & Cytokinesis: 4 haploid cells, genetically unique.

Genetic Variation in Meiosis

1. Crossing Over: Produces recombinant chromosomes.
2. Independent Assortment: Random chromosome orientation at metaphase I.
3. Random Fertilization: Any sperm can fertilize any egg.

Summary

• Meiosis and fertilization ensure genetic diversity, which drives natural selection.