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.