AS

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.