Overview
This lecture explains meiosis, the cellular process that creates gametes (sperm and egg cells) and generates genetic variety among siblings.
Meiosis vs. Mitosis
- Mitosis creates identical body (somatic) cells for growth and repair.
- Meiosis creates gametes (sperm and eggs) and increases genetic diversity.
- Human body cells have 46 chromosomes; gametes have 23.
Chromosome Numbers and Interphase
- Interphase occurs before meiosis, during which DNA replicates.
- After replication, there are 46 chromosomes but 92 chromatids, as chromosomes exist as sister chromatids.
Stages of Meiosis
- Meiosis has two rounds of division: Meiosis I and Meiosis II, each with PMAT stages (Prophase, Metaphase, Anaphase, Telophase).
Meiosis I
- Prophase I: Chromosomes condense, pair as homologous chromosomes, and crossing over (exchange of genetic material) occurs.
- Metaphase I: Homologous chromosome pairs line up in the middle of the cell.
- Anaphase I: Homologous chromosomes are separated and pulled away by spindle fibers.
- Telophase I & Cytokinesis: Two new nuclei form, and the cell divides into two.
Meiosis II
- Prophase II: Chromosomes condense; no homologous pairing or crossing over occurs.
- Metaphase II: Chromosomes line up single file in the middle.
- Anaphase II: Sister chromatids are pulled apart to opposite sides.
- Telophase II & Cytokinesis: Nuclei reform; total of four unique haploid cells are produced.
Genetic Variety and Errors
- Crossing over and independent assortment during meiosis cause genetic variation in gametes.
- Nondisjunction, the failure of chromosomes to separate correctly, can cause genetic disorders.
Key Terms & Definitions
- Meiosis — process producing gametes with half the usual chromosome number.
- Mitosis — process creating identical body cells.
- Gametes — reproductive cells (sperm and egg).
- Homologous Chromosomes — chromosome pairs with same types of genes.
- Crossing Over — exchange of genetic segments between homologous chromosomes.
- Nondisjunction — error where chromosomes fail to separate properly.
Action Items / Next Steps
- Watch the detailed video on chromosome numbers before and after interphase as recommended.
- Review the stages of meiosis and the role of crossing over.