Overview
This lecture explains meiosis, the process that produces sperm and egg (gamete) cells and leads to genetic variety among siblings with the same parents.
Mitosis vs. Meiosis
- Mitosis creates identical body cells for growth, repair, and replacement.
- Meiosis creates gametes (sperm and egg cells) that are genetically unique.
- Human body cells have 46 chromosomes; gametes have 23 chromosomes.
Purpose and Process of Meiosis
- Meiosis reduces chromosome number by half, from 46 to 23.
- Fertilization combines two gametes (each with 23 chromosomes) to restore 46 chromosomes in the offspring.
- Meiosis involves two rounds of division (Meiosis I and II), unlike one round in mitosis.
Interphase (Preparation)
- Interphase occurs before meiosis, where the cell grows and duplicates its DNA.
- Chromosome duplication means 46 chromosomes become 92 chromatids, but still counted as 46 chromosomes by centromeres.
Stages of Meiosis I
- Prophase I: Chromosomes condense, pair up with homologous chromosomes, and crossing over (exchange of genetic material) occurs.
- Metaphase I: Homologous pairs align in the middle of the cell.
- Anaphase I: Homologous chromosomes are pulled away to opposite sides.
- Telophase I: Two nuclei form, followed by division into two cells.
Stages of Meiosis II
- Prophase II: Chromosomes and spindle fibers form; no crossing over occurs.
- Metaphase II: Chromosomes align single-file in the center.
- Anaphase II: Chromatids are pulled apart to opposite sides.
- Telophase II: Four nuclei form and cells split, creating four unique gametes.
Genetic Variation and Significance
- Crossing over and independent assortment during meiosis ensure each gamete is genetically unique.
- Meiosis explains why siblings look different despite having the same parents.
- Nondisjunction can occur if chromosomes fail to separate properly, leading to genetic disorders.
Key Terms & Definitions
- Meiosis — Cell division process creating gametes with half the chromosome number.
- Gametes — Reproductive cells (sperm and egg) with 23 chromosomes.
- Homologous chromosomes — Chromosome pairs with the same genes in the same locations.
- Crossing over — Exchange of genetic material between homologous chromosomes during Prophase I.
- Nondisjunction — Failure of chromosomes to separate correctly during meiosis, causing genetic disorders.
- Chromatid — One of two identical halves of a duplicated chromosome.
- Interphase — Cell stage before division, involving growth and DNA replication.
Action Items / Next Steps
- Review the stages of meiosis and compare them to mitosis.
- Study the significance of crossing over and independent assortment.
- Watch the detailed video on chromosome numbers before and after interphase for further clarification.