Overview
This lecture explains the process of meiosis, highlighting its role in genetic variation and comparing it to mitosis. The stages, outcomes, and importance of meiosis in sexual reproduction are detailed.
Purpose of Meiosis
- Meiosis creates gametes (sperm and egg cells) with half the chromosome number (23 in humans), enabling genetic variety.
- Unlike mitosis, which produces identical body cells for growth and repair, meiosis leads to genetically different cells.
- Fertilization restores the chromosome number to 46 in humans when egg and sperm unite.
Chromosome Numbers & Interphase
- Most human body cells have 46 chromosomes; gametes have 23 chromosomes.
- Interphase occurs before meiosis (and mitosis), where DNA is replicated, but chromosome number by centromere count remains 46.
- After DNA replication, there are 92 chromatids, but still 46 chromosomes because sister chromatids are attached.
Phases of Meiosis
- Meiosis includes two rounds of division (Meiosis I and Meiosis II), each with stages: Prophase, Metaphase, Anaphase, Telophase (PMAT) labeled I or II.
- Reduction division: chromosome number halves from 46 to 23.
Meiosis I
- Prophase I: Chromosomes condense, pair with homologous chromosomes, and crossing over (exchange of genetic material) occurs.
- Metaphase I: Homologous chromosome pairs align in the cell's middle.
- Anaphase I: Homologous chromosomes are pulled apart to opposite sides.
- Telophase I and Cytokinesis: Two nuclei form; cell divides into two.
Meiosis II
- Prophase II: Chromosomes condense; no crossing over.
- Metaphase II: Chromosomes align single file in the center.
- Anaphase II: Chromatids are pulled apart to opposite sides.
- Telophase II and Cytokinesis: Nuclei form; cells split again, producing four non-identical gametes.
Genetic Variation
- Crossing over and independent assortment during meiosis create genetically unique gametes.
- Siblings are genetically different because each sperm and egg cell is unique.
Errors in Meiosis
- Nondisjunction occurs when chromosomes fail to separate properly, leading to genetic disorders.
Key Terms & Definitions
- Meiosis — Cell division producing gametes with half the chromosome number.
- Mitosis — Cell division creating identical body cells.
- Gametes — Sperm and egg cells with 23 chromosomes in humans.
- Homologous Chromosomes — Chromosomes of similar size and gene content.
- Crossing Over — Exchange of genetic material between homologous chromosomes in prophase I.
- Nondisjunction — Error where chromosomes don’t separate, causing extra or missing chromosomes.
Action Items / Next Steps
- Review detailed videos on chromosome numbers before and after DNA replication.
- Understand the key differences between meiosis and mitosis for exam preparation.