Overview
This lecture explains the process of meiosis, how it differs from mitosis, and its role in generating genetic diversity among offspring.
Meiosis vs. Mitosis
- Mitosis creates identical body cells for growth and repair.
- Meiosis makes gametes (sperm and egg cells) that are genetically unique.
- Human body cells have 46 chromosomes; gametes have 23 chromosomes.
- Meiosis reduces chromosome number by half, ensuring fertilized eggs have 46 chromosomes.
Chromosome Numbers & Interphase
- Interphase is the stage before meiosis where DNA replicates.
- After DNA replication, there are still 46 chromosomes, but each has a sister chromatid (92 chromatids).
- Chromosome count is based on the number of centromeres, not chromatids.
Stages of 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 middle of the cell.
- Anaphase I: Homologous chromosomes are pulled to opposite sides of the cell.
- Telophase I: Two new nuclei form, leading to two cells after cytokinesis.
Stages of Meiosis II
- Prophase II: Chromosomes condense, but no homologous pairs or crossing over.
- Metaphase II: Chromosomes align single file at the cell's center.
- Anaphase II: Chromatids are separated and pulled apart.
- Telophase II: Nuclei reform, resulting in four cells after cytokinesis.
Results and Genetic Variation
- Meiosis produces four unique gametes, each with 23 chromosomes.
- Genetic variation arises due to crossing over and independent assortment.
- Siblings differ because each gamete is unique.
Errors in Meiosis
- Nondisjunction is the failure of chromosomes to separate properly, leading to genetic disorders.
Key Terms & Definitions
- Meiosis — cell division producing gametes with half the chromosome number.
- Mitosis — cell division producing identical body cells.
- Gametes — reproductive cells (sperm and egg).
- Chromatids — identical copies of a chromosome post-DNA replication.
- Crossing over — exchange of genetic material between homologous chromosomes.
- Homologous chromosomes — chromosomes of the same size with genes for the same traits in the same locations.
- Nondisjunction — error in chromosome separation during meiosis.
Action Items / Next Steps
- Review detailed video on chromosome numbers before and after interphase.
- Study the phases of meiosis and key vocabulary for better understanding.