Lecture on Meiosis
Introduction
- Discussion on how siblings can look different despite having the same parents.
- Focus on the process called meiosis, often confused with mitosis.
- Mitosis: Produces identical body cells (e.g., skin, stomach cells). Important for growth and repair.
- Meiosis: Contributes to genetic variety. Produces sperm and egg cells (gametes).
Chromosome Numbers
- Humans typically have 46 chromosomes in body cells.
- Sperm and egg cells have 23 chromosomes each.
- Fertilization combines these to restore the 46 chromosomes.
Meiosis: A Reduction Division
- Starting cell: 46 chromosomes.
- Ending cells (sperm/egg): 23 chromosomes.
Preliminary Stage: Interphase
- Occurs before meiosis and mitosis.
- Cell growth, DNA replication, and cell processes happen.
- Duplicates chromosomes (46 to 92 chromatids) while maintaining 46 centromeres.
Stages of Meiosis
Meiosis I
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Prophase I
- Chromosomes condense and pair up with homologous chromosomes.
- Crossing over: Exchange of genetic material between homologous chromosomes leads to genetic variety (recombinant chromosomes).
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Metaphase I
- Chromosomes align in pairs in the middle of the cell.
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Anaphase I
- Homologous chromosomes are pulled apart.
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Telophase I & Cytokinesis
- Formation of two new nuclei.
- Cytoplasm divides to form two new cells.
Meiosis II
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Prophase II
- No crossing over.
- Chromosomes and spindle fibers form.
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Metaphase II
- Chromosomes align single-file in the middle.
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Anaphase II
- Chromatids are pulled apart.
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Telophase II & Cytokinesis
- Nuclei reform, resulting in four cells.
- Cytoplasm divides.
Outcome of Meiosis
- Produces sperm cells in males and egg cells in females.
- Each of the four resulting cells is genetically different.
- Contributes to genetic diversity among siblings.
Scientific Exploration
- Nondisjunction: Incorrect chromosome separation leading to genetic disorders.
- Ongoing research in the process and its implications.
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