Overview
The lecture explains the phases of meiosis, the specialized cell division process that creates genetically unique gametes (sperm and egg cells), highlighting the steps and significance of genetic variation.
Introduction to Meiosis
- Meiosis is a type of cell division called reduction division that produces gametes (sex cells).
- Gametes are sperm cells in males and egg cells in females.
- Meiosis consists of two stages: meiosis I and meiosis II, each with four phases.
Meiosis I: Phases and Processes
- Begins with a diploid cell containing two sets of chromosomes (one from each parent).
- Prophase I: Chromatin condenses into chromosomes, and DNA has already replicated.
- Synapsis occurs: each chromosome pairs with its homologous chromosome, forming a tetrad (four chromatids).
- Crossing over (recombination): homologous chromosomes exchange alleles, creating genetic variation.
- Nuclear membrane disappears, and spindle fibers appear.
- Metaphase I: homologous chromosomes line up at the cell equator and attach to spindle fibers.
- Anaphase I: homologous chromosomes are separated to opposite poles.
- Telophase I: nuclear membrane reforms, and cytokinesis produces two genetically different haploid cells.
Meiosis II: Phases and Outcome
- No DNA replication before meiosis II begins.
- Prophase II: nuclear membrane disappears, and spindle fibers form in both haploid cells.
- Metaphase II: chromosomes line up at the equator in each cell.
- Anaphase II: sister chromatids separate and move to opposite poles (now called chromosomes).
- Telophase II: nuclear membranes reform, spindle fibers disappear, and cytokinesis happens.
- Meiosis II produces four genetically unique haploid gametes.
Summary & Key Points
- Meiosis starts with a diploid cell and results in four different haploid gametes.
- Genetic variation arises due to crossing over during prophase I.
- Homologous chromosomes separate in meiosis I; sister chromatids separate in meiosis II.
- All gametes produced are haploid.
Key Terms & Definitions
- Gamete — a haploid sex cell (sperm or egg).
- Diploid — a cell with two sets of chromosomes.
- Haploid — a cell with one set of chromosomes.
- Homologous chromosomes — chromosome pairs, one from each parent, with the same genes.
- Sister chromatids — identical copies of a chromosome, joined together.
- Tetrad — group of four chromatids formed during synapsis.
- Synapsis — pairing of homologous chromosomes during prophase I.
- Crossing over (recombination) — exchange of genetic material between homologous chromosomes.
- Cytokinesis — division of the cell's cytoplasm.
Action Items / Next Steps
- Review the phases of meiosis and note what occurs in each.
- Study the importance of crossing over for genetic variation.
- Ensure you can define all key terms listed.