Overview
This lecture explains the phases of meiosis, the process of cell division that produces genetically diverse gametes, highlighting the key steps and genetic outcomes.
Introduction to Meiosis
- Meiosis, also called reduction division, produces gametes (sperm and egg cells).
- It consists of two stages: Meiosis I and Meiosis II, each with four phases.
Meiosis I: Process and Phases
- Meiosis I starts with a diploid cell containing two sets of chromosomes (one from each parent).
- Prophase I: DNA replicates, chromosomes condense, and homologous chromosomes pair up in a process called synapsis, forming tetrads.
- Genes on chromosomes may have different versions called alleles.
- Crossing over (recombination) occurs, exchanging alleles between homologous chromosomes, increasing genetic variety.
- The nuclear membrane disappears, centrioles move, and spindle fibers form.
- Metaphase I: Homologous chromosomes line up at the cell equator and attach to spindle fibers.
- Anaphase I: Homologous chromosomes are separated and pulled to opposite poles.
- Telophase I: Each pole has one chromosome of each homologous pair; chromosomes still have sister chromatids.
- Cytokinesis ends Meiosis I, producing two haploid daughter cells.
Meiosis II: Process and Phases
- DNA does not replicate before Meiosis II.
- Prophase II: Nuclear membrane disappears, spindle fibers reform in each haploid cell.
- Metaphase II: Chromosomes line up at the center of each cell.
- Anaphase II: Sister chromatids separate and move to opposite poles.
- Telophase II: Nuclear membranes reform, spindle fibers disappear, cytokinesis occurs.
- Result: Four genetically distinct haploid gametes, each with one chromosome set.
Key Points and Summary
- Meiosis begins with a diploid cell and produces four genetically different haploid gametes.
- Meiosis I separates homologous chromosomes; Meiosis II separates sister chromatids.
- Crossing over during Prophase I introduces genetic diversity.
- All gametes produced are haploid, containing only one chromosome set.
Key Terms & Definitions
- Meiosis — cell division producing gametes with half the chromosome number.
- Diploid cell — a cell with two sets of chromosomes (one from each parent).
- Haploid cell — a cell with one set of chromosomes.
- Homologous chromosomes — chromosome pairs, one from each parent, with the same genes.
- Synapsis — pairing of homologous chromosomes during Prophase I.
- Tetrad — structure of four chromatids formed during synapsis.
- Allele — different version of a gene.
- Crossing over (recombination) — exchange of genetic material between homologous chromosomes.
- Gamete — sex cell (sperm or egg).
Action Items / Next Steps
- Review the phases and main events of Meiosis I and II.
- Practice drawing diagrams of meiosis stages.
- Memorize key terms and definitions.