Overview
This lecture covers the first phase of meiosis, focusing on its stages, key processes like genetic variation, and essential terminology, with comparisons to mitosis.
Introduction to Meiosis vs Mitosis
- Meiosis and mitosis are both types of cell division.
- Mitosis involves one division, producing two identical diploid cells.
- Meiosis involves two divisions (meiosis I & II), producing four non-identical haploid cells.
- Meiosis reduces the chromosome number by half and generates genetic variation.
Interphase in Meiosis
- Cell spends most of its time in interphase before meiosis starts.
- DNA replication occurs in interphase, creating chromatids from chromosomes.
- At interphase, the nuclear membrane is visible and homologous pairs are not yet formed.
- Key structures: centrioles, centrosomes, nucleus, and microtubules (spindle fibers).
Prophase I
- Chromosomes condense and become visible, nuclear membrane breaks down.
- Homologous chromosomes pair up in a process called synapsis.
- Crossing over occurs at chiasmata, exchanging genetic material for variation.
- When paired and touching, homologous chromosomes are called bivalents.
Metaphase I
- Bivalents align at the cell equator (middle).
- Chromosomes arrange randomly, increasing genetic variation (random arrangement).
- Chromosomes remain double stranded.
Anaphase I
- Spindle fibers pull bivalents apart, separating homologous chromosomes to opposite poles.
- Chromosome number is halved (from diploid to haploid).
- Recombinant chromosomes are visible due to crossing over.
- Independent assortment happens, separating traits independently.
Telophase I
- Cytokinesis divides the cytoplasm, forming two non-identical haploid cells.
- Nuclear membrane reforms and spindle fibers disappear.
- Cells have half the original chromosome number and new genetic combinations.
Key Terms & Definitions
- Homologous pair — two chromosomes with similar genetic information, one from each parent.
- Diploid — a cell with a full set of chromosomes (e.g., 46 in humans).
- Haploid — a cell with half the chromosome number (e.g., 23 in humans).
- Bivalent — paired homologous chromosomes during crossing over.
- Synapsis — process where homologous chromosomes move together.
- Crossing over — exchange of genetic material between homologous chromosomes.
- Random arrangement — random alignment of chromosomes at metaphase I equator.
- Recombinant chromosome — chromosome with a mix of genetic material after crossing over.
- Chiasmata — point where crossing over occurs between homologous chromosomes.
- Cytokinesis — division of cytoplasm to form separate cells.
Action Items / Next Steps
- Review diagrams and be able to identify each phase and its key features.
- Learn and use meiosis-related vocabulary correctly.
- Prepare for next lesson: meiosis II.