Overview
This lesson introduces meiosis, focusing on meiosis I, its stages, and its role in producing haploid cells (eggs and sperm) for genetic diversity.
Introduction to Meiosis
- Meiosis produces haploid cells (eggs and sperm) from diploid parent cells.
- Unlike mitosis, meiosis involves two sequential divisions: meiosis I and meiosis II.
- Cells start meiosis after a single interphase, which occurs only once before meiosis I.
Stages of Meiosis I
- Interphase occurs before meiosis, including G1, S, and G2 phases (same as mitosis).
- Chromosomes duplicate during S phase; centrosomes and centrioles also duplicate.
- Meiosis I consists of: prophase I, metaphase I, anaphase I, telophase I, and cytokinesis.
Prophase I
- Nuclear membrane starts to break down; chromatin condenses into visible chromosomes.
- Homologous chromosomes (one maternal, one paternal) pair up and form tetrads.
- Crossing over occurs within tetrads, exchanging genetic material and increasing genetic diversity.
- Spindle fibers form from centrosomes and begin attaching to chromosomes’ centromeres.
Metaphase I
- Tetrads align along the metaphase plate (cell’s midline).
- Spindle microtubules attach to the centromeres of each homologous chromosome.
Anaphase I
- Homologous chromosomes within each tetrad are pulled apart to opposite poles by spindle fibers.
- Each new cell will receive a mix of maternal and paternal genetic material due to crossing over.
Telophase I and Cytokinesis
- Spindle disintegrates as sister chromatids reach opposite poles.
- Cytoplasm divides, resulting in two daughter cells, each with half the diploid chromosome number (still as sister chromatids).
Key Terms & Definitions
- Meiosis — Cell division producing haploid reproductive cells from diploid cells.
- Haploid (n) — Having one complete set of chromosomes.
- Diploid (2n) — Having two complete sets of chromosomes.
- Interphase — Cell cycle phase before division; includes G1, S, and G2.
- Tetrad — Group of four chromatids (two homologous chromosomes, each with two sister chromatids).
- Crossing Over — Exchange of genetic material between homologous chromosomes during prophase I.
- Centromere — Central region connecting sister chromatids.
- Kinetochore — Protein at centromere where spindle fibers attach.
Action Items / Next Steps
- Review the stages of mitosis for comparison with meiosis.
- Be prepared to continue with meiosis II in the next lesson.