10.3.1 Differentiate Among the Events of Meiosis I (Prophase, Metaphase, Anaphase, Telophase)
Meiosis I is the first division in meiosis, reducing the chromosome number from diploid (2n) to haploid (n). It consists of four stages:
Prophase I:
- Chromatin condenses into visible chromosomes, each with two sister chromatids joined at the centromere.
- The nuclear envelope and nucleolus disappear.
- The mitotic spindle forms (via centrosomes in animal cells or microtubule-organizing centers in plant cells).
- Homologous chromosomes pair up (synapsis), forming tetrads (four chromatids: two per homologous chromosome).
- Crossing over occurs, where segments of DNA are exchanged between non-sister chromatids of homologous chromosomes at chiasmata, increasing genetic diversity.
- This phase is the longest and most complex of meiosis I.
Metaphase I:
- Homologous chromosome pairs (tetrads) align at the metaphase plate (cell’s equatorial plane).
- Spindle fibers from opposite poles attach to the kinetochores of each homologous chromosome (one chromosome per pole).
- Random assortment of homologous chromosomes occurs, contributing to genetic variation.
- A checkpoint ensures proper spindle attachment before proceeding.
Anaphase I:
- Homologous chromosomes are pulled apart to opposite poles by shortening kinetochore microtubules.
- Unlike mitosis, sister chromatids remain attached at the centromere; only homologous chromosomes separate.
- This reduces the chromosome number from diploid to haploid in each daughter cell.
Telophase I:
- Chromosomes arrive at opposite poles, each with two sister chromatids.
- The nuclear envelope may partially reform around each chromosome set in some organisms.
- The spindle disassembles, and the nucleolus may reappear.
- Cytokinesis often occurs simultaneously, dividing the cytoplasm to form two daughter cells, each with a haploid set of chromosomes.
Key Difference in Meiosis I: The pairing and separation of homologous chromosomes (not sister chromatids) and crossing over distinguish it from mitosis.
10.3.2 Explain Events Occurring During the Second Meiotic Division
Meiosis II follows meiosis I and resembles mitosis but occurs in haploid cells. It separates sister chromatids to produce four haploid cells. The stages are:
Prophase II:
- Chromosomes, each consisting of two sister chromatids, condense again (if decondensed in telophase I).
- The nuclear envelope (if reformed) breaks down, and the nucleolus disappears.
- New spindle fibers form from centrosomes (animal cells) or microtubule-organizing centers (plant cells).
Metaphase II:
- Chromosomes align at the metaphase plate of each daughter cell.
- Spindle fibers attach to the kinetochores of sister chromatids, with each chromatid connected to opposite poles.
- A checkpoint ensures proper spindle attachment.
Anaphase II:
- Cohesin proteins at the centromere are cleaved, allowing sister chromatids to separate.
- Each chromatid (now a chromosome) is pulled to opposite poles by shortening kinetochore microtubules.
- Polar microtubules elongate, further separating the poles.
Telophase II:
- Chromosomes arrive at opposite poles and begin to decondense into chromatin.
- The nuclear envelope reforms around each chromosome set, and the nucleolus reappears.
- The spindle disassembles.
- Cytokinesis occurs, dividing the cytoplasm to produce four haploid daughter cells, each with a single set of chromosomes.
10.3.3 Compare Events of Second Meiotic Division with Mitosis
| Feature | Meiosis II | Mitosis |
| Starting Cell | Haploid (n), with chromosomes consisting of two sister chromatids. | Diploid (2n), with chromosomes consisting of two sister chromatids. |
| Purpose | Separates sister chromatids to produce four haploid cells. | Separates sister chromatids to produce two diploid cells. |
| Prophase | Chromosomes condense; spindle forms; nuclear envelope breaks down. | Same as meiosis II. |
| Metaphase | Chromosomes align at metaphase plate; spindle fibers attach to sister chromatids. | Same as meiosis II. |
| Anaphase | Sister chromatids separate to opposite poles. | Same as meiosis II. |
| Telophase/Cytokinesis | Nuclear envelope reforms; cytokinesis produces four haploid cells. | Nuclear envelope reforms; cytokinesis produces two diploid cells. |
| Genetic Outcome | Four genetically diverse haploid cells due to prior crossing over and random assortment in meiosis I. | Two genetically identical diploid cells. |
| Crossing Over | Does not occur (occurred in meiosis I). | Does not occur. |
10.3.4 Recognize Significance of Meiosis
Meiosis is essential for sexual reproduction and genetic diversity, with two key roles:
Formation of Haploid Cells:
Meiosis reduces the chromosome number from diploid (2n) to haploid (n), producing cells that can function as gametes or divide further.
- In Animals: Haploid cells (e.g., sperm and egg cells in humans, each with 23 chromosomes) function directly as gametes, fusing during fertilization to restore the diploid number (46 chromosomes).
- In Plants: Haploid cells (e.g., spores in ferns or pollen/ovule cells in flowering plants) may divide by mitosis to form multicellular haploid structures (e.g., gametophytes) that produce gametes. For example, pollen grains undergo mitotic divisions to produce sperm cells in plants.
Recombination of Genes Leading to Variations:
Meiosis introduces genetic diversity through two mechanisms:
- Crossing Over (in prophase I): Exchange of genetic material between homologous chromosomes creates new combinations of alleles, leading to unique gametes.
- Random Assortment (in metaphase I): Independent alignment of homologous chromosomes at the metaphase plate results in different chromosome combinations in gametes.