Mitosis vs Meiosis: Understanding Cell Division Processes

Mitosis: Cell Division for Growth

1. Interphase: DNA and centrosomes have been duplicated.
2. Prophase: The nuclear membrane disappears, chromosomes condense and become visible, and the spindle apparatus begins to form from proteins.
3. Metaphase: The centrioles are at opposite poles of the cell. The spindle fibers align the chromosomes at the cell's equator (metaphase plate).
4. Anaphase: The spindle fibers pull the sister chromatids of each chromosome toward opposite centrosomes.
5. Telophase: The chromatids decondense back into chromatin. New nuclear membranes form around the chromatin, and the spindle breaks down.
6. Cytokinesis: The cytoplasm is divided, forming two new, genetically identical daughter cells.

Meiosis: Producing Gametes

Interphase: DNA and centrosomes are duplicated.

Meiosis I: Separating Homologous Chromosomes

1. Prophase I: Crossing over occurs: homologous chromosomes pair up, and enzymes cut and exchange DNA segments between them. These fibers are then linked to the other homologous chromosome. This is the longest phase of meiosis. At the end of Prophase I, the nuclear envelope disappears, the centrosomes travel to opposite poles of the cell, and spindle fibers start to appear.
2. Metaphase I: Homologous chromosome pairs align along the equatorial plane.
3. Anaphase I: Homologous chromosomes are separated and pulled to opposite poles.
4. Telophase I & Cytokinesis: Cytokinesis occurs concurrently. Two new haploid cells are formed. The nuclear envelope reappears, and the cells then start the second division.

Meiosis II: Separating Sister Chromatids

1. Prophase II: Centrosomes are duplicated. The nuclear envelope disappears again. Centrosomes travel to opposite poles, and spindle fibers start to appear.
2. Metaphase II: Chromosomes align at the equatorial plane. This plane is oriented perpendicular (90 degrees) to the plane of Metaphase I.
3. Anaphase II: Sister chromatids are separated and pulled to opposite poles of the cell.
4. Telophase II & Cytokinesis: Chromosomes decondense. The nuclear envelope reforms. Cytokinesis occurs, and we obtain four genetically distinct daughter haploid cells.

Nondisjunction: Chromosomal Abnormality

Nondisjunction is a chromosomal abnormality that can occur during meiosis if chromosomes fail to separate correctly.

Differences Between Meiosis and Mitosis

• Mitosis occurs in somatic cells (in both haploid and diploid organisms), while meiosis occurs only in diploid cells to produce reproductive cells (gametes).
• Mitosis involves only one cell division, while meiosis involves two divisions.
• Crossing-over occurs during Prophase I of meiosis but not typically in mitosis.
• Anaphase distinctions:
  • Mitosis Anaphase: Sister chromatids are separated.
  • Meiosis Anaphase I: Homologous chromosomes are separated.
  • Meiosis Anaphase II: Sister chromatids are separated (carrying potentially different genetic information due to crossing-over).