Meiosis: A Special Type of Cell Division

1. Meiosis-I: Reduction Division

In Meiosis-I, a diploid parent cell divides into two haploid cells by separating homologous chromosomes.

a) Prophase-I: The Longest Phase

Prophase-I is more complex than in mitosis. It has several key steps:

  1. Condensation of Chromatin:
    • Chromatin condenses into visible chromosomes, each consisting of two sister chromatids.
  2. Synapsis and Tetrad Formation:
    • Homologous chromosomes move close and pair up in a process called synapsis.
    • Each pair of homologous chromosomes forms a tetrad (four chromatids).
  3. Crossing Over and Chiasmata Formation:
    • Non-sister chromatids of homologous chromosomes exchange segments of genetic material at sites called chiasmata.
    • This crossing over increases genetic variation.
  4. Other Events:
    • The nuclear envelope disappears.
    • Spindle fibers begin to form from centrioles.
🔹 Example: Crossing over allows a child to have a mix of both parents’ traits, like father’s eye color and mother’s hair type.

b) Metaphase-I: Alignment of Tetrads

  • Tetrads (homologous chromosome pairs) align at the equator of the cell.
  • Each homologous chromosome is attached to spindle fibers from opposite poles.

c) Anaphase-I: Separation of Homologous Chromosomes

  • Spindle fibers pull homologous chromosomes apart, sending one chromosome of each pair to opposite poles.
  • Each daughter cell now has half the chromosome number but still contains sister chromatids.

d) Telophase-I: Formation of Two Haploid Cells

  • Nuclear envelopes reform around the haploid sets of chromosomes.
  • Chromosomes relax back into chromatin.
  • Cytokinesis occurs, forming two haploid daughter cells.
🔹 Example: In humans, each daughter cell now has 23 chromosomes, but each chromosome still has two chromatids.

2. Meiosis-II: Similar to Mitosis

Meiosis-II resembles mitosis, but instead of homologous chromosomes, sister chromatids separate.

a) Prophase-II

  • Chromosomes condense again.
  • The nuclear envelope dissolves, and spindle fibers form.

b) Metaphase-II

  • Chromosomes align at the equator (similar to metaphase in mitosis).

c) Anaphase-II

  • Sister chromatids are pulled apart to opposite poles.

d) Telophase-II and Cytokinesis

  • Nuclear envelopes reform around chromosomes.
  • Cytokinesis divides the cytoplasm, forming four haploid daughter cells.
🔹 Example: In humans, four sperm cells (in males) or one egg + three polar bodies (in females) are produced, each with 23 chromosomes.