Meiosis is a specialized form of cell division fundamental to sexual reproduction. It forms gametes, such as sperm and egg cells. Meiosis reduces the chromosome number by half, preparing cells for fertilization where two gametes combine to restore the full chromosome set. The process involves two successive divisions: Meiosis I and Meiosis II.
The Steps of Meiosis II
Meiosis II begins with the two haploid cells produced during Meiosis I. Each cell still contains duplicated chromosomes, meaning each chromosome is composed of two sister chromatids. Meiosis II is similar in its mechanical process to mitosis, the cell division that produces identical body cells, though its genetic outcome is different.
The first stage of Meiosis II is Prophase II, where chromosomes condense again and the nuclear envelope breaks down. Spindle fibers begin to form and extend from the centrosomes, which move to opposite poles of the cell. Next, in Metaphase II, the chromosomes align individually along the metaphase plate. Microtubules from opposite spindle poles attach to the kinetochores of each sister chromatid.
Anaphase II follows, as centromeres holding sister chromatids together split. The separated sister chromatids are pulled toward opposite poles of the cell by the spindle fibers. Finally, Telophase II and Cytokinesis occur. Nuclear envelopes reform around each set of chromosomes at the poles, and the chromosomes begin to decondense. Cytokinesis then separates the two cells into four distinct daughter cells.
The Ultimate Outcome: Gamete Formation
Meiosis II produces four haploid cells from the single original diploid parent cell. This occurs because the two haploid cells generated during Meiosis I each undergo a second division. These resulting cells are haploid, containing half the number of chromosomes of the original parent cell. For humans, this means each gamete has 23 chromosomes.
These four cells are genetically unique. This distinctiveness arises from events in Meiosis I: crossing over and independent assortment. Crossing over involves the exchange of genetic material between homologous chromosomes during Prophase I, creating new gene combinations. Independent assortment, during Metaphase I, is the random alignment and distribution of homologous chromosome pairs, leading to diverse chromosome combinations.
These four unique haploid cells are gametes. In males, these are sperm cells, while in females, the process yields one functional egg cell and smaller, non-functional polar bodies. Gametes carry genetic information from parents to offspring, and their fusion during fertilization creates a new organism with a unique genetic makeup.