Cell division is a foundational biological process. For sexual reproduction, a specialized division called meiosis creates reproductive cells (gametes). Meiosis is distinct from cell division used for growth because it reduces the genetic material by half. This process occurs through two sequential rounds, Meiosis I and Meiosis II, ensuring that fertilization results in offspring with the correct total number of chromosomes.
The Critical Role of Sister Chromatid Separation
The goal of Meiosis II is to separate sister chromatids. These are two identical copies of a chromosome, created during DNA replication, joined at the centromere. Cells entering Meiosis II are already haploid, containing one chromosome from each homologous pair. However, each chromosome remains duplicated, existing as a pair of sister chromatids. Separating these joined chromatids is the final step in reducing the genetic content to a single, unreplicated set of chromosomes.
The Four Phases of Meiosis II
Meiosis II proceeds through four phases that prepare the duplicated chromosomes for final segregation. The process begins with Prophase II, where the nuclear envelope dissolves and the chromosomes become compact again. Simultaneously, the spindle apparatus, composed of microtubules, begins to form in the two daughter cells produced by Meiosis I.
The next stage is Metaphase II, where the chromosomes align individually along the cell’s equatorial plane (the metaphase plate). Microtubules from opposite poles attach to the kinetochore—a protein structure on the centromere—of each sister chromatid. This alignment ensures that each chromatid is correctly positioned before division proceeds.
Physical separation occurs during Anaphase II. The protein complexes holding the sister chromatids together at the centromere are cleaved by the enzyme separase. Once cohesin is broken, the sister chromatids are pulled apart by shortening spindle microtubules toward opposite poles. These newly separated structures are now considered individual, unreplicated chromosomes.
The final stage is Telophase II, which begins once the separated chromatids (now individual chromosomes) arrive at the opposite poles. A new nuclear envelope forms around each set of chromosomes, and the chromosomes begin to decondense. Following nuclear division, cytokinesis divides the cytoplasm, resulting in the formation of four daughter cells.
Differentiating Meiosis I and Meiosis II
Meiosis I and Meiosis II are differentiated by the structures they separate and the resulting change in chromosome number. Meiosis I is a reductional division because it separates homologous chromosomes, reducing the total chromosome number by half. A cell that begins diploid ends Meiosis I haploid, though each chromosome is still composed of two sister chromatids.
Meiosis II is often called an equational division because the chromosome number does not change. It separates the sister chromatids, reducing the amount of DNA per chromosome while maintaining the haploid chromosome count. The mechanics of Meiosis II closely resemble mitosis (somatic cell division) because both involve sister chromatid separation. However, Meiosis II begins with haploid cells, whereas mitosis begins with diploid cells.
The two meiotic divisions use different mechanisms for centromere protection. In Meiosis I, the protein shugoshin shields the cohesin at the centromeres, allowing homologous chromosomes to separate while keeping sister chromatids joined. This protection is lost before Meiosis II, permitting the cleavage of centromeric cohesin by separase in Anaphase II for sister chromatid separation.
The Production of Haploid Cells
The entire meiotic process culminates in the production of four genetically unique, haploid cells. A haploid cell contains one set of chromosomes, half the number found in the original parent cell. This outcome is essential for sexual reproduction, as the fusion of two haploid gametes restores the full, diploid chromosome number in the offspring.
The genetic uniqueness of the four daughter cells is a consequence of meiosis. This variation arises from two events in Meiosis I: the random assortment of homologous chromosomes and the physical exchange of genetic segments (crossing over). When sister chromatids separate during Meiosis II, they are no longer identical due to crossing over, ensuring the four final gametes carry a distinct combination of genetic information.