Cell division allows for growth, repair, and reproduction in living organisms. The two primary types are mitosis, which creates genetically identical body cells, and meiosis, which produces sex cells (gametes). Meiosis is a two-part process (Meiosis I and Meiosis II) necessary to halve the chromosome number. Anaphase II is a brief phase within the second division, ensuring genetic material is correctly partitioned into the final reproductive cells.
Setting the Stage for Anaphase II
Meiosis II begins immediately following Meiosis I, which successfully separated homologous chromosomes. Meiosis I is called a reduction division because it transforms a single diploid cell into two haploid cells. However, at the start of Meiosis II, each chromosome within these two daughter cells still consists of two identical sister chromatids joined at a single centromere.
The cell quickly moves into Prophase II, where a new spindle apparatus begins to form, and then to Metaphase II. During Metaphase II, the chromosomes align individually along the cell’s central plane, known as the metaphase plate. The orientation of these chromosomes is random, which contributes to the genetic diversity of the final gametes. This precise alignment is necessary to ensure that the chromatids can be accurately pulled apart in the subsequent phase.
Each sister chromatid has a protein structure, called a kinetochore, facing opposite poles of the cell. Kinetochore microtubules, which are part of the spindle fibers, attach to these structures. The chromosomes are lined up and under tension, ready for the final separation of the genetic material.
The Mechanics of Sister Chromatid Separation
The transition from Metaphase II to Anaphase II is initiated by a precise molecular signal that triggers the breakdown of the cohesive forces holding the sister chromatids together. This signal comes from the activation of a protein complex known as the Anaphase Promoting Complex (APC/C). The APC/C tags a protein called securin for destruction, which frees an enzyme called separase.
Once active, the separase enzyme performs its specific task by cleaving the cohesin proteins that physically link the sister chromatids along their length. Cohesin is a ring-shaped protein complex that holds the chromatids tightly together, especially at the centromere region. The sudden, simultaneous cleaving of this protein “glue” is the defining event that marks the start of Anaphase II.
With the cohesin removed, the centromere splits, and the formerly paired sister chromatids become individual chromosomes. These newly separated chromosomes are immediately pulled toward opposite poles of the cell. The movement is driven by the shortening of the kinetochore microtubules, which reel in the chromosomes. Motor proteins also assist in this poleward movement, and the spindle poles move farther apart, elongating the cell body.
This coordinated action ensures that an equal and complete set of chromosomes is delivered to each end of the dividing cell. The separation of sister chromatids in Anaphase II is functionally identical to the anaphase stage of mitosis, but it occurs in a cell that is already haploid.
Completion of Meiosis and the Final Result
Following the separation and movement of the chromosomes in Anaphase II, the cell enters Telophase II. At each pole, the newly arrived individual chromosomes begin to decondense. A nuclear envelope reforms around each of the chromosome groups.
Simultaneously, cytokinesis (the physical division of the cell) takes place. The cytoplasm divides, resulting in the formation of four distinct daughter cells. Each of these four cells contains a haploid set of chromosomes.
The ultimate result of meiosis is the generation of four genetically distinct haploid gametes. Because of the random alignment in Metaphase II and the crossing over that occurred in Meiosis I, each gamete carries a unique combination of genetic information. This genetic variability is fundamental for sexual reproduction, providing the mechanism for diversity within a species.