Meiosis is a specialized form of cell division completed in two main stages, Meiosis I and Meiosis II, with the goal of producing reproductive cells, or gametes. The entire process begins with a single diploid cell and ultimately results in four cells, each containing half the number of chromosomes as the original parent cell. Meiosis II, the second division, separates the duplicated parts of the chromosomes to achieve this final haploid state. This precise separation produces four genetically unique cells for sexual reproduction.
The State of the Cells After Meiosis I
The cells following Meiosis I have already undergone a major reduction in chromosome number. After the separation of homologous chromosomes in the first division, each of the two resulting daughter cells is considered haploid (n). This means the cell contains only one complete set of chromosomes, which is a significant distinction from the original diploid state (2n).
Even though the cell is haploid, each chromosome still consists of two identical strands of DNA known as sister chromatids. These chromatids remain joined together at the centromere. The brief period between the first and second meiotic divisions is referred to as Interkinesis or Interphase II.
A defining characteristic of this intermediate stage is the absence of DNA replication. Unlike the interphase preceding Meiosis I, the cell does not duplicate its genetic material during Interkinesis. This omission ensures the chromosome number is reduced by half over the entire meiotic process. The existing sister chromatids, forming a haploid set, are destined to be separated in Meiosis II.
Prophase II: Initiating the Second Division
Prophase II marks the beginning of the second meiotic division, initiating the steps needed to separate the sister chromatids. If a nuclear envelope reformed at the end of Meiosis I, it begins to fragment and disappear during Prophase II. This breakdown allows the machinery of cell division to access the chromosomes.
The two centrosomes, which organize microtubules, start to move apart. These centrosomes migrate toward opposite poles of the cell, establishing the future axis of division. As they move, the mitotic spindle, a network of microtubules, begins to re-form in each haploid cell.
The chromosomes, which remained condensed after Meiosis I, may shorten slightly further during this stage. The new spindle apparatus forms perpendicular to the orientation of the spindle from Meiosis I, ensuring proper orientation for the final separation. This reformation prepares the cell for the precise alignment of chromosomes in the next stage.
Alignment of Chromosomes in Metaphase II
The chromosomes are prepared for separation once they reach Metaphase II, aligning along the cell’s center, called the metaphase plate. This alignment differs from Metaphase I, where homologous pairs lined up together. In Metaphase II, the individual chromosomes, each still made of two sister chromatids, line up in a single file along the plate.
Spindle fibers, extending from the opposite poles of the cell, attach to the chromosomes at specialized protein structures called kinetochores. Each sister chromatid possesses its own kinetochore. The fibers attach to the kinetochore of one sister chromatid from one pole and the kinetochore of the other sister chromatid from the opposite pole.
The single-file alignment and the bipolar attachment of the spindle fibers are the final preparations for the separation phase. This positioning ensures that when the sister chromatids separate, each new daughter cell receives a complete and equal set of single-chromatid chromosomes, achieving the final haploid goal of meiosis.