Meiosis is a specialized form of cell division fundamental to sexual reproduction. Its purpose is to generate gametes (sex cells) that carry half the number of chromosomes of the parent cell. This process involves two consecutive rounds of division, Meiosis I and Meiosis II, which reduce the chromosome number and increase genetic variation. Meiosis I separates homologous chromosome pairs, and Meiosis II separates the sister chromatids to form four unique reproductive cells.
Preparing for the Second Division
The cells produced after Meiosis I are haploid, meaning they contain one chromosome from each homologous pair. Each chromosome still consists of two identical sister chromatids attached at the centromere. The brief period between the two meiotic divisions is called interkinesis, and unlike normal interphase, no DNA replication occurs. This ensures the chromosome number remains halved before the final separation.
As the cell transitions into Prophase II, the chromosomes condense again into their tightly coiled form. The nuclear envelope around the chromosomes breaks down, and the spindle apparatus, made of microtubules, begins to reform for the second division.
Chromosome Alignment at the Equator
Metaphase II begins once the spindle fibers fully capture the chromosomes and move them toward the cell’s center. The precise alignment of individual chromosomes occurs along the metaphase plate, an imaginary line equidistant from the two poles. Each chromosome, still composed of two sister chromatids, is positioned on this central plane.
Spindle microtubules mediate this alignment by attaching to the kinetochore, a protein structure located at the centromere of each chromatid. The kinetochore of one sister chromatid faces one pole, while the kinetochore of the other sister chromatid faces the opposite pole. This specific orientation ensures the sister chromatids will be pulled apart in the next stage.
Why Metaphase II Differs from Metaphase I
The mechanics of Metaphase II differ fundamentally from Metaphase I, primarily in the structures that align at the equator. In Metaphase I, homologous chromosome pairs align, with members of each pair facing opposite poles. Metaphase II, however, involves the alignment of single chromosomes, each made of two sister chromatids, similar to alignment during mitosis.
While Metaphase I separates homologous pairs to halve the chromosome number, Metaphase II separates the sister chromatids without further reducing the chromosome number. Additionally, the genetic recombination event known as crossing over occurs exclusively in Prophase I and is absent in the second meiotic division.
Separation and Final Product
The precise alignment in Metaphase II is immediately followed by Anaphase II, the stage where the sister chromatids finally separate. The proteins holding the sister chromatids together at the centromere are cleaved, allowing the newly separated chromatids to be pulled toward opposite poles of the cell by the shortening spindle fibers. Once separated, each chromatid is considered a full, individual chromosome.
The cell then enters Telophase II, where the chromosomes arrive at the poles and begin to decondense, reverting to a less compact form. A new nuclear envelope forms around each of the four sets of chromosomes, and the spindle apparatus disassembles. Cytokinesis, the division of the cytoplasm, then occurs, physically splitting the two cells from Meiosis I into four distinct, haploid daughter cells. These four cells are genetically unique due to the combined effects of crossing over and the random assortment that occurred in Meiosis I.