Cytokinesis is an integral part of the meiotic process, occurring twice during the division of germ cells. Meiosis is the specialized cell division that produces gametes (sex cells), such as sperm and eggs, ensuring the resulting cells have half the number of chromosomes of the parent cell. Cytokinesis is the physical process that follows nuclear division, involving the division of the cytoplasm and the cell membrane to form two distinct daughter cells.
Cytokinesis Following Meiosis I
The first instance of cytokinesis follows Meiosis I, known as the reductional division because it halves the chromosome number. This process begins after homologous chromosomes separate and reach opposite poles during Telophase I. The division of the cytoplasm typically occurs concurrently with Telophase I, physically separating the cell into two distinct entities.
In animal cells, the mechanism involves forming a contractile ring of actin and myosin filaments beneath the cell membrane. Contraction of this ring creates a cleavage furrow that deepens until the cell pinches in two. Plant cells, conversely, form a cell plate in the center that expands outward until it fuses with the existing cell wall.
This first cytoplasmic division produces two haploid daughter cells. Although haploid (containing one set of chromosomes), each chromosome still consists of two sister chromatids joined at the centromere. In some organisms, the division of the cytoplasm may be partial or even skipped entirely after Meiosis I, leading to a temporary binucleated cell.
Cytokinesis Following Meiosis II
The second round of cytokinesis occurs after Meiosis II, the equational division, which resembles mitosis because it separates sister chromatids without further reducing the chromosome number. This division follows Telophase II, where the separated sister chromatids arrive at the poles of the two cells produced from Meiosis I. Cytokinesis II occurs either simultaneously or sequentially in both cells.
The mechanics of the second cytoplasmic division are the same as the first (cleavage furrow in animal cells or cell plate in plant cells). This separation finalizes the meiotic process, resulting in a total of four genetically unique and truly haploid daughter cells, each containing a single set of non-duplicated chromosomes.
In males, spermatogenesis typically results in four functional sperm cells of roughly equal size. However, in females during oogenesis, the cytoplasmic division is often grossly unequal. This asymmetric cytokinesis ensures that one daughter cell, the ovum or egg, receives almost all the cytoplasm and nutrients necessary to nourish the developing embryo following fertilization. The other three cells, known as polar bodies, receive little cytoplasm and generally degenerate.
Key Differences from Mitosis
The primary contrast between cytokinesis in meiosis and mitosis lies in the frequency of the event. Cytokinesis occurs only once in mitosis, following a single nuclear division to produce two daughter cells. In contrast, meiosis involves two sequential nuclear divisions (Meiosis I and Meiosis II), and is typically followed by two rounds of cytokinesis.
This difference in frequency leads to a distinct outcome in the final cell count: mitosis results in two cells, whereas meiosis yields four cells. The context of the division is also distinct: mitosis occurs in somatic cells for growth and repair, producing genetically identical, diploid cells. Meiotic cytokinesis occurs exclusively in germ cells for sexual reproduction, producing genetically diverse, haploid cells.
While the physical mechanism of cell pinching (cleavage furrow or cell plate formation) is the same in both processes, the timing and outcome differ. In mitosis, cytokinesis is often concurrent with Telophase, leading directly to two new cells. In meiosis, the second round of cytokinesis is crucial for the final reduction to the haploid state.