The cell cycle is the process by which cells grow and divide, culminating in the division of the parent cell into two daughter cells. This complex process is tightly regulated and involves distinct phases. The final stages involve two coordinated events: mitosis, the division of the genetic material, and cytokinesis, the physical division of the cell body. Understanding what happens when these two steps become uncoupled provides insight into both normal biological development and disease states.
The Essential Roles of Mitosis and Cytokinesis
Mitosis is responsible for the accurate separation of the cell’s genetic material, organized into chromosomes. Its purpose is to ensure that each resulting nucleus receives an identical, complete set of DNA. This is achieved through a coordinated series of phases that condense the chromosomes, align them at the cell’s center, and pull the two identical halves to opposite poles. The successful completion of mitosis creates two distinct nuclei, each containing the full complement of the genome.
Cytokinesis is a separate, typically concurrent process that follows mitosis. This stage involves the physical partitioning of the cytoplasm, organelles, and cell membrane into two individual daughter cells. In animal cells, a contractile ring of actin and myosin filaments cinches the cell membrane inward, forming a cleavage furrow that pinches the cell in two. In plant cells, a cell plate forms in the middle to create a new cell wall separating the nuclei. Cytokinesis ensures that each new cell is a separate, functional entity with its own nucleus and sufficient cytoplasmic machinery.
The Structural Result: Formation of Multinucleated Cells
When a cell undergoes mitosis but fails to complete cytokinesis, the result is a single, enlarged cell containing two complete nuclei. If this uncoupled process repeats, the cell accumulates multiple nuclei, a condition referred to as a multinucleated cell. This type of cell is significantly larger than a normal cell and may be termed a syncytium, containing many nuclei within a single cytoplasm.
The failure of the cytoplasm to divide also affects the cell’s genomic state. A normal somatic cell is diploid (\(2n\)), meaning it has two sets of chromosomes. Following mitosis without cytokinesis, the single cell contains two diploid nuclei, resulting in a tetraploid (\(4n\)) genome content. This condition of having an increased number of complete sets of chromosomes is called polyploidy. The resulting binucleated or multinucleated cell possesses an increased volume of both nuclear and cytoplasmic material, altering the normal nuclear-to-cytoplasmic ratio.
Functional Consequences and Biological Relevance
The uncoupling of mitosis and cytokinesis is not always a malfunction; in certain tissues, it is a normal, advantageous part of development. A prominent example is the formation of skeletal muscle fibers (myofibers). These long, large cells require many nuclei to manage the massive volume of cytoplasm and protein synthesis necessary for muscle function. Myofibers form when precursor cells fuse or when they undergo multiple rounds of nuclear division without subsequent cytoplasmic division. Certain plant tissues and the early embryonic development of some organisms also utilize this strategy to rapidly increase nuclear number before cellularization.
In most other cell types, the failure of cytokinesis represents a severe error with significant pathological outcomes. The creation of a tetraploid cell through failed cytokinesis is a precursor to genomic instability. These cells often possess extra centrosomes, which organize the mitotic spindle. During a subsequent division attempt, the extra centrosomes can lead to the formation of multipolar spindles, pulling chromosomes to three or more poles instead of two.
This abnormal division causes chromosomes to be distributed unevenly among the resulting daughter cells. This condition, known as aneuploidy, means the cells have an incorrect number of individual chromosomes, often leading to missing or extra copies. Aneuploidy is a hallmark of many diseases, and failed cytokinesis is recognized as a mechanism contributing to the development of various malignancies. Cells with this genomic instability frequently undergo programmed cell death or senescence, but those that survive and continue to divide can contribute to the growth and progression of tumors.