Within every dividing cell, a microscopic machine called the mitotic spindle performs a fundamental task for life: accurately separating duplicated chromosomes. This process ensures that when one cell divides, each new daughter cell receives a complete and correct set of genetic instructions. The spindle acts as a sorting system, organizing and pulling chromosomes to opposite ends of the cell. The proper function of every subsequent cell depends on the flawless execution of this process.
The Architecture of the Spindle
The mitotic spindle’s primary framework is built from microtubules, which are long, hollow filaments made of a protein called tubulin. These microtubules form the spindle fibers and are organized into three main classes based on their connections. Kinetochore microtubules attach directly to chromosomes, interpolar microtubules extend from opposite poles and overlap in the middle, and astral microtubules radiate outwards from the poles toward the cell’s edge.
The organizing hubs for these microtubules in most animal cells are two structures called centrosomes. Each centrosome migrates to an opposite end of the cell, establishing the two poles of the spindle. The structure’s assembly and function are driven by motor proteins.
Proteins such as kinesins and dyneins travel along the microtubule tracks, using ATP for energy to generate force and movement. Some kinesins work at the spindle’s midzone, pushing the poles apart by sliding overlapping interpolar microtubules against each other. Other motor proteins are responsible for pulling the chromosomes along the kinetochore microtubules. The coordinated action of these elements gives the spindle its ability to segregate chromosomes.
Assembling the Spindle Apparatus
The formation of the mitotic spindle begins in prophase, an early phase of mitosis. During this stage, the two centrosomes, duplicated earlier in the cell cycle, start to move toward opposite sides of the nucleus. As they separate, they act as nucleation sites, initiating the growth of microtubule arrays that will form the spindle fibers. This self-assembly process depends on forces generated by motor proteins acting on the new microtubules.
The process advances during prometaphase, which starts with the breakdown of the nuclear envelope. This allows the spindle microtubules to access the condensed chromosomes. The microtubules then begin a “search and capture” process, where their ends grow and shrink until they contact protein structures on the chromosomes called kinetochores.
Once a microtubule from a spindle pole attaches to a kinetochore, it becomes stabilized. The goal is for the kinetochores of sister chromatids—the two identical copies of a duplicated chromosome—to attach to microtubules from opposite poles. This bipolar attachment is necessary for proper chromosome segregation, and the chromosomes are moved by microtubules until they are positioned for the next stage.
Executing Chromosome Separation
Once the spindle is fully assembled and all chromosomes are captured, the cell enters metaphase. During this stage, the spindle’s forces bring all duplicated chromosomes into alignment at the cell’s equator. This arrangement forms the metaphase plate, an imaginary plane equidistant from the two spindle poles. Each chromosome is held under tension, with sister chromatids pulled toward opposite poles in a balanced state.
This alignment sets the stage for anaphase, the main event of mitosis. The transition is marked by the separation of the sister chromatids. Cohesin proteins, which held the sister chromatids together, are broken down.
Now considered individual chromosomes, they are drawn toward the opposite spindle poles. This movement is accomplished by two coordinated mechanisms: anaphase A and anaphase B. In anaphase A, the kinetochore microtubules shorten, pulling the chromosomes toward the poles. Simultaneously, in anaphase B, the interpolar microtubules slide against each other, pushing the spindle poles further apart and elongating the cell.
The Spindle Assembly Checkpoint
To ensure the fidelity of chromosome segregation, cells use a quality control system called the Spindle Assembly Checkpoint (SAC). This mechanism monitors the attachment of microtubules to the kinetochores of every chromosome. Its primary function is to delay the cell’s progression into anaphase until each chromosome is properly attached to the spindle from both sides, a state called biorientation. This prevents the premature separation of sister chromatids.
The SAC operates as a signaling pathway activated by any kinetochore that is unattached or improperly attached. These kinetochores catalyze the formation of the Mitotic Checkpoint Complex (MCC), an assembly of proteins including MAD2 and BUB3. The MCC then inhibits a protein complex called the Anaphase-Promoting Complex (APC/C). By inhibiting the APC/C, the checkpoint prevents the degradation of proteins holding sister chromatids together, pausing the cell cycle in metaphase.
Once all chromosomes achieve stable, bipolar attachment, the tension at the kinetochores silences the SAC signal. The inhibitory MCC is no longer produced, and the APC/C becomes active. This activation triggers the cleavage of cohesin, initiating the separation of sister chromatids and the onset of anaphase. This safety mechanism ensures that each daughter cell will receive a complete and accurate set of chromosomes.
Consequences of Spindle Errors
Failures in the mitotic spindle’s function or the Spindle Assembly Checkpoint can have severe consequences for the cell and the organism. When chromosome segregation goes awry, it results in a condition called aneuploidy, where cells have an incorrect number of chromosomes. This can occur if a chromosome fails to attach to the spindle correctly or if the checkpoint fails to detect the error, allowing the cell to divide prematurely. The resulting daughter cells may end up with too few or too many chromosomes.
Aneuploidy is a hallmark of many human cancers. The mis-segregation of chromosomes leads to genomic instability, which can drive tumor development by causing the loss of tumor-suppressing genes or the amplification of cancer-promoting genes. Cells with defective spindle checkpoints are more prone to these errors and the subsequent genetic changes that contribute to malignancy.
Beyond cancer, errors in chromosome segregation during the formation of sperm and egg cells can lead to developmental disorders. For example, Down syndrome is caused by trisomy 21, where individuals have three copies of chromosome 21 instead of two. Similar errors in early embryonic mitotic divisions can also lead to developmental issues and are a cause of miscarriages.