Cell division is more than copying DNA and splitting one cell into two. A dividing cell must organize duplicated chromosomes, move them to opposite sides, divide its nucleus, and physically separate its cytoplasm. The cytoskeleton provides much of the machinery that makes those movements possible.
The cytoskeleton is a network of protein filaments that gives cells structure and enables controlled movement. During cell division, its components are extensively reorganized. Microtubules build the spindle that separates chromosomes, actin filaments help divide the cell during cytokinesis, and intermediate filaments help the cell and nucleus withstand and reorganize the mechanical changes associated with division.
The cytoskeleton has three major filament systems
The cytoskeleton is not a single structure. It consists mainly of three filament systems with different properties.
Microtubules are hollow protein tubes made primarily from tubulin. They can rapidly grow and shrink, making them well suited for building and remodeling the mitotic spindle. They also serve as tracks for motor proteins that move cellular components.
Actin filaments, also called microfilaments, are thinner and more flexible. Together with myosin motor proteins, they generate forces that change cell shape and constrict the cell during cytokinesis.
Intermediate filaments are strong, rope-like fibers that provide mechanical stability. Among their division-related roles is supporting the cell as its shape changes and helping reorganize the nuclear envelope through changes to the nuclear lamina, a network of intermediate-filament proteins lining the inner surface of the nucleus.
These systems work together, but they are not interchangeable. Microtubules are especially important for chromosome segregation, whereas actin and myosin are central to the physical splitting of the cell.
Microtubules build the mitotic spindle
The most important cytoskeletal structure in mitosis is the mitotic spindle. It is a dynamic arrangement of microtubules that captures duplicated chromosomes and separates them into two groups.
Before mitosis begins, the cell duplicates its centrosomes in animal cells. Centrosomes act as major microtubule-organizing centers. As mitosis starts, the centrosomes move apart and help establish the two opposite poles of the spindle.
The spindle contains several populations of microtubules. Kinetochore microtubules attach to protein structures called kinetochores on duplicated chromosomes. Interpolar microtubules extend toward the opposite half of the spindle and interact with one another. Astral microtubules extend outward toward the cell cortex, where they help position and orient the spindle.
This arrangement allows the spindle to do something remarkably precise: connect the chromosomes to opposite sides of the cell and generate forces that separate the chromosome copies.
The spindle makes accurate chromosome separation possible
After DNA has been copied, each chromosome consists of two sister chromatids joined together. During mitosis, the spindle must establish a correct attachment between these chromatids and the two spindle poles.
The kinetochore is crucial to this process. It is a multiprotein structure assembled on each chromosome’s centromere. Microtubules from opposite spindle poles interact with the kinetochores so that, ideally, each sister chromatid becomes connected to a different pole.
The cell has mechanisms that monitor these attachments. Improper attachment can delay progression through mitosis, giving the cell an opportunity to correct the problem. Once the appropriate conditions are met, sister chromatids separate and are pulled toward opposite poles.
Microtubules do not simply act like rigid ropes pulling chromosomes apart. They are dynamic polymers that continually assemble and disassemble. Their changing length, together with forces generated by motor proteins and other spindle-associated proteins, helps organize and move chromosomes.
The cytoskeleton also positions the division machinery
Separating chromosomes correctly is only part of division. The cell must also determine where the physical division will occur.
Astral microtubules extend from the spindle toward the cell cortex. Their interactions with proteins at the cortex and with motor proteins help orient the spindle. This influences the location and orientation of the future division plane.
This positioning is important because the cleavage furrow—the indentation that eventually separates the daughter cells—must form in an appropriate location. In many animal cells, signals associated with the central spindle and astral microtubules help specify where the contractile machinery should assemble.
Thus, the spindle has two closely related jobs: it separates the genetic material and helps coordinate that separation with the eventual division of the cell body.
Actin and myosin pinch the cell in two
Once chromosomes have been separated, an animal cell still has to divide its cytoplasm. This process is called cytokinesis.
Near the end of mitosis, actin filaments and the motor protein myosin II assemble beneath the cell membrane at the division site. Together they form a contractile ring.
Myosin interacts with actin and generates contractile force. As the ring tightens, it pulls the cell membrane inward, creating a cleavage furrow. The furrow deepens until the two sides of the cell are connected only by a narrow bridge. Further remodeling eventually produces two separate daughter cells.
The contractile ring is therefore a cytoskeletal machine that converts molecular activity into a large-scale change in cell shape.
Cytokinesis differs in plant cells
Plant cells cannot divide by simply pinching inward because their rigid cell walls prevent the kind of membrane constriction seen in animal cells.
Instead, plant cells build a new partition between the daughter cells. Microtubules form a structure called the phragmoplast in the region between the daughter nuclei. Vesicles are directed into this region, where they fuse to form a developing cell plate.
The cell plate expands outward until it connects with the existing cell wall. It eventually develops into the new cell wall separating the daughter cells.
The cytoskeleton therefore supports cytokinesis in both animals and plants, but it does so through different physical strategies: animal cells constrict, while plant cells construct a new partition.
Intermediate filaments help the cell reorganize
Intermediate filaments are less directly involved in chromosome movement than microtubules, but they are important during the dramatic structural changes of division.
One example is the nuclear lamina, a network of intermediate-filament proteins called lamins that supports the inner surface of the nuclear envelope. At the beginning of mitosis, phosphorylation of lamins contributes to the disassembly of the nuclear lamina, allowing the nuclear envelope to break down in cells that use open mitosis.
Later, as mitosis ends, the lamina is reassembled around the separated chromosome sets as new nuclei form.
Other intermediate-filament networks also undergo changes as cells round up and alter their mechanical properties during division. These rearrangements help the cell tolerate and control the substantial changes in shape and organization that occur as one cell becomes two.
Cell division is a coordinated cytoskeletal process
The different cytoskeletal systems do not operate independently.
Microtubules organize the spindle, interact with chromosomes, and help determine the orientation and location of division. Actin and myosin generate the constriction that separates an animal cell. Intermediate filaments provide mechanical support and participate in reorganizing structures such as the nuclear lamina.
Their activities are coordinated with the cell cycle. As the cell enters mitosis, the cytoskeleton is rapidly remodeled rather than simply continuing its normal interphase arrangement. Spindle formation, chromosome attachment, chromosome segregation, spindle positioning, nuclear reorganization, and cytokinesis occur as linked stages of a larger mechanical program.
In this sense, the cytoskeleton is not merely the cell’s internal scaffolding. During cell division, it becomes a dynamic system of tracks, anchors, motors, and force-generating structures that physically organizes the process. Without that coordinated remodeling, a cell could copy its DNA but would not be able to reliably distribute the copied chromosomes and separate into two functioning daughter cells.
