At Rice University, physicist Peter Wolynes has solved a puzzle that cells navigate millions of times a day: how to reshape their DNA from a messy ball into a neat cylinder during the critical moments before dividing. The answer lies in an elegant two-motor system that acts almost like a cat's claws working in tandem on a ball of yarn, one motor pulling stubbornly at the same spot while the other darts around finding new places to tug.
When a cell prepares to copy itself through mitosis, its DNA must undergo a profound transformation. Chromosomes spend most of the cell's life in a ball shape—symmetrical in every direction, studded with little loops of DNA sticking out like yarn caught on claws. But when the moment comes to divide, these chromosomes must become cylinders, uniform structures that can be transported cleanly to daughter cells. This shape shift is essential, yet how cells achieve it has remained mysterious.
Wolynes and his team, including postdoctoral fellow Zhiyu Cao, modeled how two different types of motor proteins could drive this transformation. One motor is "processive"—it binds to a loop of DNA and pulls continuously, staying put for extended periods. The other is "nonprocessive," pulling briefly before releasing and reattaching elsewhere. When the researchers ran simulations with this specific combination, the results matched real cellular data with striking accuracy.
"Our model shows that the processivity of the motor helps break the symmetry," Cao explained. "The symmetry breaking occurring locally is what drives the deformation of the ball into the observed cylinder globally." The processive motor's relentless pulling at fixed points creates an asymmetry that ripples outward, reshaping the entire structure from sphere to cylinder.
The team published their findings in the Proceedings of the National Academy of Sciences, building on earlier theoretical work published in Nature Communications. What makes this latest model particularly elegant is its discovery of why chromosomes adopt a "smectic liquid crystalline structure"—the same flexible, layered organization seen in soapy films on hands. Unlike a hard crystal, this structure retains flexibility and mobility, crucial properties for moving chromosomes during cell division.
The model even explains a previously mysterious pattern in chromosomes called a "chromosomal jet," a structural feature that occurs when processive motors accumulate near boundaries between active and inactive DNA regions. This unification of theory with observed phenomena suggests the researchers have identified a fundamental mechanism.
Wolynes notes that the study raises a profound question: why do eukaryotic chromosomes like ours undergo these symmetry-breaking transformations while bacterial chromosomes apparently do not. The concept of broken symmetry extends far beyond cell biology—it echoes through physics and even cosmology—yet finding its mechanisms in chromosomal dynamics may unlock deeper understanding of how cells reproduce themselves faithfully. Future experiments should reveal whether other aspects of the cell's machinery also rely on this elegant interplay between two motors pulling in fundamentally different ways.