How Does a Cell Divide Evenly in Two?
If we find the cell “making sense”
it is because there is sense
woven into its very fabric.
A Cytokinetic Ratchet
by John D. Wise, PhD
In practice, if not in theory, secular culture treats Science as its sacred text. Yet once again this week, a set of “science textbooks” must yield to new observation. Reality, stubborn as ever, must correct our diagrams.
The ideal of Science, capitalized and untouchable, does not always match the provisional character of laboratory science. That is not a weakness but a strength. When done well, and within proper bounds, science (small ‘s’) earns our respect. In this case, careful researchers have uncovered a previously unknown mechanical feature of cell division, forcing a revision of long-held assumptions.
A mechanical ratchet drives unilateral cytokinesis (Kickuth et al., Nature, 28 Feb 2026). The discovery, reported in the world’s leading science journal and summarized the same day by Science Daily, (“Textbooks challenged by new discovery about how cells divide”), concerns cytokinesis, the physical splitting of one cell into two. For decades, biology textbooks have described this process using what is often called the “contractile ring” model.
Scientists have uncovered a surprising new way that giant embryonic cells divide—without relying on the classic “purse-string” ring long thought essential for splitting a cell in two. Studying zebrafish embryos, researchers found that instead of forming a fully closed contractile ring, cells use a clever “mechanical ratchet” system.
The Classical Contractile Ring
After chromosomes separate during mitosis, the mitotic spindle (a structure made of microtubules), determines the “equator” where the cell will divide. Along that equatorial midpoint, just beneath the cell membrane, the cell assembles a dense belt of actin filaments within the cell cortex, a thin but mechanically important scaffold underlying the cell’s membrane that gives the cell its shape and stiffness.
Embedded in that actin belt are myosin motor proteins. Myosin molecules pull on the actin filaments that circle the cell like a drawstring. Because the filaments are arranged in mixed orientations around the circumference, the myosin motors slide them past one another, tightening the belt. The effect resembles the drawstring of a pouch. As the belt constricts, it pulls the cortex inward. Because the cortex is mechanically linked to the membrane, the membrane follows. A cleavage furrow forms and deepens until only a thin bridge remains, which is then severed, effectively creating two distinct cellular membranes – two new cells from one.
The actin drawstring is not a static rope being pulled tight, but rather an active fibrous network, constantly assembling and disassembling as it contracts. Nor is the interior of the cell an empty fluid. The cytoplasm is a dense, viscoelastic material. Its stiffness, internal pressure, and mechanical properties all matter. In small cells, this smooth drawstring mechanism works beautifully.
When Scale Changes the Physics
But as in macro-engineering, a larger scale changes the physics of the process.
This new research focused on very large embryonic cells. In such cells, cytokinesis by continuous circumferential tightening becomes mechanically unstable. As cell size increases, cytoplasmic viscosity, internal stresses, and cortical tension behave differently. Trying to pinch a very large sphere evenly in the middle is not as simple as tightening a small pouch.
Think of the difference between pinching a soft rubber foam ball into two equal halves and trying the same thing with a water balloon. The foam ball holds its shape. When you squeeze it in the middle, the material compresses locally and resists redistribution. The deformation stays where you apply force.
A water balloon behaves differently. The liquid inside shifts. Pressure redistributes instantly. Small asymmetries amplify. Instead of forming two neat halves, the structure bulges unpredictably unless the force is applied in a carefully controlled way.
That contrast captures the mechanical issue nicely. Small cells behave more like the foam ball. Large embryonic cells, filled with massive volumes of viscous and elastic cytoplasm, behave more like the water balloon. Continuous constriction risks instability.
A Mechanical Ratchet at Work
What these researchers observed in the cytokinesis of larger cells is a ratcheting mechanism. The researchers
… discovered that the cytoplasm becomes stiffer during interphase, creating a supportive scaffold that stabilizes the actin band. During M-phase, however, the cytoplasm becomes more fluid, allowing the band to move inward between the two emerging cells. These shifts between stiffness and fluidity play a central role in enabling division.
Instead of smooth, uninterrupted constriction, the cortical actin network alternates between softer and stiffer states. During softer phases, the contractile band advances inward. During stiffer phases, that advance is stabilized. Advance and stabilize. Advance and stabilize. Step by step, the cell divides without catastrophic instability.
The telos is the same: two independent membrane-bound cells from one. Change the scale, and the mechanics must adapt.
What stands out in this report is not controversy but wonder. One of the most fundamental processes of life, repeated countless times in every organism, still reveals new layers of complexity when examined at sufficient resolution and an openness to the data.
By What Light Do We Rewrite Our Textbooks?
Interestingly, neither the Science Daily summary, nor the research paper, mentioned evolution even once. In this case, the advance came from close mechanical study of how cells actually divide. One might reasonably ask what the evolutionary narrative adds to discovery at all.
Dobzhansky famously wrote in 1973, “Nothing in Biology Makes Sense Except in the Light of Evolution.” Yet here, clarity emerged from the stubborn reality of the laboratory, not the narrative of the museum. The “light” illuminating this discovery was the sheer, breathtaking complexity of a system that “steps and holds” with mathematical precision. If we find the cell “making sense” it is because there is sense woven into its very fabric. It is worth asking what (or Who) is truly doing the illuminating?
If then the light in you is darkness, how great is the darkness! (Matthew 6:23).
John Wise received his PhD in philosophy from the University of CA, Irvine in 2004. His dissertation was titled Sartre’s Phenomenological Ontology and the German Idealist Tradition. His area of specialization is 19th to early 20th century continental philosophy.
He tells the story of his 25-year odyssey from atheism to Christianity in the book, Through the Looking Glass: The Imploding of an Atheist Professor’s Worldview (available on Amazon). Since his return to Christ, his research interests include developing a Christian (YEC) philosophy of science and the integration of all human knowledge with God’s word.
He has taught philosophy for the University of CA, Irvine, East Stroudsburg University of PA, Grand Canyon University, American Intercontinental University, and Ashford University. He currently teaches online for the University of Arizona, Global Campus, and is a member of the Heterodox Academy. He and his wife Jenny are known online as The Christian Atheist with a podcast of that name, in addition to a YouTube channel: John and Jenny Wise.