The team assembled measurements from twenty-five eukaryotic species spanning yeasts, animals and plants, drawing on published images and new morphometric analyses of spindle length, spindle width and the dimensions of the metaphase plate, the equatorial plane where chromosomes line up before segregation. When they plotted metaphase plate width against genome size on logarithmic axes, the data collapsed onto a straight line, revealing a power law with an exponent of approximately one third. The correlation was remarkably tight, with an R-squared value of 0.94. In practical terms, this means that when the amount of DNA in a genome increases a thousand-fold, the width of the structure that accommodates it grows only about ten-fold. Sublinear scaling of this kind is a hallmark of geometric constraints, and it hinted at an underlying physical mechanism rather than a purely biochemical one.

The clue to that mechanism came from the geometry of the chromosomes themselves. Previous work had shown that condensed mitotic chromosomes maintain a roughly uniform chromatin density of around ninety megabase pairs per cubic micrometre across species. The new analysis extended this observation to the level of the entire metaphase plate: plate volume scaled almost linearly with genome size, with an exponent of 0.96, and the effective density of the plate was roughly constant across organisms. The plate also maintained a conserved shape, with a width-to-thickness aspect ratio of about 1.9. Crucially, chromosome number played almost no role in the scaling. Species with similar chromosome counts but different genome sizes had very different plate widths, while species with similar genome sizes but chromosome numbers ranging from six to forty-six had nearly identical plates. It is the total volume of chromatin, not how it is parcelled into individual chromosomes, that dictates spindle geometry.

To explain the one-third exponent, the researchers built a physical model of the spindle in which each chromosome is represented as a compressible elastic sphere, attached to microtubule bundles that extend between the two spindle poles. The microtubules, through their bending stiffness, push the chromosomes inward toward the spindle axis, while the chromosomes, packed tightly together in the metaphase plate, push back against one another through steric repulsion. The equilibrium between microtubule bending forces and interchromosome pushing forces determines the positions of the chromosomes and hence the width of the plate and the spindle. Because chromosome volume is proportional to genome size, the characteristic length of a chromosome grows with the cube root of the genome, and the model naturally predicts a power-law exponent close to the observed 0.33. When the model was calibrated on human spindle parameters and extrapolated across four orders of magnitude of genome size, it predicted an exponent of 0.37, in close agreement with the comparative data.

The model made a second, counterintuitive prediction: if the total volume of chromatin is held constant while the number of chromosomes is varied, the metaphase plate width should remain roughly constant. This was confirmed by comparing species with similar genome sizes but very different chromosome numbers. Conversely, when chromosome number increases at constant individual chromosome volume, meaning more total chromatin, the model predicted a power-law widening with an exponent of 0.43, and experiments matched this behaviour. The estimated interchromosome pushing forces were in the range of hundreds of piconewtons, comparable to the pulling forces that molecular motors and microtubule depolymerization exert at kinetochores, the protein structures that link chromosomes to spindle fibres.

Experimental tests followed four complementary strategies. First, the team generated polyploid human cells, including hypotetraploid and hypooctaploid derivatives of the non-transformed RPE1 line and the tumour-derived HCT116 line, using carefully designed protocols that prevented centriole overduplication so that cells divided with bipolar spindles. Metaphase plate width increased robustly with ploidy in every case, and cell-by-cell analysis using nuclear area as a proxy for ploidy yielded a power-law exponent of 0.29, matching the cross-species value. Second, the researchers exploited natural ploidy variation in patient-derived colorectal cancer organoids and found the same scaling relationship. Third, by acutely reactivating the motor protein CENP-E in cells whose chromosomes had been stranded at the spindle poles, they watched in real time as chromosomes entered the plate, and both plate width and chromatin density rose in step with the increasing chromosome number. Fourth, forcing cells into mitosis with an unreplicated genome, so that most chromatin remained uncondensed in the cytoplasm, produced spindles that were thirty-one percent thinner without any change in cell size.

Perhaps the most dramatic test involved physically squeezing the spindles. The team compressed metaphase cells with a soft agarose gel, reducing spindle height by roughly forty-four percent within a minute. If chromosomes truly push against one another, compressing the cell should squeeze the plate sideways, and that is exactly what happened: the metaphase plate widened by about thirty percent and thickened by about fifty percent, while spindle length stayed unchanged. The angle between the outermost microtubule bundles at the poles increased, consistent with the model’s prediction that microtubules pivot around freely jointed pole attachments. When the compression was released, the spindle partially recovered its original shape. Spindles with unreplicated genomes widened less under compression, as the model predicted for softer, smaller chromatin masses. Compressed cells took longer to complete metaphase, suggesting that excessive interchromosome pushing forces impede the machinery of division.

The study also clarified that spindle length and spindle width are governed by independent mechanisms. Acute depolymerization of microtubules with nocodazole collapsed spindle length by sixty-five percent within minutes but left spindle width and plate width untouched, showing that width is maintained by the mechanical properties of chromatin rather than by microtubule-generated forces. Osmotic shocks, which alter cytoplasmic density, compressed or relaxed the plate and spindle in ways that decoupled length from width. Perturbing dozens of microtubule-associated proteins changed spindle length by up to one hundred percent while shifting width by only about twenty percent. In meiosis, the picture held as well: metaphase I plates, carrying twice the chromatin of metaphase II, were twenty-three percent wider in human oocytes and thirty-three percent wider in mouse oocytes, with spindle length differing by no more than seven percent.

The implications reach well beyond basic cell biology. The authors propose that chromosome crowding explains why animal cells round up before dividing, creating space for the spindle, and why eukaryotes with larger genomes evolved open mitosis, in which the nuclear envelope disassembles. It may also explain why polyploid cells, common in the human liver and frequent in tumours, can divide at all: the spindle simply widens to accommodate the extra chromatin, and adapted polyploid cells enlarge their spindles further with a distinct transcriptional programme. In plants, whose spindles have unfocused poles and experience weaker compression forces, the mechanism may have facilitated the repeated whole-genome duplications that drive speciation. A single physical principle, chromosomes fighting for space in the metaphase plate, appears to underpin how life divides genomes large and small.

Subject of Research: Power-law scaling of mitotic spindle dimensions with genome size across eukaryotes, driven by interchromosome pushing forces from chromosome crowding at the metaphase plate

Article Title: Power-law scaling of mitotic spindles with genome sizes across eukaryotes is driven by chromosome crowding

Article References: Gudlin, L., Vukušić, K., Novak, M., Trupinić, M., Ljulj, M., Dundović, I., Petelinec, A., Petrušić, L., Hertel, A., van Ravesteyn, T., Trakala, M., Kops, G. J. P. L., Storchová, Z., Tambača, J., Pavin, N., & Tolić, I. M. (2026). Power-law scaling of mitotic spindles with genome sizes across eukaryotes is driven by chromosome crowding. Nature Cell Biology. https://doi.org/10.1038/s41556-026-02005-8

Image Credits: AI Generated

DOI: 10.1038/s41556-026-02005-8

Keywords: mitotic spindle, chromosome crowding, genome size, power-law scaling, metaphase plate, microtubules, polyploidy, cell division, biophysics, eukaryotes, meiosis, chromatin density