At first glance, our cover image is simply colorful. But a closer look reveals something unusual. The model chromatin polymer is densely folded, but the colors—representing genomic position from one end of the chromosome to the other—remain largely separated instead of becoming thoroughly intermixed. This spatial segregation is one of the defining signatures of a fractal globule, a structural state that has become central to our understanding of chromatin organization during interphase.
Why should chromatin adopt such an organization? During interphase, chromosomes must fit inside the confined volume of the nucleus while remaining accessible for gene expression, DNA replication, and DNA repair. Experiments over the past two decades have shown that chromatin exhibits fractal packing, complex long-range connectivity, and anomalously slow (subdiffusive) motion of chromosomal loci. Although these properties have each been studied extensively, they have largely been treated as separate observations.
Our work asks a simple question: Could all of these features emerge from the same underlying physical principles?
To explore this possibility, we developed a minimal polymer model based on a self-avoiding random walk with attractive interactions and random crosslinks associated with lamin-A proteins. The interaction strength is first tuned so that the simulated polymer reproduces the experimentally observed fractal organization of chromatin parametrized by its fractal dimension, followed by tuning the crosslink concentration. Once these structural properties are established, the model naturally predicts additional features without further adjustment: the experimentally observed contact probability scaling behavior (observed in Hi-C experiments), subdiffusive dynamics of chromosomal loci in both ATP-depleted and untreated cells, and an estimate of the topological dimension of a chromatin-like connectivity network.
For us, the most satisfying aspect of this work is not that the model reproduces one experimental measurement, but that it connects several seemingly unrelated ones. The same physical framework links chromatin structure, network connectivity, and dynamics, suggesting that much of chromatin's apparent complexity may emerge from only a few generic principles of polymer physics.
The cover image of the August 4 issue of Biophysical Journal therefore represents more than an attractive polymer conformation. It illustrates the broader idea that biological complexity need not require equally complex underlying rules. Sometimes, a remarkably rich biological system can emerge from a surprisingly simple physical model.
(Technical details: The image was composed with Adobe Photoshop, by using images from one of the simulated structures (Fig. 2D) and royalty-free images. No AI assistance was used in the image-generation process.)
—Yam Ben Yaish, Sadhana Singh, and Rony Granek