A fractal model for nuclear organization: current evidence and biological implications

Abstract

Chromatin is a multiscale structure on which transcription, replication, recombination and repair of the genome occur. To fully understand any of these processes at the molecular level under physiological conditions, a clear picture of the polymorphic and dynamic organization of chromatin in the eukaryotic nucleus is required. Recent studies indicate that a fractal model of chromatin architecture is consistent with both the reaction-diffusion properties of chromatin interacting proteins and with structural data on chromatin interminglement. In this study, we provide a critical overview of the experimental evidence that support a fractal organization of chromatin. On this basis, we discuss the functional implications of a fractal chromatin model for biological processes and propose future experiments to probe chromatin organization further that should allow to strongly support or invalidate the fractal hypothesis.

Figure 1.

Examples of 2D and 3D fractals. (a) The romanesco broccoli is one of the most popular natural fractal architecture. (b) The left panel shows the first, second, third and fifth iterations for the recursive construction of a 2D Hilbert fractal. The right panel is a 3D Hilbert curve, which fractal dimension is equal to f = 3. These two examples constitute deterministic fractals. (c) The picture represents a 3D crumpled globule polymer conformation (with permission from the AAAS (11)), which is a maximally compact, knot-free and fractal architecture. (d) Dark pixels form a 2D percolation cluster, which is obtained by clustering randomly distributed elements using nearest-neighbor connections. The fractal dimension of a percolation cluster is f = 2.5.