The Loops of Life

by Michael Rubinstein (Duke University)

Tuesday 7 Jul 2026, 16:00 → 18:00 Europe/Rome

Aula Conversi (Dipartimento di Fisica-Ed.G.Marconi)

Soft matter science is a highly interdisciplinary field, spanning physics, chemistry, and biology,
that focuses on materials (e.g., polymers, gels, foams, colloids, liquid crystals) that are easily
deformed by thermal fluctuations or weak external stresses. One of its objectives is to explain
how biology works based on physics concepts. In this lecture, I will provide a physicist’s
explanation of how different cells in our body, while containing identical genetic information, are
forced to “read” different genes and perform completely different functions. The puzzle is that
DNA in the nucleus of every cell is an astronomically long linear polymer, and one of the basic
concepts of polymer physics is that long linear polymers in a dense state have fractal
dimensionality D=2 and in 3-dimensional space are heavily overlapping and highly entangled. I
will explain how the loop extrusion activity of proteins, called cohesins, forces sections of linear
DNA into topologically associated domains (TADs) with apparent fractal dimension D=4. This
physical model suggests that activity packs DNA (chromatin) in the cell nucleus into a non-
equilibrium steady-state of D=2 necklaces composed of D=4 beads. Four-dimensional objects do
not overlap in three-dimensional space. Various cells in our body differ in specific locations of
these four-dimensional beads (TADs) along the chromosomes and therefore read (transcribe) different genes. This physical model suggests new epigenetic ways of treating diseases.