  The eukaryotic cell nucleus harbors the DNA genome that is organized in a
dynamic chromatin network and embedded in a viscous crowded fluid. This
environment directly affects enzymatic reactions and target search processes
that access the DNA sequence information. However, its physical properties as a
reaction medium are poorly understood. Here, we exploit mobility measurements
of differently sized inert green fluorescent tracer proteins to characterize
the viscoelastic properties of the nuclear interior of a living human cell. We
find that it resembles a viscous fluid on small and large scales, but appears
viscoelastic on intermediate scales that change with protein size. Our results
are consistent with simulations of diffusion through polymers and suggest that
chromatin forms a random obstacle network rather than a self-similar structure
with fixed fractal dimension. By calculating how long molecules remember their
previous position in dependence on their size, we evaluate how the nuclear
environment affects search processes of chromatin targets.
