  Chromatin moves dynamically inside the cell nucleus, and its motion is often
correlated with gene functions such as DNA recombination and transcription. A
recent study has shown that during early embryogenesis of the nematode,
Caenorhabiditis elegans, the chromatin motion markedly decreases. However, the
underlying mechanism for this transition has yet to be elucidated. We
systematically investigated the impact of nuclear size to demonstrate that it
is indeed a decisive factor in chromatin mobility. To this end, we established
a method to quantify chromatin motion inside the nucleus, while excluding the
contribution of the movement of the nucleus itself, which allowed us to extract
the intrinsic mean-squared displacement (iMSD) of individual chromosomal loci
in moving nuclei from the correlated motion of two loci. We show that a simple
theoretical description, which takes into account the topological constraints
of chromatin polymers, can quantitatively describe the relationship between the
nucleus size and the chromatin motion in vivo. Our results emphasize a
regulatory role of nuclear size in restricting chromatin motion, and a generic
polymer physics model plays a guiding role in capturing this essential feature.
