  The organization of interphase chromosomes in a number of species is starting
to emerge thanks to advances in a variety of experimental techniques. However,
much less is known about the dynamics, especially in the functional states of
chromatin. Some experiments have shown that the motility of individual loci in
human interphase chromosome decreases during transcription, and increases upon
inhibiting transcription. This is a counter-intuitive finding because it is
thought that the active mechanical force ($F$) on the order of ten
pico-newtons, generated by RNA polymerase II (RNAPII) that is presumably
transmitted to the gene-rich region of the chromatin, would render it more
open, thus enhancing the mobility. We developed a minimal active copolymer
model for interphase chromosomes to investigate how $F$ affects the dynamical
properties of chromatin. The movements of the loci in the gene-rich region are
suppressed in an intermediate range of $F$, and are enhanced at small $F$
values, which has also been observed in experiments. In the intermediate $F$,
the bond length between consecutive loci increases, becoming commensurate with
the distance at the minimum of the attractive interaction between non-bonded
loci. This results in a transient disorder-to-order transition, leading to a
decreased mobility during transcription. Strikingly, the $F$-dependent change
in the locus dynamics preserves the organization of the chromosome at $F=0$.
Transient ordering of the loci, which is not found in the polymers with random
epigenetic profiles, in the gene-rich region might be a plausible mechanism for
nucleating a dynamic network involving transcription factors, RNAPII, and
chromatin.
