Progressive chromosome shape changes quantified during cell divisions

Mitotic chromosomes give genome portions the required compaction and mechanical stability for faithful inheritance during cell divisions. Here, we record human chromosome dimensions from their appearance in prophase over successive times in a mitotic arrest. Chromosomes first appear long and uniformly thin. Then, individual chromosome arms become discernible, which continuously shorten and thicken - the longer a chromosome arm, the thicker it becomes. The observed chromosome arm length to width relationship can be described by a power law with progressively increasing exponent. In the search for a molecular explanation of this behavior, the popularloop extrusionmodel provides no obvious means by which longer arms become thicker. Instead, we find that simulations of an alternativeloop capturemodel recapitulate key features of our observations, including the gradually developing arm length to width relationship. Our analyses portray chromosomes as out-of-equilibrium structures in the process of transitioning towards, but on biologically relevant time scales not typically reaching, steady state.