Synopsis: Scenes from a cytoplasm

Time-lapse images of DNA molecules in a bacterial cell reveal the forces that can inhibit diffusion.
Synopsis figure
Illustration: Alan Stonebraker

The interiors of cells, such as bacteria, are busy with molecular motion. As expected, the motion of ions and small molecules is essentially Brownian, but larger molecules like proteins, DNA, or RNA, have been observed to move subdiffusively, that is, the mean square displacement of such a particle grows slower than linearly in time.

In a study published in Physical Review Letters, Stephanie Weber, Andrew Spakowitz, and Julie Theriot from Stanford University in the US are providing some of the first experimental clues of what causes subdiffusive motion within bacterial cells. With time-lapse fluorescence microscopy, they track the positions of protein-RNA complexes and of the DNA forming the bacterial chromosome. In different species and under various conditions they measure a common time dependence characterizing subdiffusive motion of specific points on the chromosome. By comparing their data to what would be expected if molecules underwent random motion in a viscoelastic environment, they suggest a general explanation for subdiffusive motion: Interactions with elastic elements tend to pull the particles back into the direction they came from.

Due to the physical, rather than biological, origin of subdiffusive motion, it is expected to be a universal property of intracellular transport. The Stanford group’s findings should thus also apply to animal cells and might have important consequences for the organization of DNA. – Karsten Kruse


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