Four-dimensional imaging of chromatin dynamics during the assembly of the interphase nucleus.

Large-scale chromatin organization is likely to play an important role in epigenetic control of gene expression. This implies that after mitosis the correct chromatin organization must be re-established in the nuclei of the two daughter cells. Here we analyze the dynamic behavior of chromatin during the transition from late anaphase to G1 in dividing HeLa cells, which express green fluorescent protein-tagged histone H2B. Time-lapse confocal microscopy was used to image the movement and the decondensation of chromatin as cell division progresses. Typically, time series of over 100 three-dimensional images (4D images) were collected, spanning a time period of up to three hours. Special care was taken to avoid photodamage, since cell cycle progression is exquisitely sensitive to photochemical damage. Quantitative analysis of the 4D images revealed that during the anaphase to G1 transition the movement of chromatin domains relative to other chromatin is remarkably limited. Chromatin dynamics can best be described as a radial expansion of the cluster of chromosomes that is present in late anaphase. We find that decondensation occurs in two phases. First a rapid decondensation by about a factor of two, followed by a slower phase in which part of the chromatin does not decondense any further, whereas the remaining chromatin decondenses further about two fold.