The ultrastructural organization of prematurely condensed chromosomes

In an effort to understand the arrangement of the basic 30 nm chromatin fibre within metaphase chromosomes, changes in the organization of prematurely condensed chromosomes (PCC) were examined as a function of progression through the cell cycle. The structural features of PCC observed under the light microscope were compared with those obtained by scanning electron microscopy. PCC with varying levels of condensation were obtained by fusing mitotic HeLa cells with interphase cells synchronized at different times in the cell cycle. PCC from G1 cells are composed of rather tightly packed bundles of tortuous chromatin fibres. The density of fibre packing along the longitudinal axis of G1-phase PCC is lower and less uniform than that of metaphase chromosomes. Early G1 PCC exhibit gyres suggesting a despiralized chromonema. The condensed domains in G1 PCC appear to be organized as supercoiled loops; whereas fibre-sparse domains consist of longitudinal fibres running along the chromosome axis. As cells progressed towards S phase, a greater proportion of highly extended regions containing prominent longitudinal fibres became evident in the PCC. The pulverized appearance of S-phase PCC under the light microscope corresponded to the highly condensed, looping fibre domains separated by more extended segments containing longitudinal fibres that are visualized using the scanning electron microscope. Active sites of DNA synthesis are implicated to be localized within extended longitudinal fibres. Post-replicative chromosome maturation extends through the G2 period and appears to involve rearrangement of the extended longitudinal fibres into packed looping-fibre clusters, which then coalesce. These observations support the model for packing DNA into chromosomes proposed in 1980 by Mullinger & Johnson. Briefly, this model suggests that the chromonema of each metaphase chromatid contains regions composed of folded longitudinal chromatin fibres as well as looping fibres that emerge from the axis at distinct foci. The final level of chromatin packing in metaphase chromosomes is attained by spiralization of the chromonema.