Structural changes in chromatin during interphase

Abstract

The structural organization of the dense chromatin in G1, S and G2 of onion meristem cells has been evaluated. A naturally synchronous subpopulation of caffeineinduced binucleate cells was employed. — Fibre size is positively correlated with cell stage in interphase. Fibre size distribution is unimodal in G1 nuclei with the peak at 12.5 nm in diameter, while in G2 the distribution is bimodal due to a new population of thicker fibres (22.5 nm in diameter). Separation between fibre centres takes place between mid G1 and mid S. — Using stereological principles, the length of chromatin fibres integrated in the chromatin patches could be estimated. This length remains constant from mid G1 to mid S, when a 1.5 fold increase in total DNA takes place. On the other hand, 40 mm/hour of chromatin fibre is integrated in the chromatin patches between mid S and mid G2. Comparable data of the relative proportion of the different nuclear components have been obtained in each interphase period. — The reported changes provide evidence of a cyclic pattern of chromatin condensation, which may be the structural support for a model of chromatin function in these cycling cells.