Evidence for existence and compactness of DNA superstructure units in mammalian cells: A microdosimetric approach to radiation-induced DNA release assayed by neutral sucrose gradient sedimentation

Experimental results obtained with the neutral sucrose gradient sedimentation techniques of analysing mammalian DNA after irradiation in vivo (Chinese hamster cells V79) are evaluated theoretically in order to verify and extend a model by Ormerod and Lehmann that describes the gradual release of free DNA from the DNA-membrane complex. The model is based on the idea of chromatin organization in the form of membrane attached superstructure units (MASSUs) defined by consecutive attachment points in intervals ofM 0 (DNA molecular weight of a MASSU). DNA sedimentation after cell lysis with sarkosyl as detergent allows good separation of the released free DNA from that remaining in the complex. The dose-dependence of both the percentage of DNA released and pertinent molecular weight parameters as measured with X-rays and derived from the model confirms it and yields the MASSU sizeM 0 = 2.4 × 109 g/mol. Furthermore, the model is evaluated with respect to high-LET radiations by a mathematical theory based strictly on microdosimetry. This approach, which could be of general interest as a contribution to theoretical microdosimetry, reduces the measurable quantities explicitly to the single event distribution of lineal energy for the gross volume,V, occupied by a MASSU in the interphase nucleus. Experimental results obtained withα-particles agree with this extended version of the model, yielding 0.2 – 0.3 µm for the diameter ofV. In summary, the following conclusions are drawn: The entire chromatin, at least the greater part of it, must be organized in regular superstructure units with no detectable variance of their lengthsM 0, which is suggested to be a general principle whose functional implications remain to be elucidated.