HIGH-ORDER DNA-STRUCTURE AS INFERRED BY OPTICAL FLUOROMETRY AND SCANNING CALORIMETRY

New quantitative insights on the native high order chromatin DNA struc ture existing within interphase nuclei are obtained by monitoring the effects of two common well-characterized fixatives, glutaraldehyde and ethanol/acetic acid mixture, at the level of the intranuclear DNA dis tribution and structures. Reproducible distinct levels of DNA fluoresc ence intensity and their intranuclear distribution are apparent in unf ixed and fixed thymocytes by using DAPI and quantitative optical micro scopy based on a charge coupled device. The fluorescent histograms cor related with the calorimetric thermograms on the very same thymocytes fixed and unfixed, establish an unequivocal baseline for the different levels of structural organization of the chromatin within the intact nucleus; namely their number, DNA packing ratio and fiber diameter. A systematic comparison among all the numerous models, being so far prop osed for the quinternary and quaternary levels of DNA folding, to iden tifies the rope or ribbon-like and the chromonema as the ones that bes t fit with the in situ distribution.