Measurement of Low-Frequency DNA Breaks Using Nucleoid Flow Cytometry

The organization of eukaryotic DNA is increasingly well understood at each extreme of chromatin organization. Advances in understanding the organization of DNA at the level of nucleosome and chromosome have outstripped knowledge of the intermediate states and function of DNA. Most would, however, agree on the following general hierarchy (1 ,2 ). The base-repeating unit of approx 146 bp, including a core histone group, is further organized into the 30-nm fiber and constrained by contact with the nuclear matrix to form loop and superloop structures capable of maintaining physical tension or supercoiling (3 –5 ). These supercoiled units of DNA subsequently adopt repetitive patterns based around a protein core template, which together are observed as chromosomes.