[Structural analysis of the chromatin loop in human chromosome by immunoelectron microscopy]

Synchronized human lymphocytes were exposed to 5-bromo-2'-deoxyuridine (BrdU) for incorporation in the late S-phase. The substituted chromosome bands were detected immunohistochemically by monoclonal anti-BrdU antibodies with the protein A-gold complex. The bands composed of many chromatin loop clusters were examined by electron microscopy without trypsin treatment. These banding patterns corresponded to the G-band by immunostaining. Chromatin loops were stretched around the chromosome, revealing the structure of coiled chromosome with trypsin treatment. The stretched loops were observed in the whole S-phase without treatment of trypsin but were not seen in the late S-phase. Variations in the degree of stretching were correlated with the duration of the BrdU treatment and the period of trypsin treatment. These results indicate that the trypsin treatment is essential to chromatin loops for their stretching in late S-phase and that the chromosome band is composed of numerous chromatin loops.