Structural analysis of human chromosome by FIB/SEM

Despite the efforts of numerous researchers over the years, inner structure of a chromosome is still controversial, although several models have been proposed to date. It is now well known that there are two important structural components to the chromosome, the chromosome scaffold and chromatin fibers. The chromosome scaffold, which is mainly composed of four different proteins, is a protein axis extending longitudinally in both chromatids. The chromatin fiber, which is composed of a DNA strand with histone proteins, is also packed in each chromatid. We used focused ion beam/scanning electron microscope (FIB/SEM) to elucidate these two structural components using human chromosomes. FIB/SEM effectively cuts the human chromosomes by its focused Ga ion beam and the cross-sections were visualized by the resolution of scanning electron microscope. As a result, the chromosome scaffold has been confirmed to be located in the central region of each chromatid. The distribution of chromatin fiber in the chromosome’s inner space was also detected. It seems to be distributed more or less randomly within a chromosome. These results strongly indicated that the nanotechnology afforded by FIB/SEM is an effective method to reveal the chromosome’s inner structure in detail.