Chromatin electron microscopy

The nucleus contains chromatin within the nucleoplasm. The chromatin is comprised in part of DNA, the genetic information that controls cellular activities through determining which proteins are produced and when. Chapter 20 of this volume describes the electron microscopy of nucleic acids. 

Zentgraft H, Bock C-T, Schenk M. Chromatin spreading, in Electron Microscopy in Molecular Biology. A Practical Approach (Sommerville J, Scheer U, eds.). IRL Press, Oxford, UK, 1991, pp. 81-100. 

Adolph KW, Kreisman LR, Kuehn RL (1986) Assembly of chromatin fibers into metaphase chromosomes analyzed by transmission electron microscopy and scanning electron microscopy. Biophys J 49 221-231... 

Images obtained by cryo electron microscopy should in principle be able to distinguish between the structural features proposed by the different models mentioned above [16]. The micrographs show a zig-zag motif at lower salt concentrations and they indicate that the chromatin fiber becomes more and more compact when the ionic strength is raised towards the physiological value (i.e., about 150 mM monovalent ions). 

Gerchman, S.E. and Ramakrishnan, V. (1987) Chromatin higher-order structure studied by neutron scattering and scanning transmission electron microscopy. Proc. Natl. Acad. Sci. USA 84,... 

Apoptotic features have been occasionally found in other cardiac disorders in humans [128-131] and experimental models [107, 132-135]. Takemura et al. [135] studied Fas-induced cardiomyocyte apoptosis by electron microscopy, and found extensive condensation of nuclear chromatin and shriveled cytoplasm, fragmented nuclei, and apoptotic bodies. Apoptotic features correlated with positivity for TUNEL and caspase-3. A distinct morphological feature was the abundance of lipid-like structures in the cytoplasm at the early phase and high incidence of plasma membrane rupture at the later phase. Apoptotic bodies were observed to be phagocytosed by neighboring cardiomyocytes. 

Shioda et al. [43,44] visualized by electron microscopy both regions of naked DNA and of DNA covered with particles in the chromosome of Halobacterium salinarium isolated from gently lysed cells. In a control experiment, they did not detect such particles in E. coli. They also reported the existence of nucleosome-like structures in S. acidocaldarius and methanogens (unpublished results cited in ref. [43]). The size of the particles detected in H. salinarium (9.5 nm) is similar to that of eukaryotic nucleosomes (10.3 nm) however, this putative archaebacterial chromatin is not as regular as eukaryotic chromatin, since not all of the DNA is covered with nucleosomes and since the length of the DNA spacer between the particles is not uniform. In contrast to these results, Bohrmann and coworkers [45] did not visualize nucleosome-like structures in isolated chromosome fibers of Thermoplasma acidophilum. These authors also reported that in situ the nucleoid of T. acidophilum appears to be highly dispersed in the cytoplasm. 

Chromatin is made up of repeating units, each containing 200 bp of DNA and two copies each of H2A, H2B, H3, and H4, called the histone octamer. These repeating units are known as nucleosomes. Strong support for this model comes from the results of a variety of experiments, including observations of appropriately prepared samples of chromatin viewed by electron microscopy (Figure 31.19), Chromatin viewed with the electron microscope... 

Yeast RSC chromatin remodeling complex structure determined by electron microscopy. The complex (red) is shown bound to a nucleosome (yellow). [Courtesy of Francisco J. Asturias, 2002, PAMS 99 13477]... 

Complementary use, in future, of small angle solution synchrotron X-ray scattering methods with electron microscopy could contribute to the clarification of the path of the linker DNA. Thus further experiments seeking a universal model for the chromatin fibre structure need to be undertaken although the existence of such a universal structure is questionable. More importantly future investigations need to be aimed at imderstanding what relevance the structure of the fibre has to the control of DNA transcription and replication. 

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