The chromonema - a forgotten level of mitotic chromosome packingстатья
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Дата последнего поиска статьи во внешних источниках: 27 мая 2015 г.
Аннотация:Chromonema-thick chromatin thread with diameter 0.2-0.3 mu m was observed in mitotic chromosomes by light microscopists in the beginning of XX century. Later, the existence of chromonemal threads was confirmed in a series of electron microscopic studies. However, this structure escaped interests of molecular biologists after the discovery of such levels of chromatin compactization, as nucleosomes, 30 nm-chromatin fibers, looped domains, scaffold elements, etc. On the other hand, in ultrathin sections of plant and animal chromosomes in early prophase and early telophase the chromonemal level is clearly visible. In metaphase chromosomes the chromonema is not so obvious due to tight compactization of chromosomes and its masking by the surrounding perichromosomal material. Electron microscopic study of cultured L 929 cells permeabilized with 0.1% Triton-X 100 has revealed that dense and thick chromatin threads about 0.08-0.1 pm in diameter - the chromonemas, can be seen in mitotic chromosomes after their treatment with chromatin condensation solution. These "chromonemal" chromosomes rapidly decondense after their treatment with hypotonic solution. During decondensation, chromosomes maintain their general shape and size and consist of loosely distributed 25 nm chromatin fibrils. When these decondensed chromosomes were returned into 3 mM CaCl2 Solution, the chromonemal structure was restored. Treatment of chromosomes with 0.6 M NaCl solution lead to loss of their chromonemal substructurization. After treatment with 2 M NaCl solution the chromosomes and their chromonemal structures were destroyed completely. Thus, chromonemal substructure of mitotic chromosomes behaves as native chromatin with respect of salt extraction. The chromonemal substructure of mitotic chromosomes can be "stabilized" by light irradiation after ethidium bromide staining. In this case hypotonic medium (10 mM Tris-HCl) does not induce the chromosomal swelling: the chromonemal level of chromatin condensation was not changed. The chromonemal structure also was not changed in "stabilized" chromosomes after salt extraction with 0.6 M NaCl solution: chromonemal structures of 80-100 nm in diameter were clearly seen on chromosome periphery. However, the chromonemal elements disappeared completely from these "stabilized" chromosomes after total histone extraction with 2 M NaCl. It seems important, that experimental detection of chromonemal level of DNP compactization in mitotic chromosomes requires chromosome loosening and un-masking, of chromonema by elimination of extrachromosomal material (matrix, or perichromosomal material). Thus, the highest level of chromatin condensation of mitotic chromosomes is typical thread-like structure - the chromonema, which includes all lower levels of chromatin compactization (nucleosomes, chromosomal fibrils, etc.). However, the manner of this single chromonemal thread packing in the body of mitotic chromosome remains unclear.