3D gene composition is like first realization in human living cells
Release date: 2017-08-01 On July 28th, the latest issue of Science published a cover article entitled "ChromEMT: Visualizing 3D chromatin structure and compaction in interphase and mitotic cells". In this study, scientists from the Salk Institute of Biology and the University of California, San Diego, unveiled the mystery of the long-term biology of DNA organization. For the first time, researchers have realized 3D gene composition in the nucleus of human living cells. Image source Science New dyes and key technologies ChromEMT If stretched out, the DNA of all the cells in our body will reach Pluto. So how does each tiny cell put a two-meter long DNA into its nucleus? The answer to this biological puzzle is the key to understanding how the three-dimensional structure of DNA in the nucleus affects human biology, including how our genome coordinates cell activity and how genes are passed between parents and children. In this latest study, scientists have for the first time presented an unprecedented view of the chromatin 3D structure in the nucleus of human living cells. In this outstanding work, the researchers identified a new type of DNA dye. This dye, combined with advanced microscopy techniques, is a new technology called ChromEMT that allows highly detailed visualization of the chromatin structure of cells in the dormant and mitotic phases. Clodagh O'Shea, co-author of the study, said: "One of the toughest challenges in biology is to understand the higher-order structure of DNA in the nucleus and to understand the link between these structures and functions. Very important, because the biologically relevant structure of DNA determines the function and activity of the gene." By revealing the structure of nuclear chromatin in living cells, this study may help to rewrite the textbook model of DNA structure. The mystery of chromatin structure in living cells Since Francis Crick and James Watson determined that the primary structure of DNA is a double helix, scientists have been wondering how DNA can be further organized to fit into the nucleus. X-ray and microscopy revealed the primary structure of chromatin tissue: DNA containing 147 bases was entangled with proteins, forming microparticles of approximately 11 nanometers in diameter - known as nucleosomes. Chromatin (chromatin is a form of interphase genetic material. Chromosome refers to a rod-like structure in which cells are condensed by chromatin at a specific stage of mitosis or meiosis) is considered to be a series of nucleosomes. composition. One problem is that no one has previously observed chromatin structure in these intermediate sizes in unbroken cells (referred to as intact cells). That is to say, in fact, the textbook model of chromatin high-order structures in intact cells has not been confirmed. To address the problem of visualizing chromatin in intact nuclei, O'Shea's team screened a large number of candidate dyes and eventually found a localized structure and 3D polymer that could be precisely manipulated through a series of complex chemical reactions. A dye that is capable of being imaged in living cells. Subsequently, O'Shea's team collaborated with Professor Mark Ellisman, a microscopy expert at the University of California, San Diego. By combining chromatin dyes with electron microscopy tomography, they created a ChromEMT (chromatine dye with electron-microscope tomography, ChromEMT). New technology. The team used ChromEMT technology to image and analyze chromatin in dormant human and dividing cells. Surprisingly, they did not observe any higher-order structures of the textbook model anywhere. Horng D. Ou, the first author of the study, said: "Chromosomes extracted from the nucleus and treated in vitro may not resemble chromosomes in intact cells. Therefore, direct observation in the body is very important." From left: Horng Ou and Clodagh O'Shea (Source: Salk Institute) Subverting people’s imagination Before the 1970s, the traditional view of chromatin structure believed that chromatin is a fibrous structure formed by histones wrapped around DNA. Until 1974, according to chromatin digestion and electron microscopy, Kornberg et al. found that nucleosomes are the basic structural unit of chromatin assembly, and proposed a “beading†model of chromatin structure, thus renewing the traditional concept of chromatin structure. . Previously, these nucleosome "beads on a string" were considered to be folded into increasing diameter (30, 120 or 320 nm, etc.) discrete fibers of increasing diameter. (Baidu Encyclopedia's description of chromatin structure is such that the bead structure is the primary structure of chromatin assembly. However, in cells, chromatin rarely exists in this stretched beaded form. When the nucleus is gently treated Under the electron microscope, chromatin fibers with a diameter of 30 nm are often seen. In the presence of histone H1, the nucleosome bead structure is helically coiled, and 6 nucleosomes per turn form an outer diameter of 25~30. Nano, a solenoid with a pitch of 12 nm. The solenoid is the secondary structure of chromatin assembly.) However, O'Shea's team observed that in dormant and dividing cells, chromatin "beads" did not form any theoretically speculated high-order structures of 30, 120 or 320 nm. Instead, it forms a semi-flexible chain. The length of this chain varies continuously between 5 and 24 nanometers and achieves different degrees of compaction by bending and shrinking. This suggests that it is the packing density of chromatin, rather than some higher order structures that determine which regions of the genome are active and which regions are inhibited. 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