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Watson and Crick proposed DNA double helix structure belongs to type B double helix, it is in the physiological salt solution extracted DNA fiber in 92% The X-ray diffraction map is based on relative humidity, which is the most stable structure of DNA molecules under water-based environment and physiological conditions. Later studies, however, have shown that the structure of DNA is dynamic. In the use of potassium or anti-ion, relative humidity of 75%, the DNA molecule's X-ray diffraction map gives the A composition, A-DNA each spiral contains 11 base pairs, and become A-DNA, the large ditch narrows, becomes deeper, the groove becomes wider and shallower. Since large and small ditches are the identification points of ef proteins when DNA functions, the identification of DNA molecules by proteins changes from B-DNA to A-DNA.
< generally speaking, A-T> rich DNA fragments are often B-DNA. When ethanol precipitation is used to purify DNA, most of the DNA passes from B-DNA to C-DNA and eventually to A-DNA. If a chain in a double strand of DNA is replaced by a corresponding RNA chain, it becomes A-DNA. When DNA is tweed, the double strand formed between the DNA template chain and the RNA chain from which it is tweed is A-DNA. Thus, it can be seen that the A-DNA < is of great significance to the expression of > genea href". In addition, B-DNA double strands are replaced by RNA chains and the double helix structure consisting of two RNA strands is also A-DNA. In addition to A-DNA, B-DNA helix, there are also B-DNA, C-DNA, D-DNA, etc. , the structural parameters of which can be seen in Table 15-4.Table 15-4 structural parameters of different right-handed double helix DNA
In 1979, Wang and Rich et al. were studying the X-ray diffraction map of the synthetic CGCGCG monocrystalline when they unexpectedly discovered that the composition of the hexamer was completely different from what was said above. It is the left-handed double helix, unlike the right-hand helix is the pitch extension (about 4.5nm), the diameter narrows (1.8nm), each spiral contains 12 base pairs, the molecular long chain of phosphorus atoms is not a smooth extension but a jagged arrangement, as "the" glyph, so called its Z-image (the first letter of zigzag). Moreover, the repeated units in this composition are twonucleotides instead of a single nucleotide, and Z DNA has only one helical trench, which is equivalent to a small trench in the B-composition, which is narrow and deep, and the large groove no longer exists (Figure 15-7). Further analysis also proved that the formation of Z-DNA is the formation of DNA single strands of radon and radon alternate arrangement. Such as CGCGCGCG or CACACACA.
Figure 15-7 Z-DNA and B-DNA
Z-DNA what is the biological significance? It should be noted that the formation of Z-DNA is usually detrimental in thermodynamics. Because the negatively charged phosphate roots in Z-DNA are too close, this produces static rejection. However, the presence of local unstable regions of the DNA chain becomes a potential unchained bit. DNA helix is a necessary part of the process of DNA replication and transcription, so this structure is thought to be related to gene regulation. For example, there is this structure in the SV40 enhancement sub-region, and there is an alternate sequence of GCs near the starting point of the mouse microvirus DNS replication zone. In addition, the characteristics of the DNA spiral upper ditch play a key role in the expression of information. Regulatory proteins interact with hydrogen atom feeds or polyseters on the other side of the base in the DNA double helix ditch to identify genetic information in DNA by interacting with specificside chains on their molecules. Large ditches have more genetic information than small ditches. The narrowness and depth of the ditch also directly affect the identification of DNA information by regulating proteins. The large ditch in Z DNA disappears, and the small ditch is narrow and deep, which makes the way the regulatory protein is recognized also changed. All these suggest that the existence of Z DNA is not only due to the alternating arrangement of radon in DNA, but also must be the result of constant adjustment and screening of DNA sequences and structures in the long evolutionary process, which has its inner and profound meaning, but people have not yet fully enough understanding.
< the discovery of the variability of > of DNA structures, or polymorphisms of DNA secondary structures, has broadened the horizons. It turns out that the most stable genetic material in living organisms can also use different attitudes to achieve its rich and multi-rich biological functions. < for more than > years, dna structure has been studied mainly by X-ray diffraction techniques, the results of which are obtained by indirectly observing the meaning of structural parameters of multiple DNA molecules. At the same time, the sample analysis conditions of this technique make the DNA molecules under test very different from the natural state. Therefore, this method is flawed in reflecting the authenticity of DNA structure. In 1989, the use of scanning tunnelsmicroscope (scanning tummeling microscopy, STM) to study DNA structures overcomes the shortcomings of these technologies. This advanced microscopic technology not only magnifies the measure by 5 million times, but also directly observes the structural details of individual DNA molecules close to natural conditions. The application of STM technology is an important advance in DNA structure research and is expected to show great potential in exploring some unknown points of DNA structure.
