The secondary structure and function of DNA.

. NetworkSection III
DISD
Secondary Structure and Function(I) DNA Secondary Structure Double Helix ModelIn 1953, Watson and Crick proposed the famous double helix structure model of DNA molecules, revealing how genetic information is stored in DNA molecules and how genetic characteristics are maintained over generations. This is a major milestone in the development of biology.before the DNA double helix model was established, as early as 1868, Miescher had extracted a complex of
nucleic acids
and
proteins
from pus cells, then known as nuclein. However, the important position of nucleic acids in life activities was not recognized until the 1950s.the 1920s, Levene studied the chemical structure of nucleic acids and proposed the four
nucleotides
hypothesis; It was confirmed that DNA is a genetic material, and in the early 1950s, Chargaff used UV terrolight grading combined with simple techniques such as paper lamination to quantitatively analyze a variety of biological DNA bases and found that the composition of DNA bases had the following laws (Table 15-3).Table 15-3 Relationship between the four bases of DNA from different biological sources
DNA sources Adenine (A) thymus (T) Ostrich (G) Cytosine (C) (A-T)/(G-C) E. coli 25.4 24.8 24.1 2 5.7 1.01 Wheat 26.8 28.0 23.2 22.7 1.21 Rat 29.7 25.6 21.9 22.8 1.21 Pig: Liver 29.4 29 .7 20.5 20.5 1.43 Thymus 30.0 28.9 20.4 20.7 Spleen 29.6 29.2 20.4 20.8
Yeast
31.3 32.9 18.7 17.5 1.079
(1) The DNA base composition of different
tissues
of the same organism is the same;
.
(3) Almost all DNA, regardless of the source of the species, has the same amount of adenine moles as the thymus molar content (A?(4) the composition of DNA bases from different biological sources is different, asshown by the differences in the ratio of A-T/G-C;. Watson and Crick studied the results of wilkins and Franklin's DNa X-ray diffraction analysis based on stereochemistry principles, and proposed a double helix pattern of DNA structure, which mainly includes the following: Double helix pattern of 15-5 DNA in FigureA. Front view: a long box with detailed instructions, S stands for DNA.B. Looking down: blackened is the base, where all the bases are sorghum, only see the side of sugar slightly triangular, the outerm is phosphoric acid and its ester bonds.(1) In a DNA molecule, two strands of DNA form a right-hand spiral structure around an imaginted common axis, with a pitch of 3.4nm and a diameter of 2.0nm. (Figures 15-5, A, B).(backbone) of a chain of two (2) chains consists of alternating, hydro-hydrochloric DNA and phosphate, located on the outer side of the double helix.(3) the base is located on the inside of the double helix, and the radon and nirconium bases in the two strands are closely related to each other with their hydrophobic, near-plane ring structure, and the plane is perpendicular to the long axis of the double helix. The radon base in one strand is connected by a hydrogen chain with the zirconium base in the same plane in the other chain, called base complementary pairing or base pairing, and the distance between the base pairs is 0.34nm. Base complementary pairings always occur between adenine and thymus (A-T) to form two hydrogen bonds, or between ostrich and cytosine (G-C) to form three hydrogen bonds. (Figure 15-6).. Figure 15-6 A-T, the hydrogen bond between G-C forms the two strands in the (4) DNA double helix, one chain is 5' →3' and the other is 3' →5' trend. A large groove and a small groove are formed spatially between the two strands, which are the basis for the protein's ability to identify the base sequence of DNA and interact with it. . The stability of the DNA double helix is maintained by the hydrogen bond between the complementary base pairs and the accumulation force between the base pairs. The complementary characteristics of the bases in the two strands in the DNA double helix logically indicate that the DNA replication process is to first split the two strands in the DNA molecule, and then use each strand as a template (parent), and synthesize the corresponding complementary chain (copy) through the principle of base complementarity to form two identical DNA molecules. Because only one of the copied pairs is pro-chain, i.e. half of the parent chain is preserved, this replication is called semi-reserved replication of DNA. It turned out that semi-reserved replication was the most basic way to transmit genetic information from organisms. . DNA double helix is an important form of nucleic acid secondary structure. The theory of double helix structure dominates the research and development of the function of modern nucleic acid structure and is an outstanding contribution in the history of life science development. (ii) Polymorphism of DNA structures The DNA double helix structure proposed by Watson and Crick belongs to type B double helix, which is based on an X-ray diffraction map of DNA fibers extracted from a physiological salt solution at a relative humidity of 92%, which is the most stable structure of DNA molecules under water-based environments 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 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
important
expression of the gene. 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-hand double helix DNA
double helix base tilt base clamp base pitch pitch per round alkali groove width /nm× large groove width nm× angle / (s) angle (°) /nm /nm base Small groove width nm large groove width nm B-DNA 0 36.0 0.337 3.4 10 0.57×0.75 1.17×0.85 C-DNA 6 38.0 0.331 3.1 9.9 3.48×0.79 1.05×0.75 D-DNA45.0 0.3030.13×0.67 0.89×0.58 A-DAN 20 3 2.7 0.256 2.8 11 1.10× 0.28 0.27×1.35
In summary, the double helix structure of DNA is always in dynamic equilibrium, and changes in DNA molecular composition are related to the spatial relative position between glycosyl and base. 1979, Wang and Rich et al., while studying the X-ray diffraction map of the synthetic CGCGCG monocrystalline, unexpectedly discovered that the composition of this 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). More importantly, the duplicate unit in this configuration is


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