Double stranded helices

Fig. 34. Enantiomeric double-stranded helixes corresponding to Figure 33 (9,202).

Sato, T., Norisuye, T., and Fujita, FI., Double-stranded helix of xanthan in dilute solution evidence from light-scattering, Polymer J., 16, 341, 1984. 

DNA DNA (deoxyribonucleic acid) is a double-stranded helix of nucleotides... 

Watson and Crick showed in 1953, using X-ray diffraction data of hydrated DNA fibres [26], that B-DNA, the most commonly encountered form, corresponds to a right-handed double-stranded helix (Fig. 4). Two polynucleotide... 

The tertiary structure of DNA is the structural level that is most relevant to 3-D reality. Traditionally, ODNs in a physiologically relevant aqueous solution are considered to be in a random-coiled ssDNA state or in the form of dsDNA helix in the presence of a complementary DNA, including the case of self-complementarity. The double helix is the dominant tertiary structure for biological DNA that can be in one of the three DNA conformations found in nature, A-DNA, B-DNA, and Z-DNA. The B-conformation described by Watson and Crick (11) is believed to predominate in cells (12). However other types of nucleic acid tertiary structures different from random or classical double-stranded helix forms can also be observed. Among them are triplexes, quadruplexes, and several other nucleic acid structures (13, 14). 

PCR uses natural enzymes to copy DNA molecules. Cells replicate their DNA when they divide, so that each daughter cell will have its own copies. The enzyme, called DNA polymerase, works by attaching to a single strand of DNA and then catalyzing the reactions that synthesize the complementary strand, which joins the other stand to make a double-stranded helix. 

The use of a 1,3-phenylene as spacer group proved to be extremely beneficial. First, 30 was used as a model and reacted with Cu(CH3CN)4-PFg in a mixture of dichloromethane and acetonitrile. Preparation of the double-stranded helix 32 turned out to be quantitative. This complex could be crystallized from acetonitrile-toluene and its structure determined by X-ray crystallography (Figure 23) [102]. 

Clustering (association or aggregation) as a result of dissimilar parts within one chain molecule is well-known in the field of biopolymers a recent example in the field of synthetic polymers is provided by poly (p-nitrophenylmethacrylate) which in dimethylformamide (105) forms threefold clusters, presumably because of the difference in polarity of backbone and ester groups. Liquori (118) has presented proof for a double-stranded helix of isotactic and syndiotactic polymethylmethacrylate. 

Figure 2 shows the energy contour map for isotactic poly( methyl methacrylate). The lowest energy minimum was found at the position corresponding to a (12/1) helix contrary to the expectation of the (5/1) helix. The minimum corresponding to the (5/1) helix is higher than the (12/1) helix by 3 kcal/mole of monomer unit. This result led to the postulation of the double stranded helix for this polymer. 

Figure 7. Double-stranded helix of isotactic poly(methyl methacrylate) (30)...

Double-Stranded Helix In most cases, DNA found in the nucleus of a cell is a helical double-stranded structure. Figure 12.63 represents the double-strand structure of a short section of DNA. The double strand is rather like a ladder. The sides of the ladder are formed by the phosphate sugar backbone. The rungs of the ladder are formed by the nitrogeneous bases. 

Nucleosides Adding the Nitrogenous Bases 34 j Nucleotides Adding the Phosphate Group 349 Forming the DNA Strand 350 Base Pairing 351 Double Stranded Helix 351 RNA 353... 

Figure 11.24. Space-filling model of the double-stranded helix ([ Ag(16) (BF4) .

Kawade (46) repeated these experiments with various RNA s (soluble, ribosomal and tobacco mosaic virus RNA s) and he found a change in a qualitative similar way of the ultraviolet absorption spectra on addition of mercuric ions. A theory as developed by Katz (45) specifies for Hg2 + 2 1 base-Hg2+ complexes which tie the double stranded helix via the bases. It was further stated that Hg2+— (thymidine) 2 complexes are the most readily formed. 

A germ line acting as a one-dimensional template and consisting of a double strand helix of nucleic acids (i.e. a seed ). 

Most DNAs contain extensive secondary structure, existing as large double-stranded helixes containing two separate, hydrogen-bonded... 

Crick (1954) proposed a two-stranded helix, somewhat reminiscent of the Watson-Crick double-strand helix which had been so successful for deoxyribonucleic acid. The model consisted of two polypeptide chains wound helically around a common axis and held together by interchain peptide hydrogen bonds. The peptide bonds were all in the cfs-configuration, and the repeating unit consisted of a pair of amino acid residues, one perpendicular and the other parallel to the fiber axis. As a result, planes... 

DNA consists of two molecules of single-stranded DNA (ssDNA) wrapped together in a fiber called the double stranded helix (dsDNA). Damage can be done to DNA by a number of separate and distinct processes, for example ... 

The continued accumulation of uracil in DNA, over many cell divisions, is thought to result — eventually — in the occurrence of residues of U across from each other in a double-stranded helix. Where uracil DNA glyeosylase processes the U residues, the result is cleavage at both strands, and a double-stranded break. Double-stranded breaks are repaired by recombinational repair. Recombinational repair is not perfect, and may yield reconnected DNA that contains mutations. Very little direct evidence exists to support the previous scenario. However, it is... 

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