Nucleic acids double-stranded helix

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). 

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

When two nucleic acid strands have complementary nucleic acid sequence, they can undergo hybridization to form double-stranded duplex structures. DNA forms a double-stranded helix composed of two complementary helical polynucleotide chains, aligned antiparallel, which are coiled around a common axis. In helix form, the anionic backbone lies on the outside of the structure with the nucleobases in the core, perpendicular to the axis and separated by a distance of 3.4 A. This B-form helix (Figure 2a) has a right-handed coil that repeats itself every 34 A with a turn every... 

The association of a nucleic acid single strand with a double helix to form a triplex has attracted much attention in the recent years, especially because of possible antisense therapeutic applications. However, theoretical studies are intricate because of the absence of crystallographic structures leading to uncertainties in the precise conformations adopted by these molecules, further known as being highly dependent on salt concentration. 

In DNA the nucleic acid bases form hydrogen bonds between them, which are responsible for the formation of the double-stranded helix. Arrange the bases guanine and cytosine to give the maximum number of hydrogen bonds. 

The term peptide nucleic acids was chosen because of the peptide bond in the polymer (see Sect. 5.2). The bond between the polyamide strand and the organic bases involves an acetyl group. The formation of DNA-like double helix structures by PNAs was described by Pernilla Wittung et al. (1994). The question arises as to whether peptide nucleic acids can in fact be synthesized under prebiotic conditions. 

Base pair (bp) The four nucleotides in the DNA contain the bases adenine (A), guanine (G), cytosine (C), and thymine (T). Two bases (adenine and thymine or guanine and cytosine) are held together by weak bonds to form base pairs. The two strands of human DNA are held together in the shape of a double helix by those bonds between base pairs. For example, the complementary nucleic acid base sequence to G-T-A-C that forms a double-stranded structure with the matching bases is C-A-T-G. 

The role of DNA in storing and transferring genetic material is dependent on the properties of the four bases. These bases are complementary in that guanine is always associated with cytosine, and adenosine with thymine. Watson and Crick, some 40 years ago, showed that the stability of DNA is due to the double helix structure of the molecule that protects it from major perturbations. Information is ultimately transferred by separating these strands which then act as templates for the synthesis of new nucleic acid molecules. 

Fig. 26. Nucleic acid structures. A The structure of the four bases in DNA, guanine (G), cytosine (C) adenine (A) and thymine (T). Uracil (U) replaces thymine (T) in RNA. B The spontaneous attraction of A for T and C for G allows the recognition of homologous sequences in aqueous solutions and the strong and specific hybridization of one sequence with its homologous sequence. C DNA forms a double helix at body temperature, which can be denatured to separate the strands by heating. D single stranded mRNA structure.

Hydrogen-bonding interactions are considerably weaker than ionic interactions and covalent bonds but have a profound effect on many chemical and physical properties [221] and determine the shapes of large molecules such as proteins and nucleic acids. Protein secondary structure is determined by H bonding between the carbonyl oxygen of one amide unit and the N—H bond of another. The two strands of the double helix of... 

The DNA double-helix consists of two strands of opposite polarity, wrapped about each other (Watson and Crick 1953). The individual nucleic acid repeating unit consists of a base attached to a sugar phosphate, which is called a nucleotide. The helix is approximately 20 A wide, the vertical separation of the bases is approximately 3.4 A, and the periodicity is 10-10.5 nucleotides/tum for simplicity,... 

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