Triplex typing

Some very unusual nucleotide bonding has been discovered in so-called triplet repeat sequences [23], Two neighboring bases in one strand complex with a single base of the opposite strand in a complex, which is called triad DNA. This type of bonding requires three bases, but unlike triplexes the three bases are within only two DNA strands. 

The formation of three-stranded nucleic acid complexes was first demonstrated over five decades ago [56] but the possible biological role of an extended triplex was expanded by the discovery of the H-DNA structure in natural DNA samples [57-59]. H-DNA is an intermolecular triplex that is generally of the pyrimidine-purine x pyrimidine type ( dot -Watson-Crick pairing and cross Hoogsteen base paring) and can be formed at mirror repeat sequences in supercoiled plasmids [59]. 

Previous work has suggested that aminoglycoside specificity may occur in nucleic acid forms that display features characteristic of an A-type conformation (RNA triplex, DNA-RNA hybrid duplex,RNA duplex, DNA triplex, A-form DNA duplex, and DNA tetraplex ), rather than in naturally occurring RNA. However, conflicting results have been reported regarding the conformation of the triplex and of the Watson-Crick duplex within these triplexes. Both... 

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). 

Both rust and oxide scales are usually mixtures of iron oxides vith other Fe (e. g. siderite) and non-Fe compounds (CaCOs). In some cases there is a more or less random mixture of components, vhereas in others, the different oxides are arranged in layers to form duplex or triplex scales. Layer-type rust arises as a result of potential or chemical gradients across the film. As these gradients vary ivith film thickness, the composition of the rust changes with the distance from the metal. On the whole, if Fe " and Fe" are present, the oxide containing Fe" is found in the inner layer of the rust. 

Substituted cyclopentadienones react with iron carbonyls to form stable, diamagnetic 7r-co triplexes of the type [Fe(CO)3(cyclopentadienone)] (215). The proposed structure is shown in (XX). These complexes undergo reactions typical of metal carbonyls, e.g., displacement of carbon monoxide by tertiary phosphines, but the carbonyl group of the ligand does not show reactions characteristic of a keto-group. These complexes are also formed by interaction of acetylenes with iron carbonyls (see Section VI,C). Interaction of tetracyclone and Fe3(CO)i2 gives unstable complexes which contain the sandwich anion [Fe(tetracyclone)2]2 analogous to the anion (XXV) (215). 

The extra hydrogen-bonding sites can, however, also form further base pairs, as observed for poly(A) poly(U). In the Watson-Crick A-U pair, one of the adenine N(6)-H donors and the N(7) acceptor are still free and can form another A-U pair of the Hoogsteen type (Fig. 16.16). This is in fact observed if the Watson-Crick duplex poly(A) poly(U) is exposed to high salt conditions, it disproportionates to form a triplex poly(A)-2poly(U) and a single strand poly(A) [539] ... 

C4 photosynthesis is found in both monocotyledons and dicotyledons. Most of the economically important ones are found in the Poaceae (grasses), and there are some interesting intermediates between C3 and C4, such as Panicum milioides. The genus A triplex contains both C3 and C4 types which are still closely enough related to interbreed. 

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