Duplex structures

Dezincification of brasses This may occur, particularly in stagnant or slowly-moving warm or hot waters relatively high in chloride and containing little carbonate hardness. Dezincification of a brasses is inhibited by the usual arsenic addition (see Fig. 4.12), but two-phase brasses are liable to severe attack in some waters . In such waters the use of duplex-structure brass fittings should be avoided. 

DNAs are soluble only in aqueous solutions and their fibrous crystals can be prepared by slow evaporation from the aqueous solution. Duplex structures in the fibers have been studied by X-ray diffraction [2,3] and sohd state NMR [4-6]. Orientation of DNA strands by using hydrodynamic flow gradients in the dilute aqueous solution [7,8] and lyotropic liquid crystal... 

Ti-6A1-4V is probably the most widely used Ti alloy in the world. It is an alloy with a duplex structure containing solid solutions based on the a, c.p.h. A3 and / , b.c.c. A2 allotropes of Ti. In its final heat-treated form it consists predominantly of a and its high strength is partly derived from its final microstructure which is manipulated by a series of thermomechanical treatments that include hot isothermal forging just below its P transits temperature (T ). The interest is, in the first place, to predict and how the amounts of a and P vary with temperature. 

Most DNAs Exist as Double-Helix (Duplex) Structures... 

Duplex Structures Can Form Supercoils DNA Denaturation Involves Separation of Complementary Strands... 

Because of the different orientations of the bases in Z DNA, this DNA conformation requires that the sequence of purine and pyrimidine bases in each chain strictly alternate. An alternating sequence of G and C, or T and G or A and C residues can adopt a Z conformation. Of course, in all cases the opposing strand must contain a sequence of bases that is complementary as in all DNA duplex structures. An alternating A-T DNA sequence cannot adopt the Z confer-... 

Kinetic analysis indicates that renaturation is a two-step process. In the slow step effective contact is made between two complementary regions of DNA originated from separate strands. This rate-limiting step called nucleation is a function of the concentration of complementary strands. Nucleation is followed by a relatively rapid zippering up of adjoining base residues into a duplex structure. The steps involved in denaturation and renaturation are depicted in figure 25.14. 

Some DNA-binding proteins specifically bind to Z DNA. How could these proteins help stabilize DNA in the Z configuration (Hint How do single-stranded DNA-binding proteins destabilize duplex structures )... 

From the complementary duplex structure of DNA it is a short intuitive hop to a model for replication that satisfies the requirement for one round of DNA duplication for every cell division. Such a proposal was made by Watson and... 

Crick when they proposed the duplex structure for DNA (fig. 26.1). First, the double helix unwinds next, mononucleotides are absorbed into complementary sites on each polynucleotide strand and finally these mononucleotides become linked to yield two identical daughter DNA duplexes. What could be simpler Subsequent biochemical investigations showed that in many respects this model for DNA replication was correct, but they also indicated a much greater complexity than was initially suspected. Part of the reason for the complications is that replication must be very fast to keep up with the cell division rate, and it must be very accurate to ensure faithful transfer of information from one cell generation to the next. 

Reactions catalyzed by purified recA protein in vitro. RecA catalyzes a number of different reactions between DNA strands, all of them involving the unwinding and winding of base-paired structures, (a) D-loop formation by interaction between supercoiled circular duplex DNA and single-stranded DNA. (b) Strand exchange between a gapped circular duplex structure and a linear duplex structure, (c) Complex formation between two helices, one of which is gapped. 

DNA unwinding by exoV enzymes. This is best observed in vitro in the presence of ATP and Ca2+. The Ca2+ inhibits the exonucleolytic activity of the enzyme. The ATP provides the energy that drives the enzyme in a concerted manner into the duplex structure. The duplex unwinds in front of the path of the enzyme and rewinds in back of the enzyme. 

D loop. An extended loop of single-stranded DNA displaced from a duplex structure by an oligonucleotide. 

Heteroduplex. An annealed duplex structure between two DNA strands that do not show perfect complementarity. Can arise by mutation, recombination, or the annealing of complementary single-stranded DNAs. 

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