deoxyguanosine as in Z DNA
FIGURE 6-13 Syn and anti positions of guanine in B and Z DNA. In right handed B DNA, the glycosyl bond (colored ried) connecting the base to the deoxynbose group is always in the anti position, while in left-landed I OKA it rotates in the direction of the arrow, forming the syn conformation at the punne (here guanine) residues but remains in the regular anti position (rto rotation) in the pyrimidine residues. (Source: Adapted from Wang A. J. H. et a!. 1982 CSHSQB 47 41 Copyright £> !9e2 Cold Spring Harbor Laboratory Press Used with permission.)
108 The Structures of DNA and Rl\JA
change from anti to syn also causes the ribose group to undergo a change in its pucker. Note, as shown in Figure 6-13, that C3' and C2' can switch locations, in solution alternating purine-pyrimidine residues assume the left-handed conformation only in the presence of high concentrations of positively charged ions (for example, Nal) that shield the negatively charged phosphate groups. At lower salt concentrations, they form typical right-handed conformations. The physiological significance of Z DNA is uncertain and left-handed helices probably account at most for unly a small proportion of a celt's DNA. Further details of the A, B, and Z forms of DNA are presented in Table 6-2.
DNA Strands Can Separate (Denature) and Reassociate
Because the two strands of the double helix are held together by relatively weak (noncovalent) forces, you might expect that the two strands could come apart easily, indeed, the original structure for the double helix suggested that DNA replication would occur in just this manner. The complementary strands of the double helix can also be made to come apart when e solution of DNA is heated above physiological temperatures (to near 100r" C) or under conditions of high pll, a process known as denaturation. However, this cumplete separation of DNA strands by denaturation is reversible. When heated solutions of denatured DNA are slowly cooled, single strands often meet their complementary strands and reform regular double helices (Figure 6-14). The capacity to renature denatured DNA molecules permits artificial hybrid DNA molecules to be formed by slowly cooling mixtures of denatured DNA from two different sources. Likewise, hybrids can be formed between complementary strands uf DNA and RNA. As we shall see in Chapter 20, the ability to form hybrids between two single-stranded nucteic acids, called hybridization, is the basts
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