Principles of Biochemistry Volume 3 - A. Lehninger 1985

Molecular Mechanisms of Genetic Information Transmission
DNA: Structure of Chromosomes and Genes
DNA double helices can undergo denaturation, i.e., unwind

Let us now examine some of the chemical and Physical Properties of DNA that stem from its double-helical Structure. Solutions of DNA isolated with great care exhibit high viscosity at pH 7.0 and room Temperature (20–25 °C). If such a solution is heated above 80–90 °C or subjected to extreme pH values, the viscosity drops sharply, indicating A change in the physical state of the DNA. As we have seen, high temperatures and extreme pH lead to the Denaturation, or unraveling, of Globular Proteins (Sec. 6.12). Similarly, high temperatures and extreme pH values cause the denaturation, or unwinding, of double-stranded DNA helices by disrupting the Hydrogen Bonds between paired bases and the hydrophobic interactions that hold the stacked bases together. As a result, The Double Helix unravels into random, disordered single-stranded coils until the two strands finally separate completely. During denaturation (also referred to as melting), the covalent bonds in the backbone of the molecule remain intact (Fig. 27-14).

If an undenatured double-stranded DNA segment consisting of a dozen or more complementary NUCLEOTIDES still prevents the strands from separating completely, the denaturation process can be easily reversed. This means that when the temperature and pH are returned to physiological values, the unwound Regions of the two strands spontaneously re-associate to form the original duplex (annealing, Fig. 27-14). However, if strand Separation is complete, renaturation proceeds in two phases. The first phase is relatively slow because the two strands must “find” one another through random collisions and form short complementary double-helical regions. The second phase occurs much more rapidly as the remaining bases successively align and form complementary Base Pairs. The two strands then “zip up,” reforming the double helix.

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Fig. 27-14. Stages of reversible DNA Denaturation and renaturation.



Last update: 06/08/2026

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