Double helix melting

Thirdly, experiments concerned with the whole process of conformational change and the unzipping process (e.g., [49]) studied the mechanical stability of individual double-stranded DNA molecules. It was foimd that the B-S transition of A-DNA occurred at 65 pN, followed by a second conformational transition during which the DNA double helix melted into two... 

C13-0103. The melting point of DNA, which is the temperature at which the double helix unwinds, increases as the amount of guanine and cytosine increases relative to the amount of adenine and thymine. Explain this observation. 

The double helix can be denatured by heating (melting). Denatured DNA, like denatured protein, loses its structure, and the two strands separate. Melting of DNA is accompanied by an increase in the absorbance of UV light with a wavelength of 260 nm. This is termed hyperchromicity and can by used to observe DNA denaturation. DNA denaturation is reversible. When cooled under appropriate conditions, the two strands find each other, pair correctly, and reform the double helix. This is termed annealing. 

Double-helical DNA can be denatured by conditions that disrupt hydrogen bonding and base stacking, resulting in the melting of the double helix into two single strands that separate from each other. No covalent bonds are broken in this process. Heat, alkaline pH, and chemicals such as formamide and urea are commonly used to denature DNA. 

The absorption coefficients of polynucleotides are different from those calculated from the sum of the mononucleotides in part this reflects the secondary structure. The abrupt increase in the absorption of DNA at the melting point, where the secondary structure changes from the double helix to a random coil, is well known. It is therefore... 

Melting Temperature. The double helix of polynucleotides described above becomes thermodynamically unstable at particular temperatures (with specified conditions of solute concentration, pH, etc.) and is transformed into the open random-coil arrangement. This transformation is rather sharp, and can be measured by the concurrent changes in a number of physical properties of the nucleic acid, such as the optical absorption coefficient. The midpoint of the transition region is called the melting point. 

The double helix is somewhat destabilized after the platination, as reflected by the decrease of the melting temperature of the duplex by 10-20°C at NMR concentrations (3 mM). 

Like proteins, nucleic acids can undergo denatur-ation. The strands of the double helix of DNA are separated and the double-stranded regions of RNA molecules "melt." Denaturation can be accomplished by addition of acids, bases, and alcohols or by removal of stabilizing counter ions such as Mg2+. The product is a random coil and denaturation can be described as a helix —> coil transition. Denaturation of nucleic acids by heat, like that of proteins, is cooperative (Chapter 7, Section A,3) and can be described by a characteristic melting temperature. 

The double helix is somewhat destabilized, compared with the unplatinated duplex, as seen from the decrease in melting temperature of about 10-20°. 

For any type of DNA, as the heat is applied, the transition between the double helix and random coil is sudden and occurs at a typical temperature called the melting temperature Tm. The Tm of a DNA depends on its G and C content the higher G + C, the higher the Tm because it takes more energy to... 

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