Elementary biophysics: selected topics elementarybiophy00epst Year: 1963 LIGHT ABSORPTION 57 - â s-g <u o Ota - Temperature Fig. 24. A sketch of the optical density () of a DNA solution as the temperature is slowly raised from room temperature to about 100°C. Tm is the melting temperature, defined as the temperature at which half the change is found. The increase in is known as the hyperchromic effect. drogen bonds, the strands will be held together. It is possible to raise the temperature so high that all the bonds will be broken at once. If, at this temperatu
Elementary biophysics: selected topics elementarybiophy00epst Year: 1963 LIGHT ABSORPTION 57 - â s-g <u o Ota - Temperature Fig. 24. A sketch of the optical density () of a DNA solution as the temperature is slowly raised from room temperature to about 100°C. Tm is the melting temperature, defined as the temperature at which half the change is found. The increase in is known as the hyperchromic effect. drogen bonds, the strands will be held together. It is possible to raise the temperature so high that all the bonds will be broken at once. If, at this temperature, the solution is suddenly chilled (by placing the tube of DNA solution in an ice bath), the chances are greater that the individual strands will fold up and make internal hydrogen bonds than that they will have time to find the complementary strands to re-form the DNA. This is sketched in Fig. 23. If the light absorption depends on the chemical state of the substance, it would be possible for the solution to absorb different amounts of light in each of the situations shown in the figure. In fact, when DNA is 'melted' there is a 40 increase in light absorption at the peak absorp- tion wavelength (260 mfi) for nucleic acids. Figure 24 shows the light absorption of a DNA solution at various temperatures. The DNA is said to have a melting temperature Tm at the temperature halfway between the native and melted (or denatured) states. It turns out that the melting temperature Tm is quite characteristic of the DNA of various organisms and that related organisms have melting temperatures which are very close to each other, so that this light absorp- tion measurement has phylogenetic significance. The increase of light absorption is known as the hyperchromic effect, and is useful in a number of experimental situations. Since RNA and single-stranded DNA have very small hyperchromic effects, it is possible to deduce their presence or absence by light absorption measurements. (b) Matc
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