Sugar structures in DNA and RNA

Although RNA and DNA are similar in their overall properties, the presence of the 2 -hydroxyl group of ribose in RNA and its replacement by 2 -deoxyribose in DNA make a large contribution to the different 

The A-T base pair has two hydrogen bonds each base serves as H-donor for one bond and as H-acceptor for the other. 

The G-C base pair has three hydrogen bonds G is an acceptor for one for these, and a donor for two. This has important consequences for the thermal melting of DNAs, which depends on their base composition. 

Melting and helix formation of nucleic acids are often detected by the absorbance of ultraviolet light. This process can be understood in the following way The stacked bases shield each other from light. As a result, the absorbance of UV light whose wavelength is 260 nanometers (the Amo) of a double-helical DNA is less than that of the same DNA, whose strands are separated (the random coil). This effect is called the hypochromicity (less-color) of the double-helical DNA. 

Finally, the Tm of a DNA depends on how well its bases match up. A synthetic DNA double strand made with some mismatched base-pairs has a lower Tm compared to a completely double-stranded DNA. This last property is important in using DNA from one species to detect similar DNA sequences of another species. For example, the DNA coding for an enzyme from human cells can form double helices with mouse DNA sequences coding for the same enzyme however, the mouse-mouse and human-human double strands will both melt at a higher temperature than will the human-mouse hybrid DNA double helices. 

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