Twist, nucleic acid conformation

The linear sequence of nucleotides linked by phosphodiester bonds constitutes the primary structure of nucleic acids. Like polypeptides, polynucleotides can twist and fold Into three-dimensional conformations stabilized by noncovalent bonds. Although the primary structures of DNA and RNA are generally similar, their three-dimensional conformations are quite different. These structural differences are critical to the different functions of the two types of nucleic acids. 

Nucleic acids have two kinds of pentoses 2 -deoxy-D-ribose and D-ribose. Both of them are present in their furanose form in nucleic acids (Fig. 16.3). These sugar residues can easily bend and twist into different conformations thus making nucleic acids dynamic in structure. Depending on the kind of pentose, nucleotides are subclassified into deoxyribonucleotides and ribonucleotides (Fig. 16.4). Ribonucleotides might contain A, G, C, and U while deoxyribonucleotides contain A, G, C, and T. The bases of DNA and RNA are important for the structure and e distribution of nucleic acids. 

Very important furanose molecules (five-membered rings) are commonly foimd in nature. For example, D-ribose and D-deoxyribose are foimd as the building imits of nucleic acids, and fructose is a constituent of sucrose. These rings are not planar, either one (envelope form) or two (twist form) atoms are out of the plane containing the others. The different envelope and twist conformations... 

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