Thus although the spatial requirements of B-DNA potentially allow four complementary base pairs to be formed (i.e., G-T, G-C, A-T, and A-C), only the G-C and A-T base pairs are normally found in DNA. The hydrogen bonding between complementary base pairs is such that the most energetically stable DNA configuration is achieved when adenine pairs with thymine and guanine pairs with cytosine. The B-DNA helix makes one full revolution approximately every 10.1 to 10.6 nucleotides (Wolfe 1993). The average distance between adjacent nucleotides in the same strand of DNA (the vertical rise) is between 0.321 to 0.337 nm (Wolfe 1993). Other DNA conformations have diameters that range from about 1.8 to 2.3 nm (Wolfe 1993). The average diameter of B-DNA is about 2.0 nm. B-DNA is the most common form of DNA found in living organisms. At least six different DNA conformations (designated A, B, C, D, E, and Z) have been identified, but only the A, Z, and B conformations are found in nature (Wolfe 1993). ![]() Other DNA conformations use the same nucleotides and molecular bonds, but the three-dimensional structure of the helix is different. Idealized schematic of the DNA double helix (courtesy Dr. The helical structure described by Watson and Crick, called B-DNA, is only one of several possible conformations.įigure 1. Then, the sugar-phosphate linkages in one strand would proceed from the 5´ to 3´ carbon, and the sugar-phosphate linkages in the other strand would proceed in 3´ to 5´ direction. Suppose the double helical structure of DNA could be unwound and both strands of DNA laid side-by-side (refer to Figure 1). The alternating sugar-phosphate groups in each DNA strand form the so-called backbone of DNA, and they also confer a directionality. The structure of the DNA helix is stabilized by van der Waals forces, hydrogen bonds between complementary organic bases (a base pair), and hydrophobic interactions between the nitrogenous bases and the surrounding sheath of water. A DNA molecule is composed of two unbranched polynucleotide chains (strands) that wind about each other into a structure called a double helix, as illustrated in Figure 1. ![]() The diameter of the DNA helix is 2 nm and the vertical rise per base pair is 0.34 nm (Van Holde 1989). In DNA, the term nucleotide refers to the complete assemblage of a nitrogenous base (A, G, C, or T), a five-carbon deoxyribose sugar, and a phosphate group. Phosphate groups are attached to the third (3´) and fifth (5´) carbon atoms. ![]() Adenine and guanine are purines, and cytosine and thymine are pyrimidines. The first carbon atom, the 1´ carbon, is by definition the carbon atom covalently attached to one of four organic bases: guanine (G), adenine (A), thymine (T), or cytosine (C). The carbon atoms of the deoxyribose sugar are designated by numbering them sequentially from one to five. ![]()
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