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Which of the following statements describes both the spiral structure of double-stranded DNA and the spiral structure found in certain segments of protein?
1. They are repeating spiral structures with intervals of pleated sheets
2. They have four alternative units arranged in polymeric chains
3. They are held together by hydrogen bonding
4. They are α-helical
5. They have covalently linked backbones
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The Correct Answer is: They have covalently linked backbones
Explanation:
Double-stranded DNA is arranged in a double helix as originally deduced by Watson and Crick. The double-helical structure of duplex DNA is different than the α-helical structure of portions of proteins. The α-helical structure of proteins is formed of one chain of proteins stabilized by individual hydrogen bonds between components of the amide bonds, that is, between the carbonyl oxygens and the amide nitrogens. In contrast, the hydrogen bonding in doublestranded DNA is important to allow each strand to act as a template for the other complementary strand, with adenine bonding to thiamine and cytosine bonding to guanine. Hydrophobic stacking between bases in the hydrophobic interior of the double strand actually makes a greater contribution to the stability of the DNA double helix than does hydrogen bonding. DNA and protein helices are both composed of polymers of subunits (amino acids and nucleotides) held together by a covalently linked backbone. As pointed out above, hydrogen bonding is important to both the double helix of DNA and the α-helix of proteins. Finally, both are, in fact, spiral structures, although only the helix of proteins is an α helix.
