7. The two strands of DNA double helix are linked to each other by
a. Hydrogen bonds between sugar
b. Phosphodiester bond between sugar
c. Hydrogen bonds between bases
d. Phosphodiester bond between bases
The correct answer is c. Hydrogen bonds between bases. In the DNA double helix, the two antiparallel strands are held together primarily by hydrogen bonds formed between complementary nitrogenous bases: adenine (A) pairs with thymine (T) via two hydrogen bonds, and guanine (G) pairs with cytosine (C) via three hydrogen bonds. These non-covalent interactions stabilize the double helix structure while allowing strands to separate during replication and transcription. Phosphodiester bonds, in contrast, form covalent linkages within each individual strand’s sugar-phosphate backbone.
Option Analysis
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a. Hydrogen bonds between sugar: Incorrect, as sugars (deoxyribose) in the backbone do not form hydrogen bonds with each other across strands; they are linked covalently within strands.
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b. Phosphodiester bond between sugar: Incorrect, since phosphodiester bonds connect the 3′ carbon of one deoxyribose to the 5′ phosphate of the next within the same strand, not between strands.
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c. Hydrogen bonds between bases: Correct, as these specific bonds (2 for A-T, 3 for G-C) link complementary bases between the two strands, maintaining the helix.
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d. Phosphodiester bond between bases: Incorrect, because bases project inward and pair via hydrogen bonds, not phosphodiester bonds, which are exclusive to the backbone.
This question tests core concepts in molecular biology for exams like CSIR NET Life Sciences, emphasizing the distinction between intra-strand covalent bonds and inter-strand weak bonds essential for DNA function.
