What holds the double strands of DNA together?

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covalent bonds link the strands to each other
hydrogen bonds hold the base pairs together
ionic bonds keep the deoxyribose linked to the phosphate groups

both covalent bonds and hydrogen bonds are involved in holding the double strands of DNA together. Covalent bonds link the sugar-phosphate backbones of the strands, forming the overall structure of the DNA molecule. Hydrogen bonds, on the other hand, hold the complementary base pairs (adenine with thymine, and guanine with cytosine) together within the double helix. Ionic bonds are not directly involved in holding the double strands of DNA together.