<h2>Mainly by Hydrogen bonds and the Van der Waals forces
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Explanation:
The DNA double helix (both strands of DNA) are held closs together by i) mainly hydrogen bonds between complementary bases and ii) the Van der Waals base-stacking interaction.
The hydrogen bonds between the base pairs form the double helical structure of DNA. There is no exchange or sharing of electrons in hydrogen bonds.
G always pairs with C with three hydrogen bonds and T always pairs with A with two hydrogen bonds.
In DNA double helix structure , there's four nitrogen bases Adenine , Thymine, Cytosine and Guanine . Hydreogen bonds formed between these nitrogenous base in DNA . In DNA double helix , two hydrogen bonds exist between Adenine and Thymine while three hydrogen bond exist between Cytosine and Guanine . Pairing of nitrogen bases in DNA double helix is very specific and this specific and selective pairing results in 'complementary base pairing ' . Hydrogen bonds doesn't involve in sharing of electrons , it occur over short distance and it can be formed and broken easily . In DNA double helix , hydrogen bond is weaker but still contribute the most to holding the two strands of DNA together because hydrogen bonds present excessively in between the DNA double helix .
