What interactions contribute the most to holding the two strands of dna together?

Mainly by Hydrogen bonds and the Van der Waals forces

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.

Answer:Hydrogen Bonds

Explanation:

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.