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How do hydrogen bonds hold the two DNA strands together?

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Diagram of a DNA polynucleotide chain showing linked deoxyribose sugars, phosphate groups, and nitrogenous bases (adenine, guanine, cytosine, thymine), with an inset comparing ribose and uracil in RNA. — Nucleic acid A portion of the polynucleotide chain of deoxyribonucleic acid (DNA), with an inset showing the corresponding pentose sugar and pyrimidine base in ribonucleic acid (RNA).

In the DNA double helix, the two polynucleotide strands are held together by hydrogen bonds that form between complementary nitrogenous bases located on the interior of the molecule.12

These bases pair according to specific rules:
* Adenine (A) always pairs with Thymine (T) via two hydrogen bonds.32
* Guanine (G) always pairs with Cytosine (C) via three hydrogen bonds.32

Because the spans of these A:T and G:C pairs are nearly identical, they allow the two sugar-phosphate backbones to bridge uniformly, maintaining the stability of the double helix structure. These hydrogen bonds are essential for the structural integrity of DNA, yet they are weak enough to be broken by heat, a process known as denaturation, which allows the strands to separate for biological processes such as replication.34