What kind of attractive force occurs between the two DNA strands in a double helix?

The structure of DNA is often compared to a twisted ladder, known as the double helix. The sides of this 'ladder' are made up of alternating sugar and phosphate groups, which form the backbone of the DNA molecule. The rungs, however, are composed of pairs of nitrogenous bases.
Nucleic acids, which make up DNA, are long polymers that carry genetic information. Each nucleotide within these acids consists of a sugar, a phosphate group, and a nitrogenous base. It is the sequence of these bases that encode our genetic information.
Furthermore, the DNA double helix is antiparallel, meaning one strand runs in the 5' to 3' direction, while the complementary strand runs in the opposite 3' to 5' direction. This orientation is crucial for DNA replication and for various enzymatic processes that DNA undergoes.

Nucleic acid base pairing is an elegant feature of DNA's molecular structure, which is pivotal for both its stability and replication. There are four types of nitrogenous bases in DNA: adenine (A), thymine (T), cytosine (C), and guanine (G). Adenine always pairs with thymine, while cytosine pairs with guanine.
Base pairing is facilitated by hydrogen bonds, a not-too-strong but extraordinarily specific interaction that occurs between the paired bases. Adenine and thymine form two hydrogen bonds, whereas cytosine and guanine form three, which provides a consistent structure and enough strength to maintain the integrity of the DNA strands while still allowing them to separate when needed.
This specificity in base pairing is also the foundation for DNA replication and repair, ensuring genetic information is accurately passed down from cell to cell, and from one generation to the next.

Biological molecules, including DNA, proteins, and carbohydrates, are governed by various chemical bonds, which determine their structure and function. The primary types of chemical bonds include covalent bonds, ionic bonds, hydrogen bonds, and van der Waals forces.
In DNA, covalent bonds link the sugar of one nucleotide with the phosphate group of the next, forming the sugar-phosphate backbone. Hydrogen bonds, although weaker than covalent bonds, are instrumental in base pairing and contribute to the overall three-dimensional structure of the molecule, without compromising the ability of the helix to unzip for replication or protein synthesis.
Ionic bonds occur often in proteins, especially between amino acid side chains, and help stabilize their structure. Van der Waals forces are also present in biological molecules and contribute to protein folding by affecting how molecules fit together in three-dimensional space.