Two strands of DNA - are held together by

The two strands of the DNA double helix are primarily held together by hydrogen bonds between complementary nitrogenous bases. Specifically, adenine (A) pairs with thymine (T) through two hydrogen bonds, while guanine (G) pairs with cytosine (C) through three hydrogen bonds. These bonds are non-covalent and occur between the hydrogen bond donors (like NH groups) and acceptors (like nitrogen or oxygen atoms) on the opposing bases. While covalent phosphodiester bonds form the sugar-phosphate backbone of each individual strand, they do not link the two strands together. Other forces like van der Waals interactions and hydrophobic effects contribute to the vertical stacking and overall stability of the helix, but the horizontal pairing that bridges the two strands is the result of specific hydrogen bonding.