What type of bond primarily holds the two strands of a DNA helix together?

The two strands of a DNA helix are primarily held together by hydrogen bonds. These bonds occur between the nitrogenous bases of the nucleotides on opposite strands of the DNA. Specifically, adenine pairs with thymine through two hydrogen bonds, while guanine pairs with cytosine through three hydrogen bonds. This specific pairing is critical for maintaining the structure of the double helix and allows for the complementary nature of DNA, which is essential for processes such as DNA replication and transcription.

Covalent bonds, while important for connecting the sugar and phosphate backbone of each nucleotide within a strand, do not hold the two strands together. Ionic bonds play significant roles in other biological interactions but are not involved in holding together the bases of DNA strands. Disulfide bonds, which involve sulfur atoms and typically stabilize protein structures, are not applicable to the two strands of DNA, as these strands do not contain the necessary functional groups to form such bonds.