What are the two strands of DNA held together by?

The two strands of DNA are held together by hydrogen bonds, which are formed between the nitrogenous bases on the opposite strands. In the DNA double helix structure, adenine pairs with thymine and cytosine pairs with guanine; these base pairs are connected by hydrogen bonds.

The hydrogen bonds are crucial as they allow the DNA strands to separate during processes such as DNA replication and transcription, while still providing enough stability to maintain the overall structure of the DNA molecule. This specific type of bonding contributes to the unique properties of DNA, enabling it to carry genetic information and participate in cellular processes effectively.

In contrast, peptide bonds are associated with linking amino acids in proteins, covalent bonds refer to strong connections between atoms within a single strand of DNA (such as between the sugar and phosphate groups), and ionic bonds involve the attraction between charged molecules or atoms, which doesn't play a role in the hydrogen bonding between the DNA strands. Thus, hydrogen bonds are the correct answer as they specifically connect the two DNA strands.