Final answer:
Hydrogen bonds between base pairs hold the two strands of a double-stranded DNA molecule together, allowing it to maintain its double helix structure while also enabling it to unzip as required for replication and transcription.
Step-by-step explanation:
The bonds that hold the two strands of a double-stranded DNA molecule together, while individually weak, are the hydrogen bonds between base pairs. Each base pair is connected by hydrogen bonds, which are not as strong as phosphodiester bonds found within the sugar-phosphate backbone of the DNA, but when combined in the millions across the length of DNA, these hydrogen bonds provide significant strength to stabilize the double helix structure. The base pairs, including adenine with thymine and cytosine with guanine, are stabilized inside the double helix through this hydrogen bonding.
Due to their relatively weak nature, hydrogen bonds allow the double-stranded DNA to 'unzip' during processes such as replication or transcription, making each strand accessible as a template. This unzipping is essential for the biological functions that DNA performs, such as storing and transferring genetic information.