Final answer:
The two chains of the DNA double helix are primarily held together by hydrogen bonds, which are weaker than covalent or ionic bonds but essential for DNA's structure and function.
Step-by-step explanation:
The two chains of the DNA double helix are held together by hydrogen bonds. These are weaker types of bonds compared to covalent or ionic bonds, requiring less energy to break. However, the hydrogen bonds between the nitrogenous bases of the DNA strands are essential for the structure's stability and play a vital role in replication and other cellular processes.
Other types of bonds like covalent bonds are found within each DNA strand, linking the sugar and phosphate groups to form the backbone of the strand. In contrast, the hydrogen bonds specifically form between the nitrogenous base pairs, adhering the two strands together in the characteristic double helix shape. An example of such a hydrogen bond is between the bases adenine and thymine, which typically form two hydrogen bonds, whereas cytosine and guanine form three.