USA Biology Olympiad (USABO) Practice Exam

What type of bond holds together the two strands of DNA?

1. Ionic bonds

2. Covalent bonds

3. Hydrogen bonds

4. Disulfide bonds

The correct answer is: Hydrogen bonds

The strands of DNA are held together primarily by hydrogen bonds. Each strand of DNA is made up of nucleotide units, and these nucleotides are linked together by covalent bonds within the strand itself, forming the backbone of the DNA. However, when it comes to the interaction between the two strands in the double helix structure, it is the hydrogen bonds that connect complementary base pairs from opposite strands. For example, adenine pairs with thymine through two hydrogen bonds, and cytosine pairs with guanine through three hydrogen bonds. These hydrogen bonds are crucial because they provide the necessary stability to the DNA structure while still allowing the strands to separate during processes such as replication and transcription, which is essential for DNA function. In contrast, ionic bonds involve the attraction between charged particles, covalent bonds involve the sharing of electron pairs between atoms, and disulfide bonds primarily occur in proteins, linking cysteine residues through sulfur-sulfur bonds. Thus, the unique role of hydrogen bonds in connecting the two strands of DNA is central to the molecule's double helical structure and function.