AAMC FL Practice Exam

What type of bond connects complementary base pairs in DNA?

1. Phosphodiester bonds

2. Hydrogen bonds

3. Covalent bonds

4. Ionic bonds

The correct answer is: Hydrogen bonds

The correct answer is that hydrogen bonds connect complementary base pairs in DNA. In the structure of DNA, the two strands are held together by these bonds formed between the nitrogenous bases on opposite strands. For instance, adenine pairs with thymine, while guanine pairs with cytosine. Specifically, adenine and thymine are connected by two hydrogen bonds, and guanine and cytosine are linked by three hydrogen bonds. This type of bonding is crucial for the stability of the DNA double helix while allowing for the strands to separate easily during processes such as replication and transcription. The relative weakness of hydrogen bonds enables the DNA strands to dissociate when necessary, which is vital for genetic expression and reproduction. In contrast, phosphodiester bonds are found within a single strand of DNA, linking the sugar of one nucleotide to the phosphate group of the next nucleotide, thereby forming the backbone of the DNA strand. Covalent bonds also primarily refer to the stronger bonds within individual nucleotides rather than between them. Ionic bonds are not involved in base pairing within the structure of DNA; they are generally weaker and occur in different contexts, such as interactions between charged molecules.