Explain why a purine base (Adenine & Guanine) must be paired with a pyrimidine base (Thymine &

Cytosine)

Because hydrogen bonds are not as strong as covalent bonds, base pairings can easily be separated, allowing for replication and transcription. Because purines always bind with pyrimidines - known as complementary pairing - the ratio of the two will always be constant within a DNA molecule.