Adenine pairs with thymine because

Adenine pairs with thymine because one is a purine base (adenine) and the other is a pyrimidine base (thymine). The structure of DNA requires that the nitrogenous bases, which make up the "rungs" of the DNA ladder, pair in a specific way: purines (adenine and guanine) must pair with pyrimidines (thymine and cytosine). Adenine has a complementary hydrogen-bonding pattern with thymine, which allows them to pair together, forming two hydrogen bonds between them. This specific pairing between the bases is important for DNA replication and the accurate transmission of genetic information.