Answer: Adenine
Step-by-step explanation:
The structure of the DNA double helix is complex in nature. There are two strands of DNA that are wound around each other. The nitrogenous bases are bonded with hydrogen bonding and base complementarity. According to the Chargaff's rule of base complementarity adenine always pairs with the thymine and guanine with cytosine. These nitrogenous bases are paired on the basis of hydrogen bonding. Adenine bonded with thymine through two hydrogen bonds whereas the cytosine pairs with guanine via three hydrogen bonds. During DNA denaturation these hydrogen bonds are broken whereas during DNA replication these hydrogen bonds are formed between the nitrogenous bases.