* Structure: Purines (adenine and guanine) have a double-ring structure, while pyrimidines (cytosine and thymine) have a single-ring structure.

* Hydrogen Bonding: The specific shapes of purines and pyrimidines allow them to form stable hydrogen bonds with each other. Adenine (A) forms two hydrogen bonds with thymine (T), and guanine (G) forms three hydrogen bonds with cytosine (C).

* Space: If two purines tried to pair, they would be too bulky and wouldn't fit within the double helix structure of DNA.

Therefore, purines can only pair with pyrimidines in DNA and RNA. This is known as Chargaff's rule, which states that the amount of adenine always equals the amount of thymine, and the amount of guanine always equals the amount of cytosine.