Here's a breakdown:

* Adenine (A) always pairs with Thymine (T) (in DNA) or Uracil (U) (in RNA) through two hydrogen bonds.

* Guanine (G) always pairs with Cytosine (C) through three hydrogen bonds.

These hydrogen bonds are relatively weak individually, but collectively they form a strong bond between the two strands of DNA or RNA, holding the molecule together.

Why is this important?

* Genetic Code: The specific pairing of nitrogen bases forms the basis of the genetic code. The sequence of bases determines the order of amino acids in proteins, ultimately controlling the traits of an organism.

* DNA Replication: During replication, the two strands of DNA separate, and each strand acts as a template for the creation of a new complementary strand. The pairing rules ensure that the new strands are exact copies of the original.

* RNA Transcription: During transcription, DNA's genetic code is copied into RNA. Again, the pairing rules ensure accurate copying of the information.

Let me know if you'd like to dive deeper into any of these topics!