* Adenine (A) pairs with Thymine (T) in DNA.

* Adenine (A) pairs with Uracil (U) in RNA.

* Guanine (G) pairs with Cytosine (C) in both DNA and RNA.

This pairing is based on the specific hydrogen bonding between the nitrogenous bases:

* A-T/U: Two hydrogen bonds form between adenine and thymine/uracil.

* G-C: Three hydrogen bonds form between guanine and cytosine.

This complementary base pairing is crucial for the structure and function of nucleic acids:

* DNA replication: The two strands of DNA separate, and each strand serves as a template for the synthesis of a new complementary strand, ensuring accurate duplication of the genetic information.

* RNA transcription: DNA's genetic code is transcribed into mRNA, using complementary base pairing to create a messenger molecule carrying the genetic information to the ribosomes.

* RNA translation: The mRNA sequence is translated into a protein, with the codons (three-base sequences) on mRNA pairing with their corresponding anticodons on tRNA, bringing the correct amino acids to the ribosome.

Therefore, the base pairing rules are fundamental for the accurate replication, transcription, and translation of genetic information.