* Adenine (A) pairs with Thymine (T)

* Guanine (G) pairs with Cytosine (C)

This is known as complementary base pairing. The two strands of DNA are held together by hydrogen bonds between the bases. Adenine and thymine form two hydrogen bonds, while guanine and cytosine form three hydrogen bonds.

These pairing rules are essential for:

* DNA replication: During replication, the two strands of DNA separate, and each strand serves as a template for the synthesis of a new complementary strand.

* Transcription: During transcription, the DNA sequence is copied into RNA, using the same base pairing rules, except uracil (U) replaces thymine in RNA.

* Genetic code: The order of the bases in DNA determines the order of amino acids in proteins. The genetic code is read in groups of three bases, called codons.

Let me know if you'd like more detail on any of these processes!