* Adenine (A) always pairs with Thymine (T)

* Guanine (G) always pairs with Cytosine (C)

This rule is essential for the structure and function of DNA.

Here's why:

* Complementary base pairing: The specific pairing of A with T and G with C creates a "complimentary" strand of DNA. This means each strand holds the information needed to create the other strand.

* Double helix structure: The base pairing, along with the sugar-phosphate backbone, allows the DNA to form a double helix structure. This structure is crucial for compacting and protecting the genetic information.

* Accurate replication: During DNA replication, the two strands separate, and each strand serves as a template to create a new complementary strand. The base pairing rule ensures that the new strands are exact copies of the original strands, preserving the genetic information.

* Protein synthesis: The sequence of bases in DNA determines the sequence of amino acids in proteins, which are essential for all biological processes.

The base pairing rule is a fundamental principle of molecular biology and underpins all of DNA's functions.