Final answer:
In the structure of a DNA double helix, adenine pairs with thymine and guanine pairs with cytosine. This is a fundamental rule known as Chargaff's rules that ensures the fit and sequence alignment of the DNA strands.
Step-by-step explanation:
In a DNA double helix, the base adenine (A) always pairs with thymine (T), and the base guanine (G) always pairs with cytosine (C). This is known as the base pairing rule or Chargaff's rules, named after the biochemist Erwin Chargaff. This rule is fundamental to the double helix structure of DNA as it guarantees the base pairs will fit into the helix and the sequence of one strand defines the sequence of the other.
Learn more about DNA Base Pairing