ChargaffRegeln

ChargaffRegeln, commonly known as Chargaff's rules, are two foundational observations about DNA base composition named after the biochemist Erwin Chargaff. They describe regularities in the amounts of nucleotides within double-stranded DNA and helped establish the concept of base pairing.

The first rule states that in double-stranded DNA the amount of adenine (A) equals thymine (T), and the amount of cytosine (C) equals guanine (G). This parity reflects the complementary pairing of A with T and C with G, the basis for the DNA double helix discovered later by Watson and Crick.

The second rule, often called Chargaff's second parity rule, notes that DNA base composition varies between species but is constant within a species. In practical terms, the total purine content (A+G) is balanced with the total pyrimidine content (C+T), and different organisms exhibit characteristic GC contents. This species-specific variability in base composition informed comparative genomics and evolutionary studies.

Historically, Chargaff's analyses in the 1940s and 1950s revealed the regularities of base pairing and species-specific base composition, providing crucial constraints that complemented the DNA structure model. Limitations of the rules include their applicability mainly to well-formed, double-stranded DNA; single-stranded DNA, RNA, and many viral genomes do not necessarily adhere to the same constraints. Nevertheless, Chargaff's rules remain a foundational concept in molecular biology and genomics.