Chargaffs

Chargaffs, or Chargaff's rules, are a set of empirical observations about the base composition of DNA formulated by the Austrian-American biochemist Erwin Chargaff in the mid-20th century. Through quantitative analysis of DNA from a wide range of organisms, Chargaff identified consistent patterns in how the four nucleotides are distributed.

The first parity rule states that in any double-stranded DNA molecule, the amount of adenine (A) equals thymine (T), and the amount of cytosine (C) equals guanine (G). In other words, A = T and G = C, so purines and pyrimidines balance within the double helix. This observation strongly supported the idea of base pairing between complementary strands.

The second parity rule extends the observation to individual DNA strands: in many cases, the amount of A roughly equals T and the amount of G roughly equals C within a single strand, even though the molecule is not yet paired. This is an empirical rule and not universal, but it highlights a broader balance in nucleotide composition.

Impact and significance: Chargaff's rules provided crucial evidence for complementary base pairing and influenced the development of the Watson–Crick model of DNA as a double helix. They also revealed that base composition varies among species, arguing against a universal DNA composition and establishing a foundation for later studies in genomics and molecular evolution.

Today, Chargaff's rules remain a foundational concept in genetics, used as a general guideline for interpreting DNA sequences and understanding mutational and evolutionary patterns, with acknowledged exceptions in certain genomes and conditions.