Mol.bio lect.02 Chargaffs Rule

Chargaff’s rule is a fundamental principle in Molecular Biology that describes the base composition of DNA. 📘 The rule states: In any double-stranded DNA molecule: The amount of adenine (A) is equal to thymine (T) The amount of guanine (G) is equal to cytosine (C) So: A = T G = C 🧬 Why this happens This rule exists because of base pairing in DNA: Adenine pairs only with thymine Guanine pairs only with cytosine This pairing structure was key to the discovery of the DNA double helix by James Watson and Francis Crick. 📊 Example If a DNA sample contains: 30% adenine → it must also contain 30% thymine Remaining 40% is split equally → 20% guanine and 20% cytosine ⚠️ Important note Chargaff’s rule applies to double-stranded DNA. It does not necessarily apply to: Single-stranded DNA RNA If you want, I can give you practice questions or show how this rule appears in exams.