Chargaff's Rules for Base Pairing in DNA

Proposed by: Erwin Chargaff (1950)

• Species Specificity: DNA base composition is identical for all individuals within a single species, but it varies between different species.

• Equivalence Rule: Regardless of the DNA source, purines and pyrimidines are always present in equal amounts: A + G = T + C.

Base Pairing & Proportions:

• Adenine (A) always pairs with Thymine (T). Therefore, they are equimolar (A = T).
• Cytosine (C) always pairs with Guanine (G). Therefore, they are equimolar (G = C).

Base Ratios:

• The total amount of (A + T) is not necessarily equal to the total amount of (G + C). For instance, in human DNA: A = 30.9%, T = 29.4%, G = 19.9%, and C = 19.8%.
• The ratio of (A + T) / (G + C) remains constant for any particular species but will differ across different species.

Measuring Length: The length of a DNA molecule is represented by its total number of nucleotide pairs (base pairs).

Chemical Linkage: Two adjacent nucleotides are joined together by a phosphodiester bond. This occurs when one molecule of phosphoric acid joins with the sugar molecules of two nucleotides through an ester linkage.

Chain Nomenclature:

• 2 joined nucleotides = Dinucleotide
• 3 joined nucleotides = Trinucleotide
• Chain of up to 20 nucleotides = Oligonucleotide
• Chain of more than 20 nucleotides = Polynucleotide

• DNA base composition is unique to each species, consistently maintaining an equal overall amount of purines and pyrimidines (A + G = T + C).
• Adenine exclusively pairs with thymine in equal amounts (A = T), and cytosine exclusively pairs with guanine in equal amounts (G = C).
• The ratio of (A + T) to (G + C) acts as a constant identifier for a given species, even though the total amounts of these pairs are not necessarily equal.
• The length of a DNA molecule is determined by its total number of nucleotide base pairs, and these adjacent nucleotides are connected by phosphodiester bonds.
• Nucleotide chains are classified by their length, ranging from dinucleotides (two) to oligonucleotides (up to twenty) and polynucleotides (more than twenty).