Importance of Chargaff’s rules

A great scientist behind DNA double helix

Between 1940s and 1950s, Austrian-American biochemist Erwin Chargaff used various technique to reveal nucleotide composition of DNA molecules.

At the time, it had been already known that DNA nucleotides have three main elements which are a deoxyribose sugar, a phosphate, and one of four nitrogen bases. Chargaff isolated DNA from a number of different organisms and revealed that composition of DNA bases (percentages of A, T, G, and C) varies widely from one organism to another.

After his famous experiments, Chargaff had established two important rules.

Chargaff’s rules

1) DNA material isolated from different organs of the same organism have exactly same DNA base composition. And this pattern does not depend on the organism age or environmental factors.

Adenin-thymine and guanine-cytosine base pairs.

2) Relative proportions of the DNA bases were not random. In each species, number of adenosine bases (A) is always equals that of thymine bases (T), as are those guanine bases (G) to cytosine bases (C). The A/T ratio and G/C ratio is equal to one. In other word the proportion of pyrimidines (T + C) to purines (A + G) always equal percentage. However, ratio of (A + T)/ (G + C) differs from one species to another.

Percentage of Base in DNA  Ratios
Origin of DNA A T G C A/T G/C (A+T)/(G+C)
Human 30,4 30,2 19,2 19,7 1,01 0,97 1,04
Salmon 29,6 29,3 20,4 20,6 1,01 0,99 1,02
Turtle 28,5 27,6 22,3 21,7 1,03 1,02 1,00
Escherichia coli 25,0 26,3 24,9 25,6 0,95 0,97 0,97
Drosophila 27,0 27,9 21,8 21,9 0,96 0,99 0,97

Subsequently, following studies have confirmed these equivalencies by various techniques.  However, these results was not enough to propose a workin model for DNA structure.  On the other hand, Chargaff’s result became important evidence for Watson and Crick to elucidate molecular model of DNA double helix.

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