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Chargaff's rules
Erwin Chargaff proposed two rules which are termed as Chargaff's rules.
These rules played an important role in the discovery of the double helix
structure of DNA.
F
First rule, in any double-stranded DNA the number of guanine(G)
units equals the number of cytosine(C) units and the number of adenine(A)
units equals the number of thymine(T) units.
F
Second rule, the composition of DNA varies from one species to
another.
Hence, in dsDNA
1. The molar
ratio of A to T equals to 1. Similarly the molar ratio
of G to C equals to 1.
2. The sum of
the purines (A and G) equals that of the pyrimidines (C
and T).
sum of (A + G) = sum of (C + T)
3. The percentage
(%) of C + G does not necessarily equal the percentage (%)
of A + T.
ΓΌ Trick to remember – “PURE AS GOLD” – purines are A & G