In the 1940s, ErwinChagaff made the remarkable observation that in samples of DN
ID: 3631 • Letter: I
Question
In the 1940s, ErwinChagaff made the remarkable observation that in samples of DNA fromawide range of organisms, the mole percent of G [= G/(A+T+G+C)] wasequal to the mole
percent of C, and the mole percents of A and T were also equal.This observation was an
essential clue to the deciphering of DNA structure by Watson andCrick in 1952.
(i) What is the structural basis for Chagaff’s rules?
(ii) However, Chagaff’s rules do not appear to be universal,as genetic material (i.e., nucleic
acids) from the f174 virus has mole percents of A = 24%, T = 31%, G= 23%, and C = 22%.
Why does this viral genetic material not obey Chargaff’srule?
Explanation / Answer
(i) Chargaff's rule is basically a description of what later was shown to be fact, that Adenines only pair with Thymines and Guanines only pair with Cytosine. As a result of this rule, the number of A=T and G=C (because for every A or G there is a complementary T or C, respectively). Adenine and guanine have double-ringed chemical structures and are called purines, while cytosine and thymine are smaller and have only single-ringed structures (they are therefore called pyrimidines). Purines can only pair (through H-bonding) with pyrimidines, meaning that only A can pair with Thymine and only Guanine can pair with Cytosine. This is because pyrimidine-pyrimidine pairings are not favored energetically, as H-bonds can't be established due to the fact that they are too far apart from each other. Purine-purine pairings are also unfavorable because the molecules are so close to each other that they repel each other. The only other combinations left are A/T and G/C or A/C and T/G. AC and TG don't pair because, due to the structure of each base, the particular Hydogen acceptor and donor molecules don't match up correctly and therefore can't form H-bonds. The only pairings where the hydrogen acceptors and donors are in the correct alignment and at the correct distance to form bonds are between A/T or G/C base pairs. Hope this helps!
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