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Chargaff's Additional Discovery

Module by: Laura Martin

In addition to discrediting the tetranucleotide hypothesis and reviving the possibility that DNA maybe sufficiently structurally complex (and variable among species) to function as the molecule of heredity, Chargaff’s careful analytical work revealed one more pattern that would eventually prove very useful for describing the three dimensional structure of the DNA molecule. As Chargaff noted in his 1950 paper,

It is, however, noteworthy – whether this is more than accidental, cannot yet be said –
that in all desoxypentose nucleic acids [DNA] examined thus far the molar ratios of total
purines [A and G] to total pyrimidines [C and T] , and also of adenine to thymine and of
guanine to cytosine, were not far from 1. (Chargaff, 1950, p.206)

1. In your own words, please explain what Chargaff observed as described above in his own words.

2. Review the data in Figure 1. Do agree with Chargaff’s interpretation of his data as expressed in the excerpt above? Why or why not? Please explain and support your conclusion with data.

3. Ultimately, Chargaff’s interpretation of the data proved correct; the nucleotides A and T (and C and G) occur in equal quantities in DNA regardless of the species from which the DNA comes. Please offer a model of the physical arrangement of these molecules that could explain this recurrent pattern.

Works Cited

  • Chargaff, E. 1950. Chemical specificity of nucleic acids and mechanism of their enzymatic degradation. Experientia. 6:201-240.

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