Chargaff and DNA

In about 1950 Erwin Chargaff published a study in which he compared the base composition of DNA extracted from different species.  Because Avery had suggested in the mid-40s that DNA might have something to do with inheritance, it seemed reasonable to expect that if this turned out to be true, different organisms might have DNAs that were different from one another.  

Since in nucleotides every base is associated with a sugar and a phosphate group, Chargaff determined the percentage of each base in the DNA of a given species, by comparing the concentration of each base for 100 phosphate units.  Examine the chart to see  how the percentages of the different bases vary from species to species.  For example compare Diplococcus pneumoniae (an older name for the pneumococcus) with Mycobacterium tuberculosis.  Compare the herring (a fish) DNA with the human DNA.  Notice that the DNA from different human tissues is about the same.  

However, Chargaff noticed a regularity about the percentage of different bases in a species that soon became known as Chargaff's Rule: 

    The pyrimidines (thymine and cytosine) were about equal to the purines (adenine and guanine)
    Moreover specific purines and pyrimidines were present in equal concentrations (within the limits of measurement error)
            Adenine and Thymine were about the same
            Guanine and Cytosine were about the same

What was different from one species to another was the ratios of these two sets of bases:  A + T / G + C.  

Chargaff's Rule became an important clue for figuring out the structure of DNA.