Guanine
When DNA is extracted from E. coli cells and analyzed for base composition, it is found that 42% of the bases are cytosine. That means that there is also 42% of guanine base.
According to chargaff's rule :
DNA from any cell of all organisms should have a 1:1 ratio (base Pair Rule) of pyrimidine and purine bases and, more specifically, that the amount of guanine is equal to cytosine and the amount of adenine is equal to thymine.
