Here in this example question, it says human DNA is comprised of approximately 20% of adenine (A). It asks approximately what percentage of the nucleotides in a human DNA sample will be guanine (G). So we're going to list A, T, and then we have G and C. These are our base pairings. We're going to say collectively, they represent 100% of all the nitrogenous bases within a given species, in this case, a human being. Remember that the percentages between A and T have to be the same. So if Adenine is 20%, that means Thymine has to be 20%. So this equals 20%. This equals 20%. So we have 40% from just these two.
If we subtract that from 100%, that means we have 60% remaining. The 60% remaining represents G and C bases. Again, they have to be equal to each other in percentages as well. So of the 60%, 30% would have to be G, and 30% would have to be C. Again, according to Chargaff's rule, their percentages have to be equal. If A is 20%, T has to be 20%. They roughly have to be the same percentage. And then if you subtract that from 100%, that'll tell you your percentage left for G and C. They themselves also have to be roughly equal to each other. So just divide this number by 2 you would see that each one will be 30%. If we look at our options, the answer would have to be A. A human's DNA sample of guanine would have to be 30%.