Because the reason why airplane have closed windows is because we can't breathe in carbon dioxide when we go up. Of course there is oxygen just not a lot as we go up. Why would there be helium in the air? We would sound weird when we skydive.
Chargaff's rules state that DNA from any species of any organism should have a 1:1 stoichiometric ratio of purine and pyrimidine bases, that the amount of guanine should be equal to cytosine and the amount of adenine should be equal to thymine.
