So when the PH of the cell equals the PKA of guanine, so when the PH is 9.7, the concentration of the protonated guanine will equal to the concentration of deprotonated guanine. That is exactly half of the guanine bases will be deprotonated. Now, that's a problem because guanine participates in hydrogen bonds which are responsible for holding the two strands of DNA together. And as we increase the PH, we basically decrease the amount of h bonds because we decrease the number of h atoms on the guanine and that destabilizes that entire double helix structure of DNA. And so within this range we see this very drastic decrease in the number of DNA molecules that exist in a double helix form because all those guanian molecules lose that h atom. So we see within this range we have a very, very drastic change in the y value in the percent of those DNA molecules that exist in a double helix form.