When DNA is heated to 90-95℃ (according to the proportion of GC base pairs in DNA, the denaturation temperature will be higher if the proportion is high, and lower if the proportion is high, because there are three hydrogen bonds between GC base pairs and only two hydrogen bonds between AT), the hydrogen bonds between DNA double helix will be opened, thus the double strand will be untied, but this denaturation is different from protein's high temperature denaturation, and this denaturation is reversible. When the temperature drops below 65℃, the untied double-strand will form a double-helix structure again. Therefore, the DNA of S-type pneumococcus killed by heating still has genetic function and can transform R-type bacteria. Therefore, R-type bacteria are reduced or completely transformed into S-type bacteria, and the number of S-type bacteria is increased.