What did Avery, MacLeod, and McCarty identify as the heritable substance?

Avery, MacLeod, and McCarty conducted groundbreaking experiments in the early 1940s that demonstrated DNA is the substance responsible for heredity. They were able to isolate and purify the transforming principle from Streptococcus pneumoniae bacteria, revealing that DNA could transfer genetic information and thereby change the characteristics of a cell. Their findings provided strong evidence that DNA, rather than RNA, proteins, or carbohydrates, is the molecule that carries genetic information.

In their experiments, they showed that when they treated heat-killed virulent strains of bacteria with enzymes that degraded proteins and carbohydrates, the transforming ability remained intact, suggesting that these biomolecules were not responsible for heredity. It was only when they used enzymes that degraded DNA that the transformation was inhibited, confirming that DNA was essential for the transfer of genetic traits. Thus, they conclusively identified DNA as the heritable substance in living organisms.