 Do you have the same color eyes as your mom or the same hair color as your dad? Just by observing our appearance, we get a sense that some traits are heritable. They get passed on from parent to offspring. But how does heritable information get transmitted from one generation to the next? In the 1940s, this was a complete mystery. We are going to tell you the story of three scientists, Oswald Avery, Colin MacLeod and Macklin McCarty, who set out to solve this puzzle. Their pioneering research provided proof that deoxyribonucleic acid or DNA contains the blueprint of who we are and what we look like. That DNA is in fact the heritable information. This finding may seem obvious now, but back in the 1940s it was incredibly controversial. Most scientists thought that DNA was too simple to contain all of the information for every single heritable trait and instead, they thought proteins, which are large and complex molecules, must be the substance passed on from parent to child. At the start of our story, our three scientists wanted to follow up on an important finding from a scientist named Frederick Griffith. Griffith worked with a type of bacteria that came in two strains, one that looked smooth and one that looked rough. If he injected the smooth strain into mice, they died. But if he injected the rough strain, the mice lived. To his surprise, if he first killed the smooth bacteria, these dead bacteria did not kill the mice. But if he injected a mixture of dead smooth bacteria and live rough bacteria, the mice died. This meant that amazingly something from the smooth strain was transferred to and permanently inherited by the rough strain, a process he called transformation. And this caused the rough bacteria to become deadly. This exciting finding left a big puzzle that Avery McLeod and McCarty wanted to solve. What was the heritable information that caused the transformation of the rough strain into the deadly smooth bacteria? The scientists purified a chemical substance from the smooth bacteria and they repeated Griffith's results with it. When they applied the substance to the rough bacteria, those rough bacteria transformed into smooth bacteria. And this trait was heritable and permanent. But the questions still remained, what type of molecule was this heritable information? At the time most people, including Avery, thought the answer was protein. But they found that enzymes that destroy proteins could not block the activity of the substance. It still made rough bacteria look smooth. This meant the substance was not protein. So could it be DNA instead? When they added an enzyme that destroys DNA, but does not destroy protein, the transforming activity disappeared. Avery and colleagues confirmed that the heritable information had a chemical signature that matched DNA and that their preparation had very little contaminating protein or other molecules. The researchers came to the surprising conclusion that DNA was in fact the heritable information. Despite the beautiful evidence from these experiments, many scientists questioned whether some contaminating protein might account for the results. It took another experiment by Hershey and Chase in 1952, described in another video, and the discovery of the double helical structure of DNA in 1953 to convince everyone beyond any doubt that DNA is the molecule of heridity. Once people were convinced that DNA was the heritable information, it sparked a wave of new research questions that served as the foundation for biotechnology revolutionizing modern medicine. Many new treatments for human diseases rely on our ability to work with DNA and even pass genetic information from organism to organism. One impactful example is in treating patients with type 1 diabetes who require daily insulin injections. Previously, insulin from livestock was used, but now human insulin DNA is transferred to bacteria which act as biological factories to produce human insulin for patients. So the early discovery of DNA transformation by Avery McLeod and McCarty has had long lasting benefits for humankind.