In this video, we're going to introduce the Griffith experiment. And so way back in 1928, a scientist named Frederick Griffith conducted a very important experiment. And his experiment helped to identify that some unknown genetic factor controls the traits of organisms. Now, of course, today, we know that the genetic factor that controls the traits of organisms is DNA. But way back in the early 1900s, it was unknown what the genetic factor actually was. Now, Frederick Griffith, what he did in his experiment was show that bacteria have the ability to transform this unknown genetic material. Now this term transform is a little bit different than the way that you might use transform in your everyday language. And so, transform or transformation in this context refers to the ability for these bacteria to uptake external DNA. And of course, uptaking external DNA is going to result in both a genotypic and a phenotypic change. And that is really what's meant by transformation in this context, the uptake of external DNA. And so Frederick Griffith showed that bacteria have the ability to transform genetic material or uptake external DNA from the surrounding environment causing a genotypic and phenotypic change.
And so down below, what we have is an image that shows Griffith's experiment and how bacteria can transform this genetic material, which at the time, the genetic material was again unknown. And so in this experiment over here on the left, we have this little key and there are really three types of bacteria that we're going to be looking at here. And the first one here is the lethal smooth bacteria, otherwise known as the S strain. And so the S strain, as its name implies here, is going to be lethal, and that's because, it has a smooth surface that has what's known as a capsule around it. Then what we have is another bacteria, which is a nonlethal rough bacteria, otherwise known as the R strain. And the R strain, as its name implies, has a rough surface, and that rough surface is going to be important, because it makes it non-lethal, meaning that it will not kill its host. And then the third and final type of bacteria that we have here are heat-killed S strain. And so really, this is the same S strain that we have up above here, except it's been exposed to a tremendous amount of heat that actually kills the S strain, completely kills it so that it's not capable of reproducing.
And so, over here on the right-hand side, what we're showing you are the experiments that Griffith conducted to show that bacteria can transform or uptake external DNA. And so, in experiment number 1, what he did was he used only the lethal smooth bacteria, the S strain. And he took this S strain that's lethal and he injected it into mice. And, of course, because the S strain is lethal, when he injected it into mice, the mice, of course, died, sadly. And then in experiment number 2, what he did was he took the nonlethal rough bacteria or the R strain. And, of course, because it's nonlethal, it's not going to kill the host. And so when he injected the R strain into the mice, of course, the mice remained alive. And, in the third experiment, which of course involves the lethal smooth bacteria or S strain that has been subjected to a tremendous amount of heat that actually kills it. And so because this heat-killed S strain is actually dead, when he took it and injected it into the mice, it was not able to kill the mice, and so the mice remained alive.
Now, this is where the interesting part comes into play with Griffith's experiment, and that is that he took experiments number 2 and number 3, and he combined them into experiment number 4. So what he did was he took the R strain, which alone does not kill the mice, and he took the heat-killed S strain, which alone does not kill the mice, and he combined them both into experiment number 4. And, of course, he thought that these two things that did not kill the mice on their own must not kill the mice. However, what he found was results that were surprising and different than what he expected. And so these two things here, the R strain and the heat-killed S strain, even though on their own they do not kill the mice, together when he injects them into the mice, together when he injects them into the mice, it actually kills the mice, and the mice died. And on top of all of that, from the dead mouse, he was able to extract living S strain even though he never injected living S strain into the mouse. He was able to extract it from the dead mouse, which was very strange.
And so what Griffith was able to conclude from this experiment is that the living R strain was actually able to transform. Transform the living R strain was able to transform the genetic material from the heat-killed S strain. And so this living R strain was able to uptake the DNA that was released into the environment when the S strain were killed. And, even though the S strain were killed, they released their genetic material, and their genetic material was uptaken by the R strain that was alive. And, when the R strain transformed the genetic material from the heat-killed S strain, it converted the R strain into an S strain. And, of course, the S strain is lethal and kills the mice. And so, at this time, Griffith did not know what this genetic material was that the R strain was transforming. However, he knew that there was some genetic factor that was being transformed by the R strain.
Now it wasn't until later work that was conducted by the scientists named Oswald Avery, Maclyn McCarty, and Colin MacLeod, who identified that transforming substance in Griffith's experiment as DNA. And so really, these scientists here, together, were the first ones to show that DNA is the genetic material. However, many scientists still remained skeptical that DNA was actually the genetic material, and that's because very little was known about DNA at the time, and much more was known about proteins. And so many scientists were skeptical that DNA could be the genetic material, and many scientists felt that proteins were still the better candidate for the genetic material. And so further experiments had to be done to solidify the fact that DNA is actually the genetic material, not proteins. And so we'll talk more about those experiments as we move forward in our course. But for now, this here concludes our introduction to Griffith's experiment, and we'll be able to get some practice applying these concepts as we move forward in our course. So I'll see you all in our next video.
