In this video, we're going to introduce the Griffith experiment. Way back in 1928, a scientist named Frederick Griffith conducted a very important experiment. His experiment helped to identify that some unknown genetic factor controls the traits of organisms. Of course, today, we know that the genetic factor that controls the traits of organisms is DNA. But in the early 1900s, it was unknown what the genetic factor actually was. Frederick Griffith showed that bacteria have the ability to transform this unknown genetic material. The term "transform" in this context refers to the ability of these bacteria to uptake external DNA, and uptaking external DNA results in a genotypic as well as a phenotypic change. 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.
In an image below, which is not displayed here, Griffith's experiment is illustrated to show how bacteria can transform genetic material. In this experiment, there are three types of bacteria discussed: the smooth bacteria, otherwise known as the S strain, which is lethal because it has a smooth surface with a capsule around it; a nonlethal rough bacteria, known as the R strain, which has a rough surface making it nonlethal; and the heat-killed S strain, which is the same as the S strain but has been exposed to a tremendous amount of heat that kills it, making it incapable of reproducing. Griffith's experiments demonstrated that the bacteria could transform or uptake external DNA.
In experiment number 1, he used only the lethal smooth bacteria, the S strain, and injected it into mice, resulting in the death of the mice. In experiment number 2, he used the nonlethal rough bacteria, the R strain, and when he injected it into the mice, the mice remained alive. In the third experiment, he took the heat-killed S strain, which, being dead, did not kill the mice when injected. The most interesting part comes with experiment number 4, where Griffith combined the R strain and the heat-killed S strain. Surprisingly, when these were injected together into the mice, the mice died, and living S strain bacteria were extracted from the dead mice, despite never injecting living S strain into them.
The conclusion was that the living R strain was capable of transforming the genetic material from the heat-killed S strain. The R strain uptook the DNA released into the environment when the S strain were killed, and transformed the genetic material from the heat-killed S strain, converting the R strain into an S strain, which is lethal and kills the mice. Griffith did not know at the time that the genetic material was DNA. It wasn't until later work by scientists Oswald Avery, Maclyn McCarty, and Colin MacLeod that the transforming substance in Griffith's experiment was identified as DNA. Despite their findings, many scientists remained skeptical that DNA was the genetic material since much more was known about proteins at the time. Further experiments had to be done to solidify the fact that DNA is the genetic material, not proteins. We'll discuss more about those experiments as we move forward in our course. This 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. I'll see you all in our next video.
