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Learn the toughest concepts covered in Microbiology with step-by-step video tutorials and practice problems by world-class tutors.

14. DNA Replication

The Griffith Experiment


The Griffith Experiment

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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 19 hundreds, it was unknown. What the genetic factor, actually, Waas 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, uh, transform or transformation in this context refers to the ability for these bacteria to uptake external DNA. And, of course, up taking external DNA is going to result in a Gina tip IQ as well as a FINA tip IQ 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 Gina typically and fina tip it change. And so down below. What we have is an image that shows Griffiths 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 uh, types of bacteria that we're gonna be looking here on. 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 non lethal 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 string that we have above here, except it's been exposed to a tremendous amount of heat that actually kills the S string 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 on dso an experiment number one. What he did was he used on Lee 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 an experiment. Number two. What he did was he took the non lethal rough bacteria or the are strain. And of course, because it's non lethal, it's not going to kill the host. And so, when he injected the are strain into the mice, of course the mice remained alive. And so the mice here is not are not dead. And then in the third experiment, what he did was he took the heat killed s strain which, of course, is the lethal smooth bacteria or s train 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 Griffiths experiment. And that is that he took experiment number two and experiment number three and he combined them into experiment four. So what he did was he took the are 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 four. And of course, he thought that, um, 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 are strain and the heat killed s strain. Even though on their own, they do not kill them Eyes together. When he injects them into the mice. It actually kills the mice and the mice die. 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 train 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 our strength was actually able to transform living. Our strain was able to transform the genetic material from the heat killed s strain. And so this living our strain was able to uptake the DNA that was released into the environment when these s train were killed. And so even though the S train were killed, they released their genetic material, and their genetic material was up taken by the R strain that was alive. And when the are strain, uh, transformed the genetic material from the heat killed s strain, it converted the our string into an s train. And, of course, the string is lethal and kills the mice. And so at this time, Griffith did not know what this genetic material was that the our strain was transforming. However, he knew that there was some genetic factor that was being transformed by the our string. Now it wasn't until later work that was conducted by the scientists named Oswald Avery Macklin McCarty and Colin Mac Lloyd, who identified that transforming substance in Griffiths experiment as DNA. And so really, these scientists here together where 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 toe 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 Griffiths 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

The bacteria that Griffith experimented with were termed "R"and "S"bacteria because:


In his transformation experiments, what phenomenon did Griffith observe?