Establishing DNA as transformation principle

So to review how we got at least to this video: in 1865, Mendel first shares his laws of inheritance. He observes that there are these heritable factors, these discreet heritable factors that would be passed down from parent to offspring according to certain rules. He came up with the laws of inheritance: the law of segregation, the law of independent assortment, and the law of dominance. But as we've said multiple times, the work at the time that it was first shared wasn't taken that seriously. In fact, a lot of people didn't pay attention to it, and it wasn't until the early 1900s that it was rediscovered.

But even when it was first rediscovered around 1900, people did not know what the molecular basis for these heritable factors that Mendel talked about was. In 1902, we have the first really solid theory for what the molecular basis for those inheritable factors actually could be. This is when Boveri and Sutton come up, and they independently did their work, but they both came to the same theory at around the same time. They came up with the chromosome theory, now called the Boveri–Sutton chromosome theory, and their work was based on observing how cells divide, especially meiosis, and then seeing how these chromosomes seem to pair up, then segregate, then independently assort and get passed on to their offspring.

They said, "Hey, these chromosomes, on a physical level, on a molecular level, seem to be behaving in ways that are very similar to the heritable factors that Mendel talked about." So, it was a very strong theory. Then we get to 1911, where that theory gets some more evidence put behind it. Thomas Hunt Morgan, whom we talked about, used his fruit flies to see how that mutant trait would pass on from one generation to another. The only plausible explanation that he could come up with was that it was being passed on on the X sex chromosome.

Him and his team continued to do more and more work to establish that chromosomes indeed seem to be the basis, the physical location for these heritable factors that Mendel first talked about in 1865. But even Morgan and his team, when they looked at chromosomes, you know, a lot of times now when we think of chromosomes, we think of chromosomes as being made up of DNA, and that is true. But chromosomes were also made up of other things, including proteins.

In the early days, when people said, "Hey, it looks like chromosomes are really the basis or the location for these heritable factors, for these genes," when people looked at these two different molecules, they said, "Hey, it's probably the proteins that are actually responsible for encoding the information of inheritance." Proteins were what people knew were these complex molecules that, in some ways, you could say encoded information, while at the time they thought DNA were these kind of boring molecules that surely couldn't encode information.

The first strong evidence that DNA is actually where the genetic information is encoded doesn’t happen for several more decades. We start along that path with Griffith, right over here, famous for Griffith's experiment, where he does something really interesting. He, by himself, his experiments in 1920, or actually he conducts and publishes in 1928, they aren't responsible in and of themselves for establishing DNA to be the molecule that's actually the basis of inheritance, but they start an interesting path of inquiry.

These gentlemen in 1944 are finally able to establish that DNA is where these heritable factors are actually encoded. So what was Griffith's experiment? Well, he was studying strains of bacteria, and he saw that the two variants of a certain strain of bacteria had the rough strain and the smooth strain. If he injected the rough strain into a mouse, the mouse lived; if he injected the smooth strain into a mouse, the mouse died. It was because the smooth strain had this protective coating on it that made it harder to attack by the mouse's immune system.

So that by itself, well, that's interesting. This is the virulent strain; this is the one that's actually going to kill the mice. Now, if he took this smooth strain, the virulent strain, and he heated it up so that the bacteria were killed, and then he injected those, this is the heat-killed smooth strain. If he injected those into the mouse, the mouse still lived because those bacteria were dead.

But then he did something very, very interesting. He took this heat-killed smooth strain, he took some of that, and he took some of the live rough strain, put them together. Now, common sense would tell you, "Okay, this blue stuff that's not going to kill the mouse, and this killed smooth strain that's not going to kill the mouse either. So if I mix it up, that shouldn't kill the mouse." But it did kill the mouse, which was fascinating.

He came up with this theory of a transformation principle. Even though he killed the smooth strain, there must have been some type of material, some type of molecule that still got transferred from the dead bacteria to the live bacteria, essentially transforming the live bacteria into the smooth strain, allowing them to kill the mouse. He came up with this idea of some kind of transformation principle.

You can imagine it took some time, several years, over ten years now—almost two decades. Avery, MacLeod, and McCarty said, "Hey, what is this transformation principle? Why don't we use Griffith's experiment? Let's keep taking, instead of just taking the whole heat-killed smooth strain, let's try to break it up into its components and isolate the different components." They kept doing the experiment until they had an isolated molecule or an isolated component that seemed to do the trick.

So, they were trying to isolate the transformation principle, and they did just as I described. They took the heat-killed smooth strain, tried to separate the different constituents out. They used certain washes to wash away certain components, enzymes that would destroy certain components, and eventually—and this is very meticulous work—you can imagine they take the whole dead heat-killed smooth strain and start to separate it into its various components.

They're using different chemical techniques to separate all of the constituents that were in that original heat-killed smooth strain. Instead of running this last phase of the experiment with the entire heat-killed smooth strain, they did it with the rough strain mixed with each of these components separately. They kept running the experiment and said, "Hey, look, when we have this component right over here and we tried to run the experiment, the mouse still lives."

The mouse still lives, so this one did not transform the rough strain, and maybe this one also did not transform the rough strain. But then eventually they were able to isolate something that did transform the rough strain, so the mouse dies. It did transform the rough strain into the smooth strain, and they took this material and started applying all sorts of tests to it. They looked at the molecular components and when they looked at the ratios of nitrogen and phosphorus, they said, "Hey, this seems to have ratios that are consistent with DNA," which is a molecule they already knew about, and it was not ratios that would have been consistent with proteins.

They ran chemical tests and said, "Hey, there doesn’t look like there’s a lot of protein in this thing that we isolated, or even RNA, which is another molecule that they knew. Enzymes that would have degraded proteins or RNA did not degrade this stuff. But if the enzymes that degraded DNA did degrade this stuff," and so they were able to come up with the idea that DNA was the transformation principle.

This is a really, really big deal. Think about this quest that we've been going through for the better part of a hundred years: inheritable factors, well, where are they located? Hey, it looks like they're on the chromosomes. We start having evidence that they're on the chromosomes, but chromosomes are made up of DNA and proteins.

It wasn't until we start with Griffith's experiment and then Avery, MacLeod, and McCarty come along and say, "Hey, let's identify what was it exactly about the heat-killed smooth strain? What's the component in it that actually transformed the other strain?" It was DNA. What was fascinating is when you mixed that DNA from the heat-killed smooth strain with the rough strain, that DNA was able to mix in with the DNA of the rough strain and allow it to start producing these smooth protein coats that allowed it to be more virulent, so the mouse's immune system couldn't attack it as well.

So it's really fascinating on a lot of levels. You know, the whole takeaway from this is how did we get to DNA being the important part of the chromosomes, at least in terms of encoding the actual genetic information? But it's also a cool way to think about how magical DNA is—that if you mix it in, if you mix in the DNA of one strain with a live version of another strain, that you actually might be able to transform that strain. So you're actually, you know, in some ways, they were doing very basic genetic engineering here.