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Current time:0:00Total duration:10:19

Video transcript

so to review how we got at least to this video in 1865 Mendel first shares his laws of inheritance he he observes that there are these heritable factors that these discrete heritable factors that would be passed down from parent to offspring according to certain rules and he came up with the laws of inheritance law of segregation law of independent assortment law of dominance but as we've said multiple times that 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 what that what the basis of these factors were and in 1902 we have the first really solid theory for what the molecular basis for those inheritable factors actually could be this is when Bovary 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 Bovary Sutton chromosome theory and their work was based on observing observing how cells divide especially meiosis and then seeing how these chromosomes seemed to pair up then segregate then independently assort and get passed on to their offspring and they said hey these chromosomes on a physical level on a molecular level seem to be behaving in a ways that are very similar to the heritable factors that Mendel talked about so it was a very strong theory and then we get to 1911 where that theory gets some more evidence put behind it Thomas Hunt Morgan we talked about it he used his fruit flies to to see how that mutant trait that would pass on from one generation to another and the only plausible explanation that he could come up with is that it would be it was being passed on on the xx chromosome and him and his team continued to do more and more work to establish that chromosomes indeed seemed 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 are also made up of other things including proteins and 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 it said hey it's probably it's probably the proteins that are actually responsible for encoding the information of inheritance proteins were these where people knew were these complex molecules that in some ways you could say encoded information well at the time they thought DNA was were these kind of boring molecules that surely this couldn't this couldn't encode information and so the first evidence strong evidence that DNA is actually where the where the the genetic information is encoded doesn't happen for several more decades and we start along that path with Griffith right over here famous for Griffiths experiment where he does something really interesting and he by himself his experiments in 1920 or that he publishes in 1928 or he actually conducts and publishes in 1928 they aren't responsible in and of themselves for establishing DNA to be the the molecule that's actually the basis of inheritance but they start an interesting path of inquiry where these gentlemen in nineteen forty four are finally able to establish that DNA is where the the these inherited these heritable factors are actually encoded so what was Griffiths experiment well he was he was studying a strain of strains of bacteria and he saw that the same the two variants on a certain strain of two variants of bacteria you had the rough strain and the smooth stage 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 and 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 that's interesting this is this is the virulence drain this is the one that's actually going to kill the mice now if he took this smooth strain the virulence drain and he heated up so they were they were those bacteria were killed and then he injected those so 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 very interesting he took this the heat killed smooth strain he took some of that and he took some of the live rough strain put together now common sense would tell you is like okay this blue stuff that's not going to kill the mouse and this this killed 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 and so he came up with this theory of a transformation principle even though he killed the smooth strain here there must have been some type of material some some type of molecule that still got transferred from the dead bacteria to the live bacteria and essentially transformed the live bacteria into the smooth strain allowing them to kill the mouse and so he came up with this idea of some kind of transformation principle and so you can imagine and it look it took some time several over over ten years now almost two decades Avery McCarty and MacLeod said hey well what what is this transformation principle why don't we use Griffiths experiment and let's keep instead of just taking you know period the whole heat killed smooth strain let's try to break it up into its components and let's try to isolate the different components and keep doing the experiment until we have an isolated molecule or an isolated component that seems to do the trick so they were trying to isolate the transformation principle and they did just as what I described they took the heat killed smooth strain they would try to separate the different kind of the different constituents out there's different there's you can you can separate them out physically you should you could use certain washes that would wash away certain components you could use enzymes that would destroy certain components and eventually and this is very meticulous work so you can imagine they take they take the stuff you know the whole dead heat killed smooth strain and they start to separate it out into its various components so that might be that might be one component right over there this is another let me do it in these different colors this is another component right over there this is another component right over there they're using different chemical techniques to separate all of the constituents that were in that original heat killed smooth strain and then instead of running this last this last phase of the experiment with the entire heat killed smooth strain they do it with the rough strain mixed with each of these components separately and then they kept running the experiment and they would say look when we have this component right over here and we tried to run the experiment the mouse still lives the mouse to live 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 the something that did transform the raw strain so the mouse the mouse dies and so it did transform the rough strain into the smooth strain and so they took this material and they start applying all sorts of tests to it they could look at the the molecular components of it and when they looked at the ratios of nitrogen and phosphorus they say 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 to say 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 and this is a really really really big deal think about this quest that we've been going for the better part of a hundred years in heritable factors well where are they located hey it looks like they're on the chromosomes we started having evidence that they're on the chromosomes but chromosomes are made up of DNA and proteins and it wasn't until the start with Griffiths experiment and then Avery McCarty and MacLeod 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 and it was DNA and what was fascinating is when you mixed that DNA from the rough killed rough from the heat killed smooth strain with the rough strain that DNA was sucked was was able to mix in with the DNA of the rough strain and allows it to start producing these smooth protein coats that allowed it to be more violent 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 one 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 just just on something how magical DNA is that if you mix it in if you mix it 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 very very basic genetic engineering here
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