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The discovery of the double helix structure of DNA

Video transcript
in 1865 mendel often considered the father of modern genetics comes up with a structured way of thinking about these inheritable factors which we now call jeans and then as we go into the early nineteen hundred's his work was rediscovered in people start to say okay we have we see how some of these trades get passed on in these somewhat predictable ways we can put some structure rounded what is the actual biological mechanism for that how it how are these traits included at a cellular or at a molecular level and so in 1902 and we talked about this in a previous video both very insulting come up with the chromosome theory based on seeing how chromosomes separate and pair during cell division and say hey those seem to be seem to match up quite well to what mental described by these heritable factors that we started having a lot more evidence for this Morgan is able to show that these mutant that a mutant eye color trade seems to seems to be passed on in a way that shows that it is on the ex sex chromosome and his him and his team to a lot more work especially with fruit flies to show that hey chromosomes are the basis for where these heritable factors are but then we even within the chromosomes people weren't sure chromosomes are made up of proteins made up of DNA what was the molecule the set of molecules that actually encoded for this for these heritable traits and at first most of the wait was on the chromosomal proteins I'd because proteins were these complex molecules that have had all of this variety that seems like it could it could code for for these heritable traits while DNA at least early on seemed like a kind of boring molecule people people assumed people assumed that there wasn't a lot of diversity in DNA assume that even if you go from one species to another that the DNA molecule was fundamentally the same so early on people actually we're on the side more of the proteins but then more and more evidence came the way of came on DNA side you had Griffiths experiment was able to show hey I could take this this heat killed bacteria but if I mix it with some other living bacteria that somehow how there's some transformation principle that transformed the living bacteria into the into the type of species that I had in the heat killed and you gonna go to 1944 and Avery mccarty in McLean Bible so some pretty good evidence that that the actual principle that the thing that was left in that heat killed bacteria was probably DNA was probably DNA and then we get even more conclusive evidence with experiments of Hershey and chase and we have a whole video on this where they say what what is what is it that viruses inject into bacteria to hijack their their their genetic system and they say hey it's not proteins it is DNA that does it so they provide much more conclusive evidence on the side of beauty but even at that point we as a community is a civilization still don't know what the actual structure of DNA was and we are we also did not know how did that structure actually code for all of these heritable factors and the work culminates with Watson Crick but it was dependent on all of the people I mentioned and more than one person who should get special credit for one getting a little bit more evidence on the side of DNA and helping Watson Crick actually there several people but in particular target off and rosalind Franklin who don't probably and rosalind Franklin particular probably does not get as much credit as she deserves charge-offs 21 that show that DNA actor is more interesting than people appreciated he noticed that the frequencies the frequencies of the nitrogenous bases of AD mean guanine cytosine and thymine in DNA varies across species varies across across species and something that somehow coding for what makes a species or species well it would have to vary across BC so that makes DNA interesting and then the other thing that he noticed this was key for Watson and Crick work is that the frequency of guanine is equal to the frequency of cytosine Indian and the frequency Adnan is equal to the frequency of time in DNA and so he is a clue that these somehow are associated with each other they somehow pair with each other and so we get to the early fifties we had all this evidence that DNA is the molecular basis you have charged off with his rules culture our gas rules and then you have rosalind Franklin and she's doing she's doing she's she's imaging x-ray imaging diffraction patterns from x-rays beamed into crystals of DNA so what I mean by Crystal of DNA a crystal is taking a bunch of molecules in a ring and arranging them in a regular pattern so crystal DNA had DNA molecule that one DNA molecule that you have another DNA molecule that you have another DNA molecule and then you be meckstroth Adam and x-rays are key because their wavelength the wave of an x-ray is small enough is to capture the features atomic level so you beam x-rays and then the X-rays defect and then you capture the pattern of that diffraction and depending on the structures in the actual molecules will have different diffraction patterns and Franklin's famous diffraction pattern is shown right over here and when she immediately saw this they had some of the telltale cues for a helical structure now I wouldn't read too much into this if you're not an expert reader of x-ray diffraction patterns it does not it is this is a direct image of a of a DNA molecule but they knew they she already knew in fact people in this community are you that this X pattern was a telltale sign was a telltale sign for for a helical structure of some kind and then they are also able to look at the different the other clues here to think about what the spacing different between different molecules and even the spacing between the different turns of the helix structure now once again this diffraction pattern is not a direct image and it takes a lot of it is a very well developed expertise to backwards map how things will distract into the thing that actually cause the diffraction and they would take it from multiple different angles to get a better if anyone actually today you have computers doing this it could take a diffraction pattern and start to construct what you know the electron clouds of what the actual molecule looks like so frankly this is painstaking work and she already had a sense that it was probably she knew as a helical structure but she was waiting to get a little bit more evidence before she published her work now at the same time you have Watson and Crick here who were trying to solve the structure and they got ahold of Franklin's work with the help of Maurice Wilkins here who Franklin worked with and they were able to establish that it wasn't a single helix but it was a double helix and you actually had these base pairs forming the rungs of the double helix and that was really interesting because that showed how DNA could replicate itself how could contain actual information I'm going to much much more depth into future videos now the sad part about this story is wilkins Watson and Crick went on to win a Nobel Prize Franklin unfortunately she died very young and you're not allowed to receive the nobel prize if you if you passed away so she's very deserving of it this work which is a lot of people consider to be one of the biggest discoveries in human in science was based on what she did arguably had Watson Crick not had access to her work they would not have been able to figure it out and if she just stuck to what she was doing without other people having sex is work she might have been able to get to that same conclusion so she is one of the people who sometimes the history of science overlooks but this isn't all you know this isn't a not give credit to Watson Crick either there are still a lot of very powerful intuitive leaps that they had to make to come up with this double helix structure antiparallel double-helix structure where they go in opposite directions but the bridge with the with these nitrogenous bases pairing with each other and this is a big big big big deal throughout most of human history we knew that trade through passed on betrayed seemed like this magical mystical thing how do I include my you know i i kno lap is like my dads but how's that actually encoded in in my DNA and now we're able to see that a lot of what we consider about ourselves to be us is encoded in these molecules included in these base players so it's beautiful it's incredible it's it's it's shedding light on one of the biggest mysteries of what makes humans actually all life life