Shining a laser through a light bulb can reveal the structure of DNA

this is one of the most important images in biology because it helped us to figure out that DNA had helical structure and this image also tells us some of the important details of that helical structure it was generated by a scientist called rosalind Franklin using a technique called x-ray crystallography and in this video I want to demonstrate how the helical structure of DNA can be deduced from an image like this by doing the reverse I'm gonna create an image like this starting off with an object that has helical structure in this case I'm not gonna used in itself I'm gonna use the filament of a light bulb and I'm not going to use x-rays I'm gonna use visible light so I'm going to use a red laser so first of all x-ray crystallography is a diffraction phenomena so let's first of all demonstrate diffraction we're going to shine our laser through two slits this is a classic experiment Young's double slit experiment you may have seen it before so we'll just very quickly go over it so you shine a laser through the two slits and so beyond the slits you have now two wave fronts here I'm representing the peaks of the wave in whites and the troughs of the wave in green and in different regions those waves will either constructively interfere each other or destructively interfere with each other for example in this position here because there's a slight difference in travel time from the two slits you find that the peaks from one wave meet the troughs from the other wave and vice versa so they cancel each other out you don't see any light in this region but in this region the difference in travel time just so happens to make it the the peaks from one slit meet the peaks from the other slit they add to each other they constructively interfere and you do get lights in those regions so if you were to map out all the points of maximum constructive interference and maximum destructive interference it would look like this so if you were to put a screen in front of the two slits you will see what's called a diffraction pattern where you have several peaks and troughs in brightness here's the pattern that I got with my laser you'll notice that this pattern is very bright in the middle and it very quickly dips away to quite dim spots as you move outwards if instead of having two slits you have multiple slits known as a diffraction grating then the brightness falls off much more slowly you get a nice even distribution of dots this is an obvious thing to say but it becomes important later look if I tilt the diffraction grating then obviously it tilts the pattern now look what happens if I take a second differential grating and I put it in front of the first one at 90 degrees to it we get this grid of dots and that makes sense really because every dot from the first diffraction grating is now split into several dots by the second diffraction grating but look what happens if I tilt that second diffraction grating laser pointers often come with interchangeable tips and these tips just have different types of diffraction grating in them to produce different patterns of laser light this tip for example I haven't taken it part I don't know what kind of diffraction grating or gratings are inside but if I shine a laser through it then this is the pattern I get and look I can twist it to change the pattern so although I don't know what's inside here I can deduce it because look the pattern is the same as the pattern I got when I was manually moving to diffraction gratings around so it must be the same thing going on in here and that's what x-ray crystallography is all about really it's about looking at the pattern you get and then working backwards to figure out what kind of crystal must I be shining my x-rays through to produce this pattern if you didn't already know the structure of DNA you might assume looking at this diffraction pattern that DNA has a structure something like two diffraction gratings held at an angle to each other because both of these images have that cross of dots but DNA doesn't have that structure so what's going on this is where it gets interesting so I smashed an incandescent bulb to reveal the filament inside and that when you shine a laser through the filament you get this pattern and you can just about see that same cross of dots in the middle that we got when we were playing with two diffraction gratings one in front of the other and that makes sense because if you look clothes at the filament it's essentially like a spring isn't it and the laser has to pass through two sides that the first side of the spring that it passes through is like a diffraction grating that's tilted in one direction and when it passes out the other side of the spring it's like it's passing through a diffraction grating that's tilted in the other direction so it's just like the setup we've got with these two diffraction gratings that are tilted at an angle look what happens when I stretch and compress the filament it changes the angle between those two lines of dots so you can work out from the angle between those lines the pitch of the spring how tightly coiled it is and the same is true for DNA to get this to work the wavelength of the light you use needs to be roughly of the same order of magnitude as the distance between the coils of the spring so in the case of DNA you can't use red laser light you can't use the visible light at all you have to use x-rays so they're of the same order of magnitude as the distance between the atoms in DNA which is why it's called x-ray crystallography two ways in which this analogy breaks down that are really interesting the first is it's x-ray crystallography so you're dealing with crystals who'd have thought you could grow DNA crystals turns out if you've got fewer than a hundred base pairs you can actually get these molecules to crystallize so the image you're seeing here is generated when you shine x-rays through a repeating pattern of helical molecules whereas we've got a single helix of bulb filament the other thing is that DNA is a double helix whereas we've got a single helix in our setup you can actually see evidence of the double helix in the image because it turns out if you've got a double helix like the one in DNA then you would expect some of the dots to disappear from the image and that's what's going on here these missing dots suggest that you have a double helix isn't that amazing so two scientists called Watson and Crick analyzed the image there was no franking created and they were able to figure out all this stuff you can figure out from the angle of the dots the the ratio between the width of DNA and the pitch of DNA where the pitch is like the length of a full turn of the helix it turns out to be about 1.7 so the wizard DNA is 2 nanometers and the length of a full turn is 3.4 nanometers and with this information and the fact that DNA is a double helix Watson and Crick were able to work out the arrangement of the various components of DNA essentially how do these nucleotides fit together necessarily figured out the structure of DNA absolutely amazing at this point a lot of you might be thinking well hold on how does this analogy work at all because well in your light bulb filament you've got nice gaps between the spirals for the laser light to pass through but DNA isn't like that there aren't any gaps so if you look at the space inside a DNA molecule it's full of these base pairs that reach into the middle well it turns out that these base pairs are pretty much transparent to x-rays the only thing that x-rays can see is the phosphor molecules in the backbone spirals of DNA so it still works by the way when I was looking at this whole crystallizing DNA thing I found some amazing images and time-lapse footage of DNA crystals growing isn't this beautiful you use the fact that DNA can turn polarized light and you use to protoid filters different angles to create this kind of rainbow effect so you can really see the details of the crystals as they grow these were made by Linden Gledhill he does amazing things with cameras and microscopes and science just really good I recommend you check out his work the link is in the description my channel has been growing quite a bit recently which is amazing like I'm so happy about where the channel is and where it's going well I think I'm less happy about is all the extra work that comes along with essentially running in a JIT of a business I feel like I've got less time to do the actual thing that I enjoy doing which is making videos and I want more time to do that so I've had to really up my productivity game you know I've said this before a big advocate of online video learning specifically Skillshare Skillshare is an online learning community that offers membership with meaning there's so much to explore in there and you know it's not just passive you'll be doing projects and stuff along the way and there's this community of like-minded creatives that you can learn from they've got these productivity courses I've been working through a few of them one that I really like is the productivity master class because it's all about systems you know and I love systems it's not just like productivity and business-related staff there's a lot of creativity related stuff like you know things that you might like illustration UX UI design photography videography web development there's so much stuff on there one thing I like is that the classes are about an hour long in total and they're broken down into these short videos you know I'm really busy so I can just fit them in whenever I've got time and you get access to absolutely everything for ten dollars a month and you know it's a new year it's 2020 think about picking up a new skill or maybe dive deeper into something you already love because Skillshare are sponsoring this video you can get two months absolutely free by going to school dot forward slash Steve mold five the link is also in the description so check it out today I hope you enjoyed this video if you did don't forget to hit subscribe and I'll see you next time [Music]

Channel: Steve Mould

Views: 333,341

Rating: 4.9582481 out of 5

Keywords: Explained, understand, interference, nucleotides, spiral, double helix, base pairs

Id: dqMYWldfs_k

Length: 10min 40sec (640 seconds)

Published: Thu Jan 16 2020