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Video transcript

today we'll be talking about gel electrophoresis what is gel electrophoresis you might ask well it's a lab technique usually used in the biochemistry lab for separating out DNA or proteins based on their size and let's talk about how it works so first you need to have the gel this can be made out of different kinds of substances such as agarose and polyacrylamide both of which I'll discuss later and the electrophoresis part of it means that you need to have an electrical field passing through the gel to get the band's to move so to create an electrical field we have to have a cathode and an anode the cathode is on this side remember from general chemistry that at the cathode reduction takes place so you'll have a negative charge but on the other side where the anode is you'll have a positive charge because this is where oxidation is taking place and to power this you'll need some kind of battery to connect them and these are both usually also connected to a big box that contains the gel electrophoresis apparatus in order for charge to flow across you need something that will conduct electricity so you have a buffer that's usually composed of different ions so this is completely covering this entire gel and remember that it's always there but I'm going to erase it just because it's going to get a little messy for what I need to show you next so next you'll load a sample of DNA in order to load your sample you'll need to take a pipette and put it into one of these wells so usually you'll want to make sure that you have a die with your mixed in with your sample so that you can see it as it's running so I'll show that as a pink line here so I'd filled that well followed by yellow here and green here so that's just the color of the dye it can really be any color you want this will just help you track the movement of the Bands later on once you have these bands what will happen once you turn on the electrical field remember that DNA has a negatively charged backbone because of all the phosphate groups so what you'll actually observe is that they'll travel towards the positive end okay so maybe after a short period of time what you'll see is that it'll look something like this maybe the pink will have split up into a few bands indicating that there's a few different sized fragments in there with the yellow you might only see one band and with the green you might actually have two bands but they're so close together still that it's kind of hard to tell so how can you really see what's going on the solution is to let this run for a longer period of time and eventually what you'll see is shown here at the bottom you'll see that whatever size fragments were in your original samples have effectively split up by their size and note that in pink you'll always want to load something known as a DNA ladder a DNA ladder is like a standard this is something that you buy from a company and they tell you exactly what sizes their fragments are so that you can match them up to your unknown samples so this could be 400 base pairs 200 base pairs and 100 base pairs and as you'll note the smallest fragments travel the furthest this is because the smallest things are really easy to push with electrical field but when you have such a big molecule or rather a big DNA fragment can be hard to move so you can see that the 400 base pair doesn't move too fast or too far and what does this tell us about our unknown yellow and green samples this shows is that the yellow sample has a band that's 200 base pairs long and the green sample actually was composed of two different fragments one that was 100 base pairs and another that was 200 base pairs so what would we do with this information if you needed a particular size of DNA say for the next step of your experiment if you wanted to insert it into a plasmid or a vector you could cut this out of the gel and use it for that now let's talk about the two kinds of gels that are most commonly used the first is agarose and the second is SDS page so agarose is a gel that's usually used for separating big pieces of DNA so if you think about the pore size in the agarose it has pretty big pore so imagine it looking kind of like this the gel is pretty big there's a big holes here so that you'll be able to separate out the big pieces of DNA that come through however if you're trying to separate out little pieces and won't be that obvious because they'll all just race through these giant holes so remember that this is for big DNA fragments usually this is for DNA that's bigger than 50 base pairs sds-page on the other hand can be used for very small things so imagine that being a much finer weaving with smaller pores although this can be used for small pieces of DNA can also be used for proteins you might be wondering what does SDS page even stand for the SDS part is sodium dodecyl sulfate this is a chemical agent that denatures proteins disrupting any non-covalent interactions they may have this makes it so that the charge of the proteins isn't a factor when they're separating out onto the gel and they're only being separated strictly by size the page part is polyacrylamide gel electrophoresis or we'll just leave it at GE so poly acrylamide is the substance that gels made out of so how can we remember the difference between these two types of gels remember that SDS page is for small DNA or protein so as for small and as for SDS and agarose is for bigger fragments of DNA so today we've talked about how you would set up a gel electrophoresis why it works and how you would want to pick the substance that your gels made out of if you are really doing this in the lab now that you have your fragments of known size of DNA or protein you could either sequence them or use them in other molecular techniques