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Current time:0:00Total duration:13:36

Video transcript

okay roll it you know this is it is the longest word in the world like anywhere any language more than a hundred and eighty-nine thousand letters if you were to write it down though I don't know why you would it would fill up more than 100 pages and if you could actually save without like breaking your face it would take about five hours what the Frick is this word well it is the name of the longest known protein on earth and it's actually in you right now because of its enormous size it was given the name Titan by scientists and that's with two eyes and it's protein that helps give someone like the springiness to your muscles today we're going to be talking about DNA and how it along with three versions of its cousin RNA unleash chemical kung fu to synthesize proteins just like this so this is going to take a while to explain so how about we make ourselves some hot pockets mmm my favorite ham and cheese every time I talk about I wonder how do they do it how do they pack exactly the same flavor and every foil cardboard wrapped food-ish item clearly there's got to be some super secret instruction manual kept and a location known only to two people and since I'm talking about biology here that brings up a related question how did I get built from the DNA instructions and biological molecules we've been talking about today that's what I'm going to do not actually make hot pockets or a person but I'm going to be talking about DNA transcription and translation which is how we get made into the delicious things that we are today though hopefully none of us know how delicious people are animals plants and and also hot pockets really are nothing more than salty water carbohydrates fats and you know proteins combined in precise proportions following very explicit instructions let's say I want to make my own hot pocket I would have to one break into the layer of the hot pocket company holding the secret manual to read the instructions on how to make the machinery to produce the hot pocket and the proportions of the ingredients three quickly write down that information in shorthand before I get caught by the hot pocket police four go home follow the instructions build machinery mix the ingredients together until I have a perfect hot pocket and that is how we get us very simply inside the cell's nucleus the DNA instruction manuals copied gene by gene by transcription onto a kind of RNA and then taken out of the layer where the instructions are followed by the process of translation to assemble amino acid strings into polypeptides or proteins that make up all kinds of stuff from this tighten down here to the keratin in my hair but most the polypeptides to get made aren't structural proteins like hair they're enzymes which go on to act like the assembly machinery breaking down and building and combining carbohydrates and lipids and proteins that make up variations of cell material so enzymes are just like whatever ingenious machinery they use the factory to make this okay let's start out in the lair I mean the nucleus the length of DNA that we're going to be transcribing onto an RNA a molecule is called our transcription unit and let's say in today's example that it's going to include the gene that transcribes for our friend Titan which in humans at least occurs on chromosome 2 now which transcription has a sequence just above it on the Strand and that's called upstream biologists call that upstream on the Strand and that sequence sort of defines when the transcription unit is going to begin the special sequence is the promoter and it almost always contains a sequence of two of the four nitrogenous bases that we talked about in our last episode adenine thymine cytosine and guanine specifically the promoter is a really simple repetition we got thymine adenine thymine adenine and then a a a and then on the other side a T because you know how this works right this is called the Tata box it's nearly universal and it helps our enzyme figure out where to bind to the Strand now you'll remember from our episode about DNA structure that DNA strands run in one of two directions depending on which end of the strand is free and which end has a phosphate bond one direction is 5 prime to 3 Prime and the other direction is 3 prime to 5 Prime in this case upstream means toward the 3 prime end and downstream means toward the 5 prime end so the first enzyme in this process is RNA polymerase and it copies the DNA sequence downstream of the Tata box that's towards the 5 prime end and copies it into a similar type of language messenger RNA lookaside so you'll notice that to read the DNA in order to make enzymes we need an enzyme in the first place so it kind of gets chicken egg here we need the enzyme to make the DNA in the DNA to make the enzyme so where did RNA polymerase come from in the first place if we haven't made it yet what an excellent question it turns out that all of these basic necessities get handed down from your mom she packed quite a lot more into her egg than just her DNA so you know we had a healthy start so thanks mom so the RNA polymerase binds to the DNA at that tata box and begins to unzip the double helix working along the DNA chain the enzyme reads the nitrogenous bases those are the letters and helps the RNA version of those nitrogenous bases floating around in the nucleus to find their match now you might also recall from our previous episodes that nitrogenous bases only have one counterpart that they can bond with but RNA which is the pink one here doesn't have sign mean like DNA does which is the green in the blue instead it has uracil so u appears here in T's place as a partner to add as it moves the RNA polymerase Ruiz it's the DNA behind it and let's our new strand of messenger RNA peel away eventually the RNA polymerase reaches another sequence downstream called a termination signal that triggers it to pull off now some finishing touches before this info can safely leave the lair first a special type of guanine is added to the 5-prime and that's the first part of the game RNA that we copied and that's called the 5 prime cap on the other end it looks like I fell asleep with my finger on the a key of my keyboard but another enzyme added about 250 adenine Zhan to the 3 prime end this is called our poly a tail these caps on either end of the mRNA package make it easier for the mRNA to leave the nucleus they also help protect it from degradation from nearby passing enzymes while also making it easier to connect with other organelles later on but that's still not the end of it as if to try to confuse me to protect the secret hot pocket recipe the original recipe book also contains lots of extra misleading information so just before leaving the nucleus that extra information gets cut out of the RNA in a process called RNA splicing and it's something like editing this video this process is really complicated but I just have to tell you about two of the key players because they have such cool names one thus nerfs which are small nuclear ribonucleoproteins these are a combination of RNA and proteins and they recognize the sequences that signal the start and end of the areas to be spliced nerps bunched together with a bunch of other proteins to form the spliceosome which is what does the actual editing as it were breaking the junk segments down so their nitrogenous bases can be reused in DNA or RNA and sticking together the two ends of the good stuff that good stuff that gets spliced together by the way are called the exons because they will eventually be expressed the junk that gets cut out are just the intervening segments or the introns the material and the introns will stay in the nucleus and get recycled so for instance tighten down there is thought to have hundreds of exons when it's all said and done probably more than 360 which maybe more than any other protein and it also contains the longest intron in humans some 17,000 base pairs long man Titan it is just a world-record holder so now that it's been protected and refined the messenger RNA can now move out of the nucleus okay so a quick review of our hot pocket mission possible caper so far we broke into the lair containing the instructions we copied down those instructions in shorthand we added some protective coatings and then we cut out some extra notes that we didn't need and then we escaped back out of the lair now I have to actually read the notes make the machinery and assemble the ingredients this process is called translation so next rewind your memory you would just watch that video again to the episode about animal cells do you remember the rough endoplasmic reticulum I hope you do those little dots on the membranes are the ribosomes and the processed messenger RNA gets fed into a ribosome like a dollar bill into a vending machine ribosomes our mixture of protein and second kind of RNA called ribosomal RNA or rRNA and they act together as a sort of workspace our RNA doesn't contribute any genetic information to the process instead it has binding sites that allow the incoming mRNA to interact with another special type of RNA the third in this caper called transfer RNA or tRNA drr and i really might as well be called translation RNA because that's what it does it translates from the language of nucleotides into the language of amino acids and proteins on one end of the tRNA is an amino acid on the other end is a specific sequence of 3 nitrogenous bases these 2 ends are kind of matched to each other each of the 20 amino acids that we have in our body has its own sequence at the end so if the tRNA has the amino acid methionine on one end for instance it can have UAC as the nucleotide sequence on the other end how is just like building a puzzle the mRNA slides through the ribosome the ribosome reads the mRNA three letters at a time each set called a triplet codon the ribosome then finds the matching piece of the puzzle a tRNA with three bases that will pair with the codon sequence at into the tRNA by the way it's called the anticodon so for all the terminology you need to know it and of course by bringing in the matching tRNA the ribosome is also bringing in whatever amino acid is on that tRNA okay so starting at the five prime end of the mRNA that's fed into the ribosome after the five prime cap for almost every gene you find the nucleotide sequence Aug on the mRNA the ribosome finds a tRNA with the anticodon UAC and on the other end of that tRNA is Metheny the mRNA like a mile-long dollar bill keeps sliding into the ribosomes so that the next codon can be read and another tRNA molecule with the right anticodon binds on if the codon is UUA then the matching tRNA has an AAU on one end and leave seen on the other hand if the mRNA has an AGA then the matching tRNA has a you see you on one end and an arginine on the other in each case that new amino acid gets connected on the previous amino acid starting a polypeptide Jade which is the beginning very beginning of a protein but it turns out that there are lots of different ways to read this code because uua is not the only triplet that codes for leucine uug does to an arginine it's coded for by six different triplets this is actually a good thing means we can make a few errors and copying and transcribing and translating DNA and we won't necessarily change the end product the process continues with the mRNA sliding in a bit more on the ribosome bringing in another tRNA with another amino acid that amino acid binding to the existing chain and on and on and on and on and on sometimes for thousands of amino acids to make a single polypeptide chain for example this whole word is basically just the names of the amino acids in the sequence and the order in which they occur in the protein all 34,000 350 of them but before we could make our own Hot Pockets and that string of amino acids becomes my muscle tissue we have some folding to do that's because proteins in addition to being hella big can also contort very complex and downright lovely formations one key to understanding how a protein works is to understand how it folds scientist have been working for decades on computer programs to try and figure out protein folding now the actual sequence of amino acids in a polypeptide what you see scrolling along down there is called primary structure one amino acid covalently bonded to another and that one to another into another and a single side some amino acids don't like to just hold hands with two others they're a bit more promiscuous than that so hydrogen's on the main backbone of the amino acids like to sometimes form bonds on the side hydrogen bombs to the oxygens on amino acids a few doors down when they do that depending on the primary structure they Bend and fold and twist into a chain of spirals called helix we sometimes also find several kinks strands laying parallel to another called pleated sheets all those hydrogen bonds and pleated sheets are what makes silk strong for instance so when the end are promiscuous amino acids lead to wrinkled sheets uh-huh these hydrogen bonds are what helped give these polypeptides their secondary structure but it doesn't end there remember the R groups that defy on each amino acid well some of them are hydrophobic and since the protein is in the cell which is mostly water all of those hydrophobic groups try to hide from the water by huddling together and that can bend up the chain some more other R groups are hydrophilic which if nothing else means they like to form hydrogen bonds with other hydrophilic R groups so we get more bonding the more bending and our single-file line has now taken on a massively complex three-dimensional shape it also explains why I can fix my bedhead by wetting my hair with water the water helps break some of those hydrogen bonds in the keratin which relaxes its structure that way I can comb it out and when it dries those bonds reform and voila perfect hair all of this shape caused by bonding between our groups gives our polypeptide its tertiary structure so now we have a massively contorted polypeptide chain and actually contours very precisely sometimes just one chain is what makes up the whole enzyme or protein and other proteins like hemoglobin several different chains come together to form a quaternary structure so quick review of structure sequence is the primary structure the backbone of hydrogen bonds forming sheets and spirals are the secondary structure our group bonds are tertiary in the arrangement of multiple proteins together give the quaternary structure these polypeptides are other structural proteins like this thing at the bottom here that you can find in your muscle or in my hot pocket they might also be enzymes enzymes like do stuff they can cut up biological molecules like I do with the chef's knife it can mix stuff and they can put stuff together so from that one recipe book we got all the ingredients and all the tools necessary to make me which is better than a hot pocket would you all agree
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