so in this video we are just going to be looking at the structure of the DNA okay I'm just going to be drawing out uh what we know so far is DN is a polymer and it's made up of monomers known as DNA nucleotides and DNA nucleotides are these small molecules they are made up of a phosphate group a a deoxyribose sugar which is a type of pentose sugar with five carbon and it also has nitrogenous base I've simplified the nitrogenous bases over that and there can be four possible nitrogenous bases that can exist which are nitrogen and space a adenine T thymine C cytosine or G guanine and you join this DNA nucleotides together you can get something called DNA molecule which is polymer and that's when it forms the very familiar shape that we know when I when I asked my students uh what is DNA they always tell me yeah the end is that swirly thing it's only the nucleus in our cells and it does things it affects our characteristics uh it it affects how we look like it affects our appearances but how do why does that swirly thing made up of nucleotides affect how we look it affects our skin color our height our hair type to a certain extent it might also be able to affect our personality but that's also environmental as well okay so but we cannot downplay the effect of DNA on our daily lives see here's the thing all living things have it we know that it affects our appearances and as you can see this otter smelling flowers quite a cute picture the author has DNA and the flowers have TNA as well so why does the author look like another and why does the flower look like a flower kind of weird isn't it because there's there's nothing inherently much different about the chemical structure of the DNA in the otter and the uh DNA in the flowers yet they look totally different as you can see in the picture all right I just use that as a picture because I I just came across this picture or not and I thought it was the cutest thing ever so forgive me for that all right now before we go into that we want to talk a little bit about the structure of DNA a molecule of DNA is basically made out of two strands as you can see here one in black one in red some students ask is it really black and red no it's not this is just a lot this is just for my illustration purposes and the shape of DNA is referred to as a double helix double because it's made up of two strands Helix because it's a spiraling along each other now if I were to just basically uncoil and unwind the DNA and just put it side by side like straightening them and putting it side by side we will notice that the two stands are not directly touching each other actually okay but the two strands are connected to each other by something called basis here is where the purines and pyrimidines have to be just a little bit now James Watson and Francis click understood that the people who conceptualized the shape of DNA okay they did not discover DNA by the way I would I would just like to put it up there James Watson and Francis quick you uh people knew of the existence of DNA I think around the 1930s uh James Watson and Francis click with the people who came up with the structure of DNA and that is why they want the Nobel Prize in physiology and Medicine there is no Nobel Prize for biology weirdly enough there is one for chemistry that is one for physics you don't have one for biology yet sadly sucks but anyway that's just that's just a list of uh problems uh which irk me but anyway coming back to this okay so James Watson and Francis Craig understood that there are two types of nitrogenous bases which are the purines which are the larger of the nitrogenous base made up of two living structures and the pyramiding which is a single wing structure they understood that if one base had purine on one side and the other side also had purine it would be too close to each other uh also if one side had pyramid ends and the other side also had Thailand it will be too far for them to form a chemical or bone a bond with each other therefore they understood that if one side has to have purine the other side has to have pyromiding that's when the distance is just right purine and purine are too close I reminded I remaining are too far but purine and pyramidine can form a distance that is just right for them to connect to each other and one purine base will always pair up with one pilot and they understood that if there's an adenine on one side it will always pair up with timing and if cytosine is on one side it will always pair up with guanine this is universal and I want you to understand this no matter whether you're a human a cactus an otter a bacterium or even an algae just floating in the lake somewhere that your DNA will behave the same way adenine will always pair up with timing cytosine will always pair up with guanine there are exceptions but these are extremely rare exceptions that arise due to mutation but under normal circumstances adenine pass up with thymine and cytosine always pass up with and when adenine pairs up with diamond they will form two hydrogen bonds and when cytosine pass up with guanine they will form three hydrogen bonds with each other and I'm just drawing it over there for you to see this phenomenon is referred to as something called complementary based Panic complementary base pairing means the bases have to match each other a will always want to match up with t and c will always want to match up with g in the circumstances for example let's DNA let's look at an example I'm drawing out a strand on the left side where the base sequence is a c g g c what will be the sequence on the other DNA strand obviously it will be t g c c n g and that is referred to as something called complementary based parent and I'm also going to draw up the hydrogen bonds to show it to you as a reminder a and t were form two hydrogen bonds C and G will form three hydrogen bonds now we are not done with DNA yet okay we are going to go into the detail of DNA so what I'm going to do is I'm going to fill out the b sequences and I'm just going to make it more detailed to show you how the individual nucleotides look like you can see the individual nucleotides the sequences are still the same okay and I want you to see how the nucleotides are connected to each other the nucleotides are connected to each other with the purple color line okay and when the purple color line forms that I've highlighted one strand just to show you how one strand looks like in yellow and I've highlighted the green color strand to show you how the other strand looks like it connects the nucleotides together so we know that we know that the DNA strand which I've just basically circled it's made of DNA nucleotides joint gapped by something called phosphodiester bonds phosphoriathobond is a type of covalent bond and the phosphodie is the bond basically links one nucleotide to another nucleotide so that they can form a single DNA strand now you must understand that the phosphodiester bond is between the sugar of one nucleotide and the phosphate group of the other nucleotide that is important to know I'm highlighting it I'm drawing it over there for you to see that the sugar which I've highlighted in yellow okay and the phosphate group which I've highlighted in green so that you can see how the phosphodiester bonds form between one nucleotide and the other nucleotide that is important because sometimes if they do ask you to draw a DNA strand you might have to involve the phosphodites the bonds so the placement of the phosphodie is the bonds is quite important plants are empty parallel to each other now what do I mean by any parallel they basically run in opposite directions to make it simple I just I'm just gonna I'm just going to show you a nucleotide on one side and the nucleotide on the other side notice they are Direction one is sort of facing upwards and the other one the phosphate group is facing downwards okay so uh just to give you a Direction that's how it kind of looks like that's that's where they're facing so does the DNA actually have directions yes they do okay we usually use something referred to as the five Prime and three prime Direction and this is something that confuses a lot of students they're like what makes it five what makes it three okay I've gotten a simple way of teaching this I hope you can follow me for this all right the first thing we want to do is at every corner of the DNA molecule I'm going to just put a single Blank Space okay you can see the blank space in like just basically written over that okay now what I'm going to do is I'm going to zoom in okay so and we're going to see which side is going to be 5 and which side is going to be 3. so let's zoom in into one corner of the DNA first now at this corner let's label the carbon remember the carbon in the deoxyribose sugar is supposed to be started with carbon number one carbon number two number three number four and number five and carbon number five is the closest to that blank space so we will put five Prime over that it's as simple as that that's five times and if we were to go to the bottom left corner we do the same thing again let's label the sugar carbon one carbon two carbon three carbon 4 and carbon five and we notice that carbon number three is now closest to the blank space and therefore we just put three prime over that simple as that all right and because it's anti-parallel and because the DNA runs in an antiparallel which means to say they are running in opposite directions okay that side has to be a three prime and the lower right corner has to be a five kind if you don't believe me let's check it out okay let's look at the bottom right corner let's label the common common one carbon 2 carbon 3 carbon 4 and carbon five and carbon 5 is the closest to the Blank Space over there thus that will be 5 Prime basically and of course the top right corner same thing carbon number three is closest to a blank space and therefore that is three prime that's basically it so this is why we have to ask ourselves the question why do I even care that the DNA is running in the opposite direction does it really matter um yes it does sadly I can hear the collective groan from all my students that oh God why but that's just the way it is okay so uh we are going to talk about it a little bit more in detail so you see why does it matter it matters because new nucleotides are always added in a five Prime to three prime Direction now what do I mean by that okay let's give ourselves a situation okay I'm just throwing out a nucleotide on one side okay this is a long strand of nucleotide okay as you can see over that and I've also labeled them as five Prime and three prime now I'm also going to draw out on the other side you notice that the DNA stand on the other side is quite short okay it's only made up of three nucleotides let's say I want to make that strand longer therefore I would like to add more nucleotides into the Strand so the new nucleotide does it go to the top portion or does it go to the bottom portion remember new nucleotides are always added in a 5 Prime to three prime Direction therefore in this diagram it will actually be added to the bottom that's how it works so it will be added to the Cleveland Direction and it'll just basically keep going on and on and on until the addition of nucleotides stop that's basically it to summarize DNA is basically made up of DNA nucleotides they are basically joined together by phosphodized at once right to make a single strand but DN is two strands right so the two strands are antiparallel to each other which means to say that they are running in opposite directions okay and I'm also going to label the five Prime and three times and the two strands are also connected together by hydrogen bonds via complementary base having for example if on one side it has a the other side will have t one side has G the other side has C the one side has t the other side has a and they form hydrogen bonds with each other so sorry there's a bit of a my G and C is a little bit of an angle just forgive me for that okay and the third thing that we must also know the two strands coil around each other forming a double helix this is in summary the structure of DNA the highlighted parts that I've just uh the parts that have highlighted are usually the things that you need to explain in the exam or a list in the exam if they ask you to describe the structure of DNA