[Music] nucleic acids are one of the four essential macromolecules found within living systems they make up very important structures that Life as we know it cannot exist without this includes the genetic information that we all carry within ourselves called DNA which stands for deoxy ribo nucleic acid DNA is a long molecule made up of repeating subunits called nucleotides which are a type of nucleic acid DNA is the only molecule that we know of that carries genetic information for living organisms I know what you're thinking but what about viruses that use RNA as their genetic material while this is true and some viruses carry RNA as genetic material viruses are not considered living organisms so we can still draw the line and say DNA is the genetic material aterial for all living things nucleotides are the building blocks of nucleic acids and while there are different types of nucleotides that serve different functions they do share a base structure that you need to know this base nucleotide structure is commonly drawn with circles pentagons and rectangles but know that this is only a model that represents specific components that are put together to form the nucleotide which actually looks like this there are three components that make up the nucle the First Central component is a pentos sugar molecule called ribos which could also be deoxy ribos but more on that later we represent the sugar with a pentagon because this holds true to its actual shape which is a pentagon of four carbons connected to one oxygen to make up the ring with the fifth carbon branching off from here next connected to the fifth carbon of the sugar is a phosphate group which we draw as a circle the chemical formula for phosphate is P4 with an overall charge of Min -3 lastly stemming from the first carbon in the sugar is a nitrogen base we draw this simply as a rectangle even though this is not representative of the actual structure we'll talk about those later but for right now you should be able to draw and label this simple version of a nucleotide it will show up in one form or another on the IB exam we now know how to draw nucleotides but when we look at structures like DNA and RNA they are composed of many nucleotides bound together not just one in DNA and RNA the phosphate of one nucleotide can chemically bond with the sugar of another this forms a bond that holds them in place and creates what we call the backbone of these molecules so remember the backbone of DNA and RNA is made up of these repeating phosphate sugar bonds that link the nucleotides together when drawing DNA and or RNA for the IB exam these bonds must be included now in terms of of the nitrogen base as I said before there are a few different variations of this molecule that can exist within a nucleotide structure when we are talking about the structure of DNA and all that it entails there are four different nitrogen bases that are used to build its structure these four bases are a adenine t thyine g guanine and C cytosine the DNA in our cells contains billions of these bases in specific orders which is our genetic code no matter what living things we compare we all have this same base genetic code though some organisms have more than others and everything has its own variation of the sequence that is what makes us unique so remember here that the order of bases is the genetic code and the repeating sugar and phosphate parts make up the backbone and have nothing to do with holding any genetic information besides supporting and holding the bases in place let's take some time to to build our complete DNA model now that we have all of the pieces we know that DNA is built up of nucleotides which we know have those three components of a sugar phosphate and nitrogen base and we know that nucleotides can be connected to form a strand via the sugar phosphate bond that creates the backbone in addition to this information you need to know that DNA as a complete molecule is made up of two strands not just one so let's add another complete strand into the mix and before we draw it in you also need to know that the two strands of DNA that come together to create the complete molecule are parallel to each other and always face in opposite directions an easy way to tell that they are facing in opposite directions is by looking at the pentos sugar molecules if you think of the top of the Pentagon as a point you can see this left strand is pointing up while the right strand is pointing down it's very important that you get this right while drawing this model out on paper so we have two strands with the proper orientation but they need to connect and this happens with hydrogen bonds that connect the bases which hold the strands together this is a complete very simplified model of how you should draw the structure of DNA we will add a bit more detail to this as we learn a few more things throughout the video and in the HL video if you are an HL student we have talked a bit about DNA and as we move into talking more about RNA it's important that you know the difference between the two molecules both are nucleic acids but when it comes down to their overall structure there are three important differences you need to know first is the number of strands that make up each molecule DNA is made out of two strands of nucleotides connected in the middle at the bases via hydrogen bonds where RNA is only a single strand of nucleotides this this is by far the easiest and most obvious way to tell them apart next the bases that make up these two strands are slightly different we stated already that DNA contains the bases adenine guanine cytosine and thyine RNA also contains adenine guanine and cytosine but the fourth base is uracil not thyine lastly the pentos sugars of each structure are slightly different which is indicated by the actual name of each compound DNA or deoxy ribo nucleic acid has a deoxy ribos sugar which lacks an oxygen stemming from the second carbon meaning there is only a hydrogen atom connected to it hence the deoxy part of the name RNA or ribonucleic acid contains ribos sugar which does have the hydroxy group with the oxygen present other than that one difference the rest of the sugar components are the same and of course it is a requirement that you are able to draw and annotate these three different differences between DNA and RNA for the IB exam so get out of notebook and start sketching speaking a little more on RNA RNA is a nucleic acid polymer that is formed by the condensation of nucleotide monomers this is a fancy way of saying what we already know that RNA is a single strand of many nucleotides and when the RNA strand is being built each nucleotide is added to the Strand via a condensation reaction this condensation reaction involves the phosphate of one nucleotide and the sugar of another which is identical to the DNA backbone we discussed earlier and when they link together a water molecule is created in the process with one hydrogen coming from the sugar and the second hydrogen and oxygen coming from the phosphate group to create the H2O this creates the backbone of the RNA and links the nucleotides together thus creating a single strand we'll talk more about where and why this happens in future videos the five total nitrogen bases found in DNA and RNA are important because they carry our genetic code and in order for the code to be copied and passed down to offspring and copied to be used within a cell there must be a way for the code to be maintained this happens through complimentary base pairing which describes how only certain bases within a DNA strand or between a DNA and RNA strand can match up this is due to their structure and ability to create hydrogen bonds the rules that we need to know for nitrogen base pairing for DNA are that adenine always pairs with thyine and is connected with two hydrogen bonds and guanine always pairs with cytosine with three hydrogen bonds so that being said if we have an adenine here on the left DNA strand there must be a thyine here on the right and so on and so forth for the others RNA molecules are created by copying the DNA code and without getting into too many details here you need to know that all of the complimentary base pairs are the same with with the exception that thyine is replaced by uracil so if this was an adenine on a DNA strand that we are copying into RNA that would match with a uracil instead of a thyine because RNA does not contain any thyine bases DNA is the molecule that stores our genetic information and we need to take a second to appreciate just how amazing it is at doing its job of being able to store such a large amount of information and the instructions needed to build complex organisms first and foremost DNA has four possible bases so storing information can be done based on all the possible combinations that can be made from this sequence if we have a chain that is four bases long and we have four possibilities to put in each slot with a TG or C it means we can have 256 different possible combinations total four bases raised to the fourth power based on the length of the chain and if we have a chain of 20 bases the possible combinations would be 4 to the 20th which should be over 1 trillion different combinations the other great thing about DNA is that it is small and doesn't take up a lot of space especially when it is packaged up neatly in some nuclei for this reason DNA strands can virtually take on any length making the possible combination count for long strands like we have in our human cells seem almost Limitless the DNA code is universal between all organisms so no matter if you are looking at a plant a fish a bacteria or a human all of these living organisms use DNA as their genetic code this is no happy accident and actually provides evidence that supports that all living organisms came from a universal common ancestor in terms of evolution it would make the most sense that the structure of this code was a basis that created the diversity of life that we see on our planet instead of all of these organisms evolving or appearing independently that just happened to have the same basic molecular machinery [Music]