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 our cells called DNA, which stands for deoxyribonucleic 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 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 nucleotide.
The first central component is a pentose sugar molecule called ribose, which could also be deoxyribose 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 PO4 with an overall charge of minus three.
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 the molecule is the backbone of the molecule. 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 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, thymine, 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 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 pentose 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 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 thymine.
RNA also contains adenine, guanine, and cytosine, but the fourth base is uracil, not thymine. Lastly, the pentose sugars of each structure are slightly different which is indicated by the actual name of each compound. DNA, or deoxyribonucleic acid, has a deoxyribose 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 ribose sugar which does have the hydroxyl 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 differences between DNA and RNA for the IB exam.
So get out a 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 complementary 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 thymine 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 thymine 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 complementary base pairs are the same with the exception that thymine 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 thymine because RNA does not contain any thymine 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, T, G, 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 four to the tw- 20th which would 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 happen to have the same basic molecular machinery