Transcription
Good morning, good afternoon, good evening, ladies and gentlemen. You are welcome to my YouTube channel. Today, we are going to be discussing an introduction to nucleic acids biochemistry. In this lecture video, you are expected to learn nucleic acid, nucleotides, and types of nucleotides. And at the end of this lecture, you should be able to describe the chemical components of nucleic acid, the formation of nucleosides, formation of nucleotides, and you should be able to describe DNA and RNA.
So, what is a nucleic acid, or what are nucleic acids? Generally, nucleic acids are polymers of nucleotides; that is, nucleotides are the monomeric unit of nucleic acids. So, nucleic acids, they are polymers of nucleotides, nucleotide being the monomeric or the building block of nucleic acid. So, what are these nucleotides? Each nucleotide has three components. Each nucleotide has three components: a five-carbon monosaccharide, which is a sugar—a five-carbon sugar, pentose sugar—which can be either deoxyribose if it is in DNA or ribose if it is in RNA. We are going to use these two pictures of the deoxyribose and ribose to explain the differences between the two. And apart from the five-carbon monosaccharides, the second thing a nucleotide has is a nitrogen-containing cyclic compound, which is a nitrogenous base, which can either be purine or pyrimidine. We are going to see this in the subsequent slides. And also, the last part, the last component of the nucleotide is a phosphate group. So, each nucleotide has a five-carbon monosaccharide, five-carbon pentose sugar, nitrogen base, and a phosphate group.
So, what is the difference between ribose and deoxyribose? We said deoxyribose is found in DNA, and ribose is found in RNA. If you look at the deoxyribose here, if you start counting from this first carbon—carbon one, carbon two—in carbon two, you pay attention to carbon two. It has—because it's a five-carbon sugar—carbon one, carbon two, carbon three, carbon four, and carbon five. So, in carbon two, you look at the H here; look at it. Carbon two has H in the deoxyribose, while in ribose, carbon two has OH. So, the deoxyribose in DNA—the deoxyribonucleic acid—and ribonucleic acid has ribose sugar because it has OH at position two. So, take note of this difference now. So, this is a typical structure, or typical representation of a nucleotide. We said it has how many components? Can you remember? We just said it now: three. It has a pentose sugar, a pentose sugar, right? It has a nitrogenous base, and it has a phosphate group. So, you have one, two, three components. This makes up a nucleotide, okay? So, let's proceed now.
What are these nitrogenous bases? These nitrogenous bases are divided into two. We have the purine bases, which contain two rings. Look at them. We have adenine, which is represented as A, and guanine, which is represented as G. These letters—A, G, C, U, T—are what you will be encountering in the nucleic acids because it's going to be too ambiguous, too cumbersome to be writing the full names. So, A is for adenine, G is for guanine. So, adenine and guanine are two-membered, they have two rings, and they are members of purine bases. Now, the second nitrogenous bases group is pyrimidine bases, which consists of, or contains, only one ring. You can see we have cytosine, represented as C, uracil, represented as U, and thymine, represented as—represented as what—T. So, they have what? One ring, one ring, okay? So, adenine, guanine, and cytosine are both common to DNA and RNA, while uracil is only found in RNA, and thymine is only found in DNA. Okay? So, in RNA, the thymine that is found in DNA is being substituted with uracil. So, instead of T, RNA has uracil, but they have this C, adenine, guanine; they have them in common. The only difference is that instead of T, you have what? Uracil. While in DNA, instead of uracil, you have thymine. Please take note of that.
So, what is this phosphate group? We said this phosphate group, you know, is part of the three components of that nucleotide. So, it is a very important component of nucleic acids. It serves as the link between two sugar molecules or nucleotides in polynucleotides. When we say polynucleotides, they are polymers of nucleotides; you know, nucleic acid, that's what we are referring to as. So, it serves as a link between two sugar molecules or nucleotides. So you take notes that this phosphate group is important in nucleic acid, and it serves as a link between two molecules of what? Nucleotide in polynucleotides. So, later on, in subsequent slides, we will see how this phosphate group is used to link between sugar molecules. Okay? So, the phosphate group is a strong acid in solution. This explains why DNA and RNA are called what? Acids. It's a strong acid in solution; so, that is why DNA and RNA are called acids. It is responsible for the negative charges that are found in DNA and RNA at physiological pH. So, these are the three or four important notes that you take into consideration when we are talking about the phosphate group in nucleic acid.
So, you remember we talked about nucleotides, right? So now, what are nucleotides? You know, nucleotides, they have—we explained that nucleotides have three components: they have phosphate, phosphate group; they have pentose sugar; and they have what? Nitrogenous base. So, nucleotides—the only difference between nucleoside and nucleotide is that nucleosides do not have a phosphate group. So, they have—nucleosides have ribose or deoxyribose sugar plus the nitrogenous base. So, they don't have the phosphate group. So, if you have a nucleotide, and you remove the phosphate group, you have what we call the nucleoside. So, the combination of ribose or deoxyribose and one of the five bases produces a nucleoside. They are linked via a covalent beta-N-glycosidic bond. Take note: this is what—this is the type of bond that exists between them. They are linked via covalent beta-N-glycosidic bond. So, you can see we have here—there are some nucleosides that are found in RNA, and there are nucleosides that are found in DNA. So, this is uracil, okay? This is what? Uracil. You can see—this is uracil, this is cytidine, this is adenosine, this is guanosine, okay? So, this is from uracil. The nitrogenous base is what? Uracil, without the phosphate group. So, you have what? Uracil. The nitrogenous base here is what? Cytosine, without the phosphate group, you have what? Cytidine. So, also adenosine, guanosine—these they are found in RNA, while the nucleotides that are found in DNA, you have what? Deoxyribose, because the pentose sugar here, the O is absent; you only have what? Hydrogen instead of what? O. So, that is why it is deoxyribose, you remember? So, in DNA, you have what? Deoxythymidine, deoxycytidine, deoxyadenosine, and deoxyguanosine.
So, nucleotides now. Nucleotides are phosphorylated nucleosides; that is, a nucleoside plus phosphate group, you have what we call nucleotides, or a sugar plus base plus phosphate group—that is a nucleotide. Each nucleotide is a 5'-monophosphate ester of a nucleoside. So, it is a 5'-monophosphate ester of a nucleoside. So, the phosphate group is attached to the hydroxyl group of pentose sugar by an ester linkage, usually OH group on carbon 5, 3, or 2. So, for in this case, you can see that the attachment of the phosphate group is at carbon 5. So, this is nitrogenous base, which can be either purine or pyrimidine. This is carbon one, carbon two, carbon three, carbon four, this is carbon five. You have one phosphate. You can name it nucleoside—if you have—um—from here, without—if you can see this is—if you draw the line from here, you don't have the O, you don't have the phosphate group, so it means you have only pentose sugar and the base, which is nucleoside. So, a nucleoside monophosphate, it has one phosphate group—nucleoside monophosphate, right? Nucleoside diphosphate, right? And nucleoside triphosphate—NMP, NDP, and NTP. So, all these nucleoside monophosphate, nucleoside diphosphate, and nucleoside triphosphates, they are all nucleotides, okay?
Now, let's look at the types of nucleotides that we have. Number one, we have monophosphate nucleotides. These monophosphate nucleotides containing only one phosphate molecule. So, you can have three examples here: A, B, C, which is adenosine monophosphate. Look at the structure here: monophosphate, guanosine monophosphate, and you have what? Uracil monophosphate. So, these are the nucleotides that contain only one phosphate molecule from adenine, guanine, and uracil. So, you have uracil monophosphate, monophosphate, and monophosphate. Now, second types of nucleotide, we have the diphosphate and triphosphate nucleotides, which they contain two for diphosphate and three for triphosphate. They contain three molecules respectively. Diphosphate and triphosphate contain two and three molecules of phosphate respectively. Examples of diphosphate includes—examples of diphosphate nucleotides include ADP, which is adenosine diphosphate, GDP—guanosine diphosphate, UDP—uracil diphosphate. While ATP is adenosine triphosphate—this is the energy currency that we're talking about for living organisms. ATP, GTP—guanosine triphosphate, and UTP—uracil triphosphate—are examples of what? Triphosphate nucleotides. The third category are cyclic nucleotides. In cyclic nucleotides, the phosphate group or phosphate molecule are—they esterify two—they esterify two OH molecules on carbon 5 and carbon 3, or carbon 3 and carbon 2 in RNA. So, you have a small letter c that is usually added to denote the cyclic nucleotide. For example, 3',5'-cyclic—small letter c—it means 3',5'-cyclic adenosine monophosphate. You can have 3',5'-cyclic GMP—it's called 3',5'-cyclic guanosine monophosphate. And the fourth category are called polynucleotides. These polynucleotides are the DNA and the RNA—that is, the polymer of the nucleotides. So, here, the 3'-OH of the pentose of a mononucleotide esterifies the 5' phosphate group of the second mononucleotide to form a dinucleotide. So, the 3'-OH of the pentose of a mononucleotide esterifies the 5' phosphate group of the second mononucleotide to form a dinucleotide. So, the bond between the two nucleotides in polynucleotide is known as the 3',5'-phosphodiester bond. The next nucleotide joins the existing polynucleotide through its free 3'-OH group. Okay? So, each end of the polynucleotide—which is polynucleotide—is distinct. One has a free 5' phosphate, while the other end has a free 3' group. So, for each of these polynucleotides, there is going to be a free 5' and phosphate group. In 5', the phosphate group is what is free, while in the 3', the OH group is free. So, by convention, nucleotides or bases sequence is written in what? 5' to 3' direction. So, polynucleotides are called directional molecules. So, DNA and RNA are long linear polymers called nucleic acids that carry information in a form that can be passed from one generation to the next. So, genetic information is stored in the sequence of bases along a nucleic acid chain.
Let's look at this picture—picture explaining how the nucleic acid looks like. This is the structure. If you look at it, it has these nitrogenous bases. You have the adenine, thymine, guanine, cytosine. So, this is how it looks like. It has base pairing. We have adenine pairing with thymine, guanine is pairing with cytosine. That is how the letters are appearing: thymine with adenine, cytosine. So, A with T, T with A, right? So, G with C, C with G. That's how they appear, okay? The bond between adenine and thymine is two hydrogen bonds, while the bond between guanine and cytosine is three hydrogen bonds. You should take note of that. So, it has a sugar-phosphate backbone, and it has this groove—major and minor groove—and you have the base pairs. So, if you look at this, this is a more open structure. You have this direction, which is—look at it—5' to 3' direction. You remember at the 5', you have what? 5' phosphate group, which is free for joining, right? For chemical bond formation, and at the 3', you have the—the O—the 3' OH, right? Available for bond formation. So, adenine bonds with thymine with two hydrogen bonds; you can see one, two. While guanine and cytosine bonds with three hydrogen bonds—one, two, three; you can see. So, that is how they are pairing. It's from 5' to 3'. This is a single strand, while the other single strand making it the double strand is from—look at it—5' to 3'. So, that is how they appear, okay? So, DNA is double-stranded, okay? Now, what is the difference? Now, let's look at the difference between now—DNA and RNA, okay? You have—in DNA, in DNA, you have what? Double-stranded sugar-phosphate; this is double-stranded, okay? And the pentose sugar in DNA is what? Deoxyribose. While in RNA, you have a single-stranded molecule; it's a single-stranded sugar-phosphate, and the sugar—pentose sugar—is what? Is ribose. Double-stranded in DNA, single-stranded in RNA. Sugar in RNA is ribose, while the pentose sugar in DNA is deoxyribose. So, what do they have in common? They have cytosine, guanine, and adenine in common, but in DNA, you have thymine being the nitrogenous base, while in RNA, you have uracil replacing T. Okay? So, these are the observable features between DNA and RNA. Thank you very much. This is the end of today's lecture. I hope you have enjoyed it, and you are free—feel free, please—to ask questions in the comment section if you don't understand anything. And please like and comment this video, and also don't forget to subscribe if you have not done that. Thank you very much. See you in the next class.