Hey it's professor Dave, let's discuss nucleic acids. We learned about proteins and carbohydrates as two of the main polymers in the body, but there is one more type we need to learn about, and that's nucleic acids. We've probably heard of these, since they include DNA and RNA, and we probably know a little bit about DNA. But it's time to know a lot more, because this is the molecule that gives an organism its identity, and we want to comprehend all of the intricate processes associated with DNA, as this will allow us to combat cancer and other diseases. Just like the other polymers we learned about, first we have to understand the monomers they are comprised of, so let's take a look at nucleotides.
A nucleotide is a molecule that has three sections. first there is a monosaccharide which is always either D ribose or 2-deoxy-D ribose, and we can number the carbons 1 prime through 5 prime. the difference between these has to do with the presence or absence of a hydroxyl on carbon 2, and these sugars will specifically occur in RNA or ribonucleic acid and DNA or deoxyribonucleic acid respectively. next extending from the anomeric carbon a heterocyclic base.
it's a heterocycle because there is an element other than carbon inside the rings, in this case nitrogen, and it's a base because of the lone pairs on the nitrogen atoms. these bases can be either purines or pyrimidines, depending on whether there is one ring or two, and in DNA the bases are adenine, guanine, cytosine, and thymine, which are abbreviated as A, G, C, and T. in RNA, instead of thymine there will be uracil.
or U, which is almost the same as T, it's just missing this methyl group. These bases always have beta glycosidic linkage to the anomeric carbon. The sugar and the base combined are called a nucleoside, and these have names related to the bases.
If the base is adenine, we get either adenosine or 2-deoxyadenosine, and so forth. Now if we add the third section we mentioned earlier, which is a phosphate group on carbon 5 of the sugar, we will get a nucleotide. nucleotides include ATP, which is the currency of cellular energy that we will talk more about later, as well as cyclic AMP, a second messenger molecule released in signal transduction.
but most importantly nucleotides are the monomers that make up nucleic acids. in nucleic acids the monomers are connected by phosphate esters that link the three prime hydroxyl of one nucleotide to the five prime hydroxyl of another. As these come together, we can see that there is a backbone comprised of identical sugars and phosphate groups with the varying bases protruding from the chain. In this way, we can list the sequence of bases like GCAT, just the way we list the primary structure of a protein when we list the sequence of amino acids. Now comes the interesting part.
Before we fully understood the structure of DNA, we noticed that pairs of bases were always present in roughly equal amounts, those pairs being A and T as well as C and G. After much research, we finally understood that this is because DNA exists as two strands that pair up in base-specific manner, meaning that the two strands are complementary. Everywhere that there is a C on one strand, there is a G right across on the other strand, and everywhere there is an A, there is a T on the other side. This specific pairing happens for two reasons. First, the geometry of DNA is such that one purine and one pyrimidine will fit nicely between the strands, whereas two purines would be too wide and two pyrimidines would be too narrow.
But the specific pairing out of the two spatially viable possibilities is because of hydrogen bonding that happens between complementary bases. We can see here that an AT pair has this carbonyl interacting with this proton. and this other proton interacting with the lone pair on this nitrogen atom. A C-G pair makes three such interactions.
If we tried to bind A with C and G with T, these interactions would be impossible, so base pairing is highly specific, and a DNA molecule is millions of these base pairs extending in a double helix that looks like this, with one strand being a perfect complement to the other, allowing for an unimaginable number of possible sequences. These two strands are anti-parallel, meaning they run in opposite directions, one going from what we call the five prime end to the three prime end, whereas the complementary strand will go three prime to five prime. By contrast, RNA, which as we said is comprised of ribose sugars rather than two deoxyribose, will tend to be single-stranded, and we will learn about certain kinds of RNA later. Later we will also learn about how DNA is the genetic code that makes you what you are, and this code has to be inside every single cell in your body.
But because there is so much information to code, the DNA molecule is really, really long. If you took the DNA from just one of your cells and stretched it out, it would be over a meter long. And if you took all of the DNA molecules in all of the trillions of cells in your body, unwound them, and line them up end to end, it would stretch to the end of the solar system and back. How is there room for all of this stuff in your body?
Well DNA is stored by coiling around proteins called histones, then these coils undergo supercoiling to save even more space. One long supercoiled DNA molecule with all of the histones is called a chromosome, and all of the genetic material in the nucleus of a cell is collectively referred to as chromatin. And while this kind of image may have been familiar before, now we know how to zoom in and look at the individual monomers of this huge molecule, which will allow us to understand all the things that DNA can do. Thanks for watching, guys. Subscribe to my channel for more tutorials, and as always, feel free to email me, professordaveexplains at gmail.com.