Hi in this video we're gonna talk about eukaryotic chromosome structure. So eukaryotic chromosomes have a specific structure and the first term that you need to know is chrome aton. And chrome aton is a combination of D. N. A. And protein. And there are two types of protein. There are hetero chroma tin and you chrome aton and hetero chromosome means that it's tightly packed and you Cro magnon means that it's loosely packed. So we're looking at this, this would be hetero chrome button, the D. N. A. Is here in black and the proteins are here on these like colorful circles and this is gonna be you crow metin. Now there's lots of other things here. You don't need to know any of that. You don't need to know what these are, that's where different class. But for now just know hetero Komotini is tightly closed. You Cro magnon is open now proteins that package with the D. N. A. R. Associate with the D. N. A. Often are really important for condensing that D. N. A. To a small um chromosome that actually fits into the nucleus. And so chromosomes require a lot of packaging and therefore different levels the first level. And the one that's most talked about in your book is the nuclear zone and this is made up a group of proteins called histone proteins and D. N. A. Now there's a lot of different types of histone proteins and they do different things in the nuclear zone. So the nuclear zone has a histone core. And so there's two copies of each of the H. Two A. H two, B. H three and H four. These are histone proteins. So there's two copies of each times two. And these make up a core that the DNA wraps around. And you can see these here. These are histone, these are nuclear zones. And inside of them, these are the histone cores. These balls here. Now in between the histone core you get a histone linker. This has a histone protein called H. one and it lies in between and it connects those histone cores together. So those together make up the nuclear zone. Now from here, these nuclear zones are condensed even further into what's known as a 30 nanometer fiber. And this fiber has multiple nuclear zones in it. So, this would look something like this. Right? So, you can't see the individual nuclear zones. But if you did, if I circled in and zoomed out, right, What I would see is I see a nuclear zone nuclear zone nuclear zone, nuclear zone nuclear zone. And they would be right next to each other sort of all tied in together. And the D. N. A. Would be wrapping around them. All right. And going around couple of times. And this is packaging that D. N. A. Really tightly. You can imagine it as a string. If you were wrapping it around all these different balls, you would get you can put a lot of string very lengthy. The amount of string into a very small area. So that is the 30 nanometer fiber. It then goes up into a 250 nanometer fiber which has these these condensed even further. And you see these are starting to look like a chromosome. Um I'm not gonna zoom in and draw it. But essentially this is what it looks like. It's more condensed together and then eventually you get even more condensed and that turns into the chromosome. So there's a lot of different condensing that goes into creating these chromosome structures and fitting that entire length of D. N. A. Into such a small area. Now when you get to the chromosome level there are specific structures on the chromosome as well. The first is the centrum here and this is the constricted region of the chromosome and the spindle fibers attached during division. And that allows chromosomes to be separated properly into the gametes. The chromosome is associated with a protein complex called the kinetic or so a group of proteins that links centrum ears to the spindle fibers. So the spindle fibers don't attach directly to the centrum here. They actually attached to the kinetic or so you have central air kinetic or spindle fibers. And that is that whole complex there allows chromosomes to separate properly during division. Now the center mirror is condensed. It's constricted so is it likely to have hetero commenting or you chrome button. Do you remember? It's likely to be hetero chrome aton because it's condensed. So it's a lot of hetero chromosome right there and it has certain sequences that allow the kinetic or to attach. And in order to get that such tight hetero chrome button that is present in the centrum. Er there's actually a histone variant. Histone variants are very rare. So we talked about histone proteins up here and I said these are the these are the five H 234 and one. These are the five types of histone proteins. Well here's a variant and this is the sin H. Three and it's only found at centrum ears. Like I said. Histone variants are very weird or very rare but it's important because the central here is a very unique place. Right? So it makes sense that it has a unique histone protein on it. Now the second structure on a chromosome is the telomere and this is the end of the chromosome. And the telomere is characterized by containing telomere sequences. It's repeats of A. S. And T. S followed by two or three G. So it can be a T. T. A. T. G. Right? Is an example of a telomere sequence but usually it's the same type of sequence over and over and over again. And that sequence may vary by species but generally the sequence is just a bunch of A's. And T. S followed by Gs. Now tell America sequences contain a ton of repeats. I mean up to like 250 killer bases of repeats at these sequences. But proteins especially shelter in as a protein. It binds these sequences and prevents D. N. A. From just breaking off these repetitive sequencing and messing them up because you don't want that to happen or your chromosome shortening. Right? So the protein binds those repetitive sequences and keep those repetitive sequences strong even though they're very repetitive which is usually typically hard on the cell to maintain those repetitive sequences. And then at the very very very very end. I mean the very end of the telomere there's a G rich three prime overhang which means that the three prime, there's a single stranded region that contains a ton of Gs on it. Right? So this part here is not double stranded. It's the whole part of the whole chromosome that's not double stranded. And this which we'll talk about when we talk about replication is important in replication. So if I were to zoom in at a telomere which is at the end, you can see that here is a repetitive sequences T T A G G G. And it's gonna go keep going all the way around to the very end. So that is some structure. Let's now move on.

Okay. So now let's talk about an interesting structure called super coiling. Super coiling is a tight um tight condensation or structure of chromosomes. It's like over tight almost. So if you if you have a rope, if you imagine you had two ropes, right? And they were a double helix. So you just wrapped one rope around the other and you keep twisting it right? Keep twisting it eventually it bunches up in the middle. You can do it with rubber band or hair scrunchie if you just keep twisting it right Eventually it twists up on itself because it has so much tension in those twists that it causes that structure to become something that it's not supposed to be. And that's called super coiling. Now, positive super coiling. Cause DNA. That are over rotated and negative super coiling is DNA that is under rotated. Um And essentially both are bad. Right? And there's a class of enzymes in the cell called Topo a summer races. And these are enzymes that fix these altered rotations and D. N. A. So type one is relaxes the number of negative super coils. So there we go. And then type two, which you may also see is D. N. A. Dry race. These are the same things. Um These introduce negative super coils to remove positive super coils. So they both do different things. But essentially how they work actually is they both introduce breaks into the D. N. A. So you can imagine if you have a scrunchie that you've or two ropes that you've twisted together and twisted and it's curled in on it if you take a pair of scissors, really strong scissor for rope, right? And you were to cut one of the strands, it would release that tension a little right? And if you were to cut both of them it would completely release it. And so these two act by cutting those D. N. A strands. Now repair mechanism comes in and fixes it because I can't just say cut or you're just going to cut up your whole D. N. A. Um But in order to release those super coil ing's, these just introduce different water called nicks or cuts into the D. N. A. So here's an example of super coiling. Don't worry about all these steps. This is not important. Right? But here's D. N. A guy race down here. This is the enzyme and you can see that you have um this is really super coil D. N. A. It doesn't look like a nice double helix which is what we're used to. And so these can come in and relax it. Or if you have, if it's too relaxed, it can come in and tighten it. Let me back up. It can come in and tighten it if it needs to. But essentially these topo summer races work too. So this is the type one up here. This is a type two and these work by either somehow fixing either over relaxed or over tightened um super coil D. N. A. Super important. We're going to see these again when we talk about DNA replication as well. Um, so with that, let's move on.