Packaging of DNA

Hi in this video, we're gonna be talking about DNA packaging. So why do we need to talk about DNA packaging and why does this L. Do it? Well it does it because the packaging of D. N. A. Is necessary in order to fit the D. N. A. Into or within the confines of the cell. So there's so much D. N. A. Each cell contains around two m of it. And that's a huge problem because ourselves are considerably smaller than two m. They're usually average 5 to 8 micro meters in diameter. So how do you actually fit two m of D. N. A. Into such a very small amount? Well, you have to package it and so there are a few different packaging levels that we're going to talk about. Um And they're here below, you can see the nuclear's OEM which is here the 30 nanometer fiber which is here um some looping looping which is here and then finally the chromosome which is here um which is number five. And so we're going to go through each one of these um sort of packaging levels in order to determine how they form and what they actually do to package all the D. N. A. Into a cell. So let's move on

so the first packaging level that we're gonna talk about is the nuclear zone. And so the nuclear zone consists of D. N. A. And histone proteins. So they were it was discovered by Dean who wish you wish and labor one. I'm not even going to attempt it. You don't necessarily you don't need to know these names. Are you really going to be tested on them? Just sort of know that these people exist. Somebody discovered it. And so what they found is that what they did though is a really interesting experiment where they took D. N. A. Isolated from cells and they put in enzymes that chop up D. N. A. And what they found is that the DNA wasn't just chopped entirely to pieces. But instead it was chopped into these sort of repetitive 200 base pair of fragments. So they were like well why is the D. N. A. Being chopped into 200 base pair pregnant when if it was entirely exposed it should just be cut all the pieces. And so they they found out that or they hypothesized that the D. N. A. Was cut into 200 base pair of fragments because it was being protected by something. And so what it's being protected by our histone proteins. So what are histone proteins? They are a major class of proteins that are bound uh two D. N. A. Class of proteins bound to DNA. To form a nuclear zone. So there are actually five classes of these proteins. They're not given very creative names. Uh here they are H. One H. Two H. Two B. H three H four. Um And they're how they're classified is based on the ratio of life seemed to argentine present on the protein. So we have D. N. A. We have these five classes of histone proteins. How do these all come together to create an eclipse? Um Well how they do this is throw there are eight histone proteins per nuclear zone core. So you have to H. Two A. And two A. And two H. Two Bs. And they are bound together to form die MERS. And you also have another pair of two H three's and two H. Four that are formed. And these act as a nuclear zone core. So those are eight proteins here. So you have you have your four here because you have two pairs and your four here which you have two pairs. And so because these are positively charged proteins because of their life scene and our ginny um the negatively charged DNA just sort of easily wraps around them. Now you have this ad so that takes care of four of the classes but obviously not the fifth. So the fifth class is an H. One histone and that acts as a linker histone connecting them together. Um So each one of these nuclear zones are about 10 nanometers long. Um And you don't need to know these numbers just sort of in order to really think about it and conceptualize it in reality I added them. So in a nuclear zone there's going to be 100 and 47 base pairs of DNA wrapped around it. Um And it'll wrap 1.67 times around the histone core. Like I said, you don't need to know those numbers. Just sort of if you're thinking about, well, you know, what is the nuclear zone? How big is it? Well it's 147 base pairs. So if we're gonna look at the nuclear zone here, you have your D. N. A. This is your double helix Felix. And you can see that the D. N. A. Which is here in red is easily wrapping around this nuclear zone poor. So this is gonna have your eight histone proteins. And it's not really shown on here. But if I were to draw the H. One histone, which is a linker, it's gonna be kind of in the middle here. It's gonna be working to link these nuclear zones. Which is each one of these is a nuclear zone. So here's one, here's one um together. So this is the very first packaging level of D. N. A. From the double helix to the nuclear zone. And then you get this sort of nuclear zone string. So now let's move on

Okay, so now I'm going to talk about the next two levels of DNA packaging and that is the chromosome fiber and DNA looping. So you'll see here that I don't have a lot of information in this section, especially compared to previous section of the nuclear zone. The reason is because really not much is known about these. We know they exist, we have some images of them, but really how they form and you know what are all of these components and why they form this way is really not known. So I'm just gonna give you, you know, the very big information that is known and show you an image of what this looks like. So first thing is the formation of the chroma tin fiber. So we have our nuclear zones. Remember those are kind of the beads on a string and they're packaged into a 30 nanometer fiber. And really what facilitates this, 30 nanometer fiber creation is the H one histone protein. Remember this is the linker histone protein and it connects adjacent nuclear zones and is required for the 30 nanometer fiber formation. So these are generally packaged in zigzags round around a double helix. So if I were to show you this image, you're going to be looking at the chromosome fiber here and three A. And three B. And what you can see is that there is there is this sort of zigzag formation, you can't can't really draw on it. But if you were to look at this, you're going to see the zigzag formations of these nuclear zones connecting together. And so this forms this kind of snake like Fiber here that is you know, eventually formed into 30 nm long, which is what it said. Now if we're moving on to DNA looping, that's gonna be your images over here. Let me back out of the way. So you can see it better. So each loop contains around 50,000 to 100,000 base pairs. So we're getting much larger structures now. And these are actually these structures are maintained by by non histone proteins or proteins that are not his stones that attach the D. N. A. To this protein scaffold that allow them to form these really unique straw pictures. I mean look at this, this is this structure here and this one here, this one here, these are kind of weird. Um and we actually don't have good images of what this looks like in real life. So the models you can imagine look very weird, but it also looks weird inside the cell. And so this DNA looping condenses the D. N. A. Even further than 30 nanometers so that it can be packaged into the cell. So now let's move on

Hello everyone in this lesson. We are going to be talking about D. N. A packaging, specifically how chromosomes are made and how we utilize chromosomes in our cells. Now it's obvious that D. N. A. Is gigantic. These huge strings of nucleic acids and we're going to have to package our D. N. A. In some form. So it's just not crazy all over the place. So we all kind of understand what chromosomes are. But here we're going to go into a little bit more detail. So chromosomes are chroma tin that is packaged in a particular way. So remember crow metin we learned is going to be the D. N. A. Nucleic acids, all those bases plus the proteins associated with them. Things like nuclear soames and his stones. And it's going to include RNA. That is also associated with the D. N. A. This is going to be chroma tin and chroma tin is going to be packed into the form of a chromosome And this is going to have thousands and thousands of genes in each chromosome for that particular organism. And chromosomes are going to exist in two unique states. Two distinct states. And these are going to be the interphase chromosome state and the meta phase chromosome state. So basically the differences between these two states is going to be the density of compaction. How compacted. How tightly wound is the D. N. A. Is it loose? Is it really tightly wound? That's going to be the difference here. So interface chromosomes is referring to when the cell is in interface and this means that it's not actively dividing at the moment. So these are actually going to be less condensed DNA threads less condensed chroma tin and they're going to occupy the nucleus. And it's going to be kind of just like a loose threading of the D. N. A. It's not going to be terribly condemn. This is one of the loosest forms that D. N. A. Is in. And because it's so loose you can't really see interface chromosomes under the microscope. You really can't see where individual chromosomes are whatsoever. So it's very loose and it's not condensed at all. Now whenever the cell decides that it's going to go into cellular division, be it mitosis or mitosis then we're going to get meta phase chromosomes and meta phase chromosomes are created from the interface chromosomes and basically just condensing them down as much as we can. So meta phase chromosomes are going to be more condensed. These are going to be the stereotypical X structures that you guys will see down here in just a second. These are going to be the stereotypical X structures that we think of. Whenever we think of chromosomes and they can be seen under a microscope during cellular division these are very highly condensed forms of chromosomes. Now why would we want them to be very highly condensed? Well remember I said this is happening when the cell is dividing. And if the D. N. A. Is very tightly condensed into their own chromosomes they're much easier to move. It's much easier to separate the chromosomes than if they're all tangled and loose together in the interface chromosome state. So that's why your cells greatly condensed down the chromosomes into that nice, tightly packaged meta phase chromosome. Now chromosomes are going to have some very important structural features that we're going to talk about as well. So chromosomes contain centrum ears, kinetic ores and telomeres. In addition to all of the genes that are in your chromosomes. So first we're going to talk about a Sentra mirror. The central mirror is going to be the specialized region of D. N. A. Where to sister chroma tides meet. So this is going to look like, let's see if I can draw this for you guys, it's going to look like this. Let's say we have a duplicated from here. So it's going to have sister chroma tides. And the region where these two sister chroma tides meet is going to be the centrum here and it's going to hold the sister chroma tides together. And that's going to be where they meet and where they inevitably split during the process of DNA replicate. Excuse me. Whenever they split in anna phase depending on mitosis or mitosis, that's going to be the centrum here and these are going to be specialized regions it generally consists of large sequences of repetitive satellite D. N. A. So it's very repetitive in that area and its main job is just to allow for those sister chroma tides to attach to one another. And it's going to allow kinetic or proteins to attach. So what is the kinetic or the kinetic or is going to be a protein structure assembled on the centrum here. So what's that going to look like That is going to look like this? So we have these kinetic or proteins that attach to the area of the central here. So in red we have the kinetic course. And this is going to be where the spindle fibers attach. So the centrum here is that region that allows the chroma tides to attach. The kinetic ores are proteins that attach to the central mirror. And then the main topic, spindles or the micro tubules attached to the Connecticut. I know it's a lot of layers here but they're all very important. So this is where the spindle fibers attached during cellular division. So what is that going to look like? That is going to look like these fibers attached to the chromosomes which are going to pull them apart. So spindle fibers. So these are very important structures for cellular division because they allow those chroma tends to be separated and they allow chromosomes to be moved around. Now we also have another special structure called the telomere telomere is utilized to ensure chromosome integrity. Telomeres are also repetitive DNA sequences but there at the end of a chromosome and they're utilized to protect the chromosome from degradation, protected from deteriorating because every time time a chromosome replicates some of the ends of the chromosomes are lost whenever the chromosome is replicated. So we don't want our important genes that code for things in our bodies to be lost from our chromosomes. So what do we do? We put repetitive kind of nonsense D. N. A. At the ends of our chromosomes. That it doesn't matter if it gets lost whenever these cell replicates. So a telomere would be kind of at the end here on linear D. N. A. And we have linear DNA. These telomeres are here in black. They are going to be repetitive sequences. They're not random though the sequence that is most highly repeated in telomeres is going to look like this. This is going to be the very important sequence that is repeated over and over and over again I believe 2500 times it's repeated. And then these telomeres will be degraded over time over the life of the individual. And their job is to protect the coding DNA, the impor D. N. A. So this is going to be another representation in addition to what I already drew for you guys. This is a meta phase chromosome which means it's a very highly condensed chromosome and we can see that this region is going to be the centrum ear and there are most likely going to be kinetic or proteins in that central area. And then things like telomeres would be on the ends here where I'm highlighting in green. These would be the regions for telomeres, those protective ends of linear D. N. A. Okay, so now let's go down and we're going to talk about stereotypes, stereotypes. This is an example of a stereotype here. A carry a type is an ordered display of a full set of an organism's chromosomes. So if you want to look at all the chromosomes in an organism, you're going to want to look at the stereotype and stereotypes are commonly done for humans. And you're going to see that in deployed organisms like humans. This is a human carry a type here. So this is a human carry a type. You're going to have homologous pairs. So we are deployed organisms Which means we have two sets of chromosomes which I'm just gonna write chrome's. We have two sets of chromosomes. So there's two of each chromosome and these are called homologous pairs. So you're going to find homologous pairs in all of the chromosomes except for the male Y chromosome, which is not going to have a homologous pair, females, the X chromosomes do Have homologous pairs but in males the Y chromosome does not have a homologous pair other than that you're going to see all of these paired chromosomes as you can see in this image. So remember that human beings have 23 unique chromosomes but 46 in total. So 46 chromosomes in total. And you can see all 46 here, but they're paired up because there's two of each chromosome or 23 unique different chromosomes. Okay, everyone, I hope that was helpful. Now let's go on to our next topic.

Okay. So now we're gonna talk about the conservation of DNA packaging. So we have all these really complex ways that DNA. Is packaged in order to fit it into the cell. But that doesn't necessarily mean that every cell does it the same way. But um in we find in organisms throughout the world that packaging of D. N. A. Is highly conserved. So it is really done very similarly across organisms. So how does this happen while this happens? Because histone proteins are extremely conserved. So for instance, you don't necessarily need to know this number. But the H. Three histone protein of sea urchin and calf thymus differs by only one amino acid. So you can imagine the differences that exist between a sea urchin and a calf thymus are huge. But the age three histone protein has only changed by one amino acid. Now that's not to say that all histone pro there's only these, you know, um There's only these five histone proteins. There are actually histone variants that we're gonna talk about in future lessons and they usually are extremely important but they only have one function. So as the, you know, the H two, A. H two, B. H. H three, H four H one. Histamine that we talked about in the formation of the nuclear zone are super conserved. But there are variants that have popped up. But instead of forming the nuclear zone they actually have very unique and specific functions in different organisms. So one example of this is this H. Three, the central Merrick H. Three. And this is a histone protein that exists only at centrum ears. And it has a very specific function in assembling of kinetic or proteins. So just to take a second to look at some of the conservation um here you have different animals Here on the sign. And this is looking at the history approaching H1. And you can see that if you look at the amino acid sequence which is what this is I said sequence. Sometimes they refer to amino acid as amino acid residues. Um You can see that looking through all these organisms, it's extremely similar. There are some variations. Um you'll see them here here but even within these variations um most of them are the same. Um And so they're extremely conserved proteins which makes DNA packaging extremely conserved as well. So now let's move on.

Okay. So in this video we're gonna be talking about unusual chromosome structure. So I've talked to you a lot about the packaging of DNA and how that's normally done in organisms. But in a few organisms that contain these unique chromosomes that I'm just going to mention to you just so you know that you know not everything is just perfect in the world of cell biology. So certain organisms contain these unique like you you e chromosome structures. One of these is gonna be called politicking chromosomes and they're found in joe's ophelia. Which if you remember our fruit flies and how these form is instead of separating during division which is what happens during most forms of division, they actually linked together. So kind of the exact opposite of what should happen during division. And so when this happens they actually have this unique banding that you can just visualize under a scope. Um We don't really know what causes this banding. Why are some of them darker and some of them lighter. But it's thought that most of this banding, the darker and lighter is caused by def condensation. So the darker the band, the more condensed. So we can see an example of politician chromosomes here. Um And so these are these sort of very long chromosomes that you can see that have been linked together and they have these bands which you can actually visualize here some up close and they think that the darker version is more condensed D. N. A. Whereas the lighter portion of it is less condensed. So this is one sort of unusual chromosome structure that occurs on Earth. Another one. Another structure is called a lamp brush chromosome and this is generally found in various oocytes or ovarian cells, um but not really observed in mammals, just sort of other types of animal ovarian cells. And these are interesting because they are the largest chromosomes known. So these chromosomes, you don't actually need special, really intense microscopes to see, you can just see them with a normal light microscope that you probably have in your biology labs. Um And so that's kind of a unique feature. And so yeah, those are politician and land brush are two sort of unique chromosome structures that exist and that are important to know about in the study of cell biology. So now let's move on.

Which of the following histone proteins do not form dimers that make up the nucleosome core?

• A. H2A
• B. H2B
• C. H3
• D. H4
• E. H1

How many histone proteins are found within the nucleosome core?

• A. 2
• B. 4
• C. 8
• D. 9

Interphase chromosomes are more condensed than other forms of chromosomes?

• A. True
• B. False