Chromosomes, chromatids and chromatin (video) | Khan Academy
Therefore, every chromosome contains hundreds of thousands of nucleosomes, and these nucleosomes are joined by the DNA that runs between them (an. The term DNA, chromosome, and chromatin are three terms which have very distinct meanings in biology. DNA stands for deoxyribonucleic acid and refers to a. How do chromosomes, DNA and genes all fit together? called 'chromatin'. can use the analogy of a city to better understand the relationship between DNA .
There's a lot of words and some of them kind of sound like each other, but they can be very confusing. So the first few I'd like to talk about is just about how DNA either generates more DNA, makes copies of itself, or how it essentially makes proteins, and we've talked about this in the DNA video. Just some small section.
And, of course, it's a double helix. It has its corresponding bases. Let me do that in this color. And then, of course, it just keeps going on in that direction.
So there's a couple of different processes that this DNA has to do. One is when you're just dealing with your body cells and you need to make more versions of your skin cells, your DNA has to copy itself, and this process is called replication.
You're replicating the DNA. So let me do replication. So how can this DNA copy itself?
- Chromosomes, chromatids and chromatin
- What is the relationship of DNA, a chromosome and chromatin.?
And this is one of the beautiful things about how DNA is structured. So I'm doing a gross oversimplification, but the idea is these two strands separate, and it doesn't happen on its own. It's facilitated by a bunch of proteins and enzymes, but I'll talk about the details of the microbiology in a future video.
So these guys separate from each other. Let me put it up here. They separate from each other. Let me take the other guy. That guy looks something like that.
They separate from each other, and then once they've separated from each other, what could happen? Let me delete some of that stuff over here. Delete that stuff right there. So you have this double helix.
They were all connected. Now, they separate from each other. Now once they separate, what can each of these do? They can now become the template for each other. If this guy is sitting by himself, now all of a sudden, a thymine base might come and join right here, so these nucleotides will start lining up. So you'll have a thymine and a cytosine, and then an adenine, adenine, guanine, guanine, and it'll keep happening.DNA Replication and Packaging - Richard McIntosh (CU Boulder)
And then on this other part, this other green strand that was formerly attached to this blue strand, the same thing will happen.
You have an adenine, a guanine, thymine, thymine, cytosine, cytosine. So what just happened? By separating and then just attracting their complementary bases, we just duplicated this molecule, right? We'll do the microbiology of it in the future, but this is just to get the idea.
This is how the DNA makes copies of itself. And especially when we talk about mitosis and meiosis, I might say, oh, this is the stage where the replication has occurred. Now, the other thing that you'll hear a lot, and I talked about this in the DNA video, is transcription. In the DNA video, I didn't focus much on how does DNA duplicate itself, but one of the beautiful things about this double helix design is it really is that easy to duplicate itself.
You just split the two strips, the two helices, and then they essentially become a template for the other one, and then you have a duplicate.
What is the relationship of DNA, a chromosome, and chromatin?
Now, transcription is what needs to occur for this DNA eventually to turn into proteins, but transcription is the intermediate step. And then that mRNA leaves the nucleus of the cell and goes out to the ribosomes, and I'll talk about that in a second.
So we can do the same thing. So this guy, once again during transcription, will also split apart. So that was one split there and then the other split is right there. And actually, maybe it makes more sense just to do one-half of it, so let me delete that. Let's say that we're just going to transcribe the green side right here.
Let me erase all this stuff right-- nope, wrong color. Let me erase this stuff right here. Now, what happens is instead of having deoxyribonucleic acid nucleotides pair up with this DNA strand, you have ribonucleic acid, or RNA pair up with this. And I'll do RNA in magneta. So the RNA will pair up with it. And so thymine on the DNA side will pair up with adenine.
Guanine, now, when we talk about RNA, instead of thymine, we have uracil, uracil, cytosine, cytosine, and it just keeps going. That mRNA separates, and it leaves the nucleus. It leaves the nucleus, and then you have translation. The transfer RNA were kind of the trucks that drove up the amino acids to the mRNA, and this all occurs inside these parts of the cell called the ribosome. But the translation is essentially going from the mRNA to the proteins, and we saw how that happened.
You have this guy-- let me make a copy here. Let me actually copy the whole thing. This guy separates, leaves the nucleus, and then you had those little tRNA trucks that essentially drive up. So maybe I have some tRNA. Let's see, adenine, adenine, guanine, and guanine.
A codon has three base pairs, and attached to it, it has some amino acid. And then you have some other piece of tRNA. Let's say it's a uracil, cytosine, adenine.
And attached to that, it has a different amino acid.
Chromosomes and Chromatin
Then the amino acids attach to each other, and then they form this long chain of amino acids, which is a protein, and the proteins form these weird and complicated shapes. So just to kind of make sure you understand, so if we start with DNA, and we're essentially making copies of DNA, this is replication.
You are transcribing the information from one form to another: Now, when the mRNA leaves the nucleus of the cell, and I've talked-- well, let me just draw a cell just to hit the point home, if this is a whole cell, and we'll do the structure of a cell in the future.
If that's the whole cell, the nucleus is the center. That's where all the DNA is sitting in there, and all of the replication and the transcription occurs in here, but then the mRNA leaves the cell, and then inside the ribosomes, which we'll talk about more in the future, you have translation occur and the proteins get formed.
So mRNA to protein is translation. You're translating from the genetic code, so to speak, to the protein code.
So this is translation. So these are just good words to make sure you get clear and make sure you're using the right word when you're talking about the different processes. Now, the other part of the vocabulary of DNA, which, when I first learned it, I found tremendously confusing, are the words chromosome. I'll write them down here because you can already appreciate how confusing they are: So a chromosome, we already talked about.
You can have DNA. You can have a strand of DNA. That's a double helix. This strand, if I were to zoom in, is actually two different helices, and, of course, they have their base pairs joined up. The condensin complex is present in the axial region of each chromatid. Condensin acts on this region to constrain the stretch, possibly by catenating the centromeric DNA.
In the absence of condensin fthe centromeric chromatin can be abnormally pulled by the attached microtubules arrows causing defects in executing mitosis. Chromosomes, white; microtubules, green. Condensin loads onto the chromosomes early in mitosis and is essential for maintaining their structure and organisation, but it is not essential for global chromatin compaction. RCA regulates mitotic chromatin compaction and its activity is mediated by protein phosphorylation.
Specialised chromosome structures and chromosome organisation in interphase. The two sister chromatids white are held together at the centromere and this defines the region where the kinetochore is assembled.
The inner kinetochore red lies underneath the outer kinetochore greenwhich mediates the interaction with the spindle microtubules blue. Each sister chromatid contains a single linear DNA molecule and the terminal ends are called telomeres pink. Courtesy of J Dorrens, University of Edinburgh. In a hybrid chicken cell containing one CHO Chinese hamster ovary chromosome, hybridisation with CHO genomic DNA reveals that the single CHO chromosome present in the cell f is not dispersed within the interphase nucleus e but maintains a compact organisation and a distinct territory.
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