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Chromosomes, chromatids and chromatin

DNA replication, transcription, and translation are key biological processes. Replication involves DNA duplicating itself. Transcription involves DNA creating mRNA, and translation converts mRNA into proteins. Created by Sal Khan.

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  • leaf red style avatar for user Denna
    Before I watched this video, I only know that human has only DNA, and RNA is for lower level organism like Viruses. So, I want to ask what different between mRNA and RNA? Are they the same? And mRNA leaves nucleus for transcription and consequently translation, so it won't be the gene anymore and final result is PROTEIN.(is my conclusion true?) Please help me to answer my question. Thanks!
    (77 votes)
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    • orange juice squid orange style avatar for user Bob
      Some good questions. Unfortunately, an in-depth answer would take quite a while to give. I suspect many of the later videos in Biology here will also help, but I'll try to give you some idea of things.

      You are correct that in humans--and, indeed, in most organisms: plants, animals, even bacteria--the genetic material is DNA. However, though we use DNA as our genetic material, we also do make use of RNA for other purposes, such as protein synthesis.

      RNA and mRNA are exactly the same thing. So is tRNA. Basically, the "m" in "mRNA," which stands for "messenger" indicates what purpose this particular strand of RNA is going to accomplish within the cell. In the case of mRNA, its job is to copy the information from DNA (because DNA and RNA are compatible in the sense that you can reconstruct the exact same sequence of nucleotides (with the exception of Uracil, though that does not throw off our transcription because we just use U instead of T or T instead of U, depending on which direction we're copying) by copying one to the other.

      So, basically what happens is, we take a sequence of DNA and temporarily "unzip" it so that we can make mRNA that contains the same information. That DNA is then reassembled, so we haven't lost anything. The mRNA is just like a photocopy of the DNA that still leaves the original intact. This is an oversimplification of the process (if you take a General Biology or cell biology course at university, you'll get a much more detailed description), but in a nutshell, that's what transcription is.

      Then, the mRNA does indeed leave the nucleus (though the original DNA stays there, so again--we're not losing any information in this process), and heads over to a ribosome, which is just a fancy word for the structure in the cell where proteins are made. This is where translation happens.

      What happens in translation is that in the ribosome, we have groups of three nucleotides. Say, AAG or CUU. These small sections of RNA are called tRNA or "transfer RNA." They are attached to single amino acids, or the building blocks of proteins. Basically, the mRNA comes into the ribosome. When the first three nucleotides find a segment of tRNA that perfectly matches, they bond. Then the next three--called a codon, by the way--do the same thing, and so on, until we have a long strip of amino acids that eventually will become a complete protein.

      Since these are in groups of three, there are 64 possible combinations--far more than we need for the 20 possible amino acids. So sometimes, two different combinations or codons might code for a single amino acid. And at the end, there are a couple of codons, called stop codons, that signal the end of translation.

      So the whole sequence, in short, is DNA to mRNA to tRNA to protein. DNA stays within the nucleus. mRNA gets synthesized from that DNA and then leaves to join up with tRNA at the ribosome, where it is used to choose just the right amino acids to synthesize a protein.

      Is that helpful, or have I just confused the issue?
      (246 votes)
  • leaf blue style avatar for user James Fredenberg
    During replication, after the two split up, how would the A, C, T, and G just appear from nowhere and attach with the strand?
    (66 votes)
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  • leafers ultimate style avatar for user Carla Bertrán
    Is DNA always wrapped around histones? Or is it just when it starts to condense into a chromosome?
    (38 votes)
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    • mr pants teal style avatar for user Matthew Belliveau
      Great question! DNA can unwind from histones. When DNA is wrapped around histones, the genes cannot be transcribed into RNA (and, eventually in protein). So these enzymes come along called HATs, which chemically modify the histones in effect loosening their grip on the DNA. This allows the transcriptional machinery access to the DNA, creating RNA (and, eventually, protein).

      Other enzymes, called HDACs, have the reverse effect of HATs. They remove the chemical modifications, which allows the DNA to tightly wrap around the histones again, preventing those particular genes in that segment of DNA from being transcribed.

      This is one way in which a cell "turns on/off" certain genes that are needed/unneeded.
      (49 votes)
  • eggleston green style avatar for user tylerjpauls
    What happens if DNA gets damaged? Do you die?
    (22 votes)
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    • orange juice squid orange style avatar for user CarlBiologist
      Sometimes the damaged DNA can be repaired. If it is very bad your body will kill the damaged cell and it will be replaced with a new one. Sometimes if the damage is done in a particular way the cell will become cancerous and replicate out of control. It really depends on the type of damage.
      (51 votes)
  • blobby green style avatar for user Mahlet Sergis
    why are the phases of mitosis named interphase, proaphase, metaphase, anaphase, and telephase?
    (15 votes)
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    • orange juice squid orange style avatar for user Christoph Leberecht
      "Inter" means between in Latin. It is the time between two Mitosis.
      "Pro" means before in ancient Greek. The chromatin condenses and the cell gets ready to get separated into two cells. I don't know it exactly, but I guess it is coming from "before" the microtubules attach.
      "Meta" means after or adjacent in ancient Greek. So, after the microtubules attach and now start to pull the chromosomes apart.
      "Ana" means up or back in ancient Greek. The chromosomes are split and the chromatids move "back" to opposite poles of the cell.
      "Telo" means end in ancient Greek. In this stage the cell is cleaned up (chromosomes unwind, nuclear membrane reforms, ...) and the Mitosis comes to an end.
      (54 votes)
  • aqualine ultimate style avatar for user erbmyc
    where do the other half of the DNA come from when they replicate? like does the dna grow the new half or does the new half come from somewhere else? if so where? another replicating DNA? if so what good does it do?
    (16 votes)
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    • blobby green style avatar for user sfontenard
      Each DNA molecule is double stranded. During replication the single strands are separated and complementary bases are polymerized to form a new complementary strand. Thus, one double stranded molecule becomes two. Picture it as cutting a ladder straight down the middle, then taking each half and adding rungs (I-) to rebuild each half into a whole new ladder with half of the original ladder and a new half. The "rungs" for DNA are called bases: Adenine, Guanine, Cytosine and Thymine. A and T are able to base pair covalently and C and G pair covalently. Sugars which are attached to A, T, C and G form the backbone and the bases themselves are the rungs. Enzymatic reactions allow the sugars to be joined together to form a chain but only if the correct base is bound to the sugar. If the parent strand has a T then only a sugar containing an A will be allowed to be bound to the previous sugar molecule so that the growth of the new strand can proceed. Each cell needs two strands of each chromosome (total 46) in order to function properly. Cells only replicate their DNA when they are dividing so that each new cell will contain 23 pairs.
      (18 votes)
  • aqualine ultimate style avatar for user Noah
    Why don't brain cells reproduce?
    (10 votes)
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    • leaf green style avatar for user Praveen
      If your question is about neurons, then it is due to differentiation of the cells to its final functional state and that is called terminal differentiation. They become specialized to do a specific job and stop doing certain functions that other cells usually do. But remember that once these cells are differentiated they live for long time in the brain and if dead, they may be replaced by new neurons from neural stem cells. But as you age the ability to replace these cells become difficult.
      (22 votes)
  • blobby green style avatar for user Priit Tohver
    How do chromatids attach to each other? What type of bonds are there
    (13 votes)
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    • mr pants teal style avatar for user Matthew Belliveau
      Two sister chromatids attach at a region called the centromere. A kinetochore, a large, multimeric protein, joins the two strands at this section. You may remember hearing about the kinetochore while studying mitosis - it helps guide the DNA along the spindle fibres.

      My guess is that the kinetochore bonds to the DNA via ionic or polar bonds (i.e., it contains some sort of DNA binding domain). I can almost certainly eliminate the possibility of covalent bonds.

      This is my best guess... anyone else have any ideas?
      (10 votes)
  • blobby green style avatar for user Dylan Orellana
    How is a chromosome defined? How is a chromatid defined? A chromosome deals with genes/ a set of information while a chromatid is a physical structure?
    (4 votes)
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    • mr pink red style avatar for user sohaib
      Chromosomes:A threadlike structure of nucleic acids and protein found in the nucleus of most living cells, carrying genetic information in the form of genes

      Chromatid:Each of the two threadlike strands into which a chromosome divides longitudinally during cell division. Each contains a double helix of DNA
      (14 votes)
  • blobby green style avatar for user Adrian
    Since proteins are created outside of the nucleus, but yet histones are located inside, how do they get into the nucleus?
    (8 votes)
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    • piceratops ultimate style avatar for user Neha  Rino
      I'm not extremely clear about the process, but as far as I can tell, the proteins which are going to the nucleus have special stretches of amino acids called NSLs. These NSLs act like a ticket to the protein, and reacts with the cytoplasm, nuclear envelope and the nuclear pore complexes which let them through their respective areas. Then, the NSLs bond with the nuclear pore complex and the protein is translocated into the nucleus.
      Hope that helped!
      (3 votes)

Video transcript

Before I dive into the mechanics of how cells divide, I think it could be useful to talk a little bit about a lot of the vocabulary that surrounds 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. So let's say I have a little-- I'm just going to draw a small section of DNA. I have an A, a G, a T, let's say I have two T's and then I have two C's. Just some small section. It keeps going. And, of course, it's a double helix. It has its corresponding bases. Let me do that in this color. So A corresponds to T, G with C, it forms hydrogen bonds with C, T with A, T with A, C with G, C with G. 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? And this is one of the beautiful things about how DNA is structured. Replication. 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. Too big. 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. They're base pairs. 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. 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. Now, transcription is what needs to occur for this DNA eventually to turn into proteins, but transcription is the intermediate step. It's the step where you go from DNA to mRNA. 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. This is mRNA. Now, this separates. That mRNA separates, and it leaves the nucleus. It leaves the nucleus, and then you have translation. That is going from the mRNA to-- you remember in the DNA video, I had the little tRNA. 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. This is tRNA. That's a codon. 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. 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're replicating the DNA. Now, if you're starting with DNA and you are creating mRNA from the DNA template, this is transcription. You are transcribing the information from one form to another: transcription. 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: chromosome, chromatin and chromatid. 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. I'll just draw some base pairs joined up like that. So I want to be clear, when I draw this little green line here, it's actually a double helix. Now, that double helix gets wrapped around proteins that are called histones. So let's say it gets wrapped like there, and it gets wrapped around like that, and it gets wrapped around like that, and you have here these things called histones, which are these proteins. Now, this structure, when you talk about the DNA in combination with the proteins that kind of give it structure and then these proteins are actually wrapped around more and more, and eventually, depending on what stage we are in the cell's life, you have different structures. But when you talk about the nucleic acid, which is the DNA, and you combine that with the proteins, you're talking about the chromatin. So this is DNA plus-- you can view it as structural proteins that give the DNA its shape. And the idea, chromatin was first used-- because when people look at a cell, every time I've drawn these cell nucleuses so far, I've drawn these very well defined-- I'll use the word. So let's say this is a cell's nucleus. I've been drawing very well-defined structures here. So that's one, and then this could be another one, maybe it's shorter, and then it has its homologous chromosome. So I've been drawing these chromosomes, right? And each of these chromosomes I did in the last video are essentially these long structures of DNA, long chains of DNA kind of wrapped tightly around each other. So when I drew it like that, if we zoomed in, you'd see one strand and it's really just wrapped around itself like this. And then its homologous chromosome-- and remember, in the variation video, I talked about the homologous chromosome that essentially codes for the same genes but has a different version. If the blue came from the dad, the red came from the mom, but it's coding for essentially the same genes. So when we talk about this one chain, let's say this one chain that I got from my dad of DNA in this structure, we refer to that as a chromosome. Now, if we refer generally-- and I want to be clear here. DNA only takes this shape at certain stages of its life when it's actually replicating itself-- not when it's replicating. Before the cell can divide, DNA takes this very well-defined shape. Most of the cell's life, when the DNA is actually doing its work, when it's actually creating proteins or proteins are being essentially transcribed and translated from the DNA, the DNA isn't all bundled up like this. Because if it was bundled up like, it would be very hard for the replication and the transcription machinery to get onto the DNA and make the proteins and do whatever else. Normally, DNA-- let me draw that same nucleus. Normally, you can't even see it with a normal light microscope. It's so thin that the DNA strand is just completely separated around the cell. I'm drawing it here so you can try to-- maybe the other one is like this, right? And then you have that shorter strand that's like this. And so you can't even see it. It's not in this well-defined structure. This is the way it normally is. And they have the other short strand that's like that. So you would just see this kind of big mess of a combination of DNA and proteins, and this is what people essentially refer to as chromatin. So the words can be very ambiguous and very confusing, but the general usage is when you're talking about the well-defined one chain of DNA in this kind of well-defined structure, that is a chromosome. Chromatin can either refer to kind of the structure of the chromosome, the combination of the DNA and the proteins that give the structure, or it can refer to this whole mess of multiple chromosomes of which you have all of this DNA from multiple chromosomes and all the proteins all jumbled together. So I just want to make that clear. Now, then the next word is, well, what is this chromatid thing? What is this chromatid thing? Actually, just in case I didn't, I don't remember if I labeled these. These proteins that give structure to the chromatin or that make up the chromatin or that give structure to the chromosome, they're called histones. And there are multiple types that give structure at different levels, and we'll do that in more detail. So what's a chromatid? When DNA replicates-- so let's say that was my DNA before, right? When it's just in its normal state, I have one version from my dad, one version from my mom. Now, let's say it replicates. So my version from my dad, at first it's like this. It's a big strand of DNA. It creates another version of itself that is identical, if the machinery worked properly, and so that identical piece will look like this. And they actually are initially attached to each other. They're attached to each other at a point called the centromere. Now, even though I have two strands here, they're now attached. When I have these two strands that contain the exact-- so I have this strand right here, and then I have-- well, let me actually draw it a different way. I could draw it multiple different ways. I could say this is one strand here and then I have another strand here. Now, I have two copies. They're coding for the exact same DNA. They're identical. I still call this a chromosome. This whole thing is still called a chromosome, but now each individual copy is called a chromatid. So that's one chromatid and this is another chromatid. Sometimes they'll call them sister chromatids. Maybe they should call them twin chromatids because they have the same genetic information. So this chromosome has two chromatids. Now, before the replication occurred or the DNA duplicated itself, you could say that this chromosome right here, this chromosome like a father, has one chromatid. You could call it a chromatid, although that tends to not be the convention. People start talking about chromatids once you have two of them in a chromosome. And we'll learn in mitosis and meiosis, these two chromatids separate, and once they separate, that same strand of DNA that you once called a chromatid, you now call them individually chromosomes. So that's one of them, and then you have another one that maybe gets separated in this direction. Let me circle that one with the green. So this one might move away like that, and the one that I circled in the orange might move away like this. Now, once they separate and they're no longer connected by the centromere, now what we originally called as one chromosome with two chromatids, you will now refer to as two separate chromosomes. Or you could say now you have two separate chromosomes, each made up of one chromatid. So hopefully, that clears up a little bit some of this jargon around DNA. I always found it quite confusing. But it'll be a useful tool when we start going into mitosis and meiosis, and I start saying, oh, the chromosomes become chromatids. And you'll say, like, wait, how did one chromosome become two chromosomes? And how did a chromatid become a chromosome? And it all just revolves around the vocabulary. I would have picked different vocabulary than calling this a chromosome and calling each of these individually chromosomes, but that's the way we have decided to name them. Actually, just in case you're curious, you're probably thinking, where does this word chromo come? I don't know if you know old Kodak film was called chromo color. And chromo essentially relates to color. I think it comes from the Greek word actually for color. It got that word because when people first started looking in the nucleus of a cell, they would apply dye, and these things that we call chromosomes would take up the dye so that we could see it well with a light microscope. And some comes from soma for body, so you could kind of view it as colored body, so that's why they call it a chromsome. So chromatin also will take up-- well, I won't go into all of that as well. But hopefully, that clears a little bit this whole chromatid, chromosome, chromatin debate, and we're well equipped now to study mitosis and meiosis.