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Current time:0:00Total duration:18:23

Chromosomes, chromatids and chromatin

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 in the DNA and the DNA video so let's say I have a little I'm just gonna draw a small section of DNA say a van a G T let's say I have two T's I have and then I have 2 C's just some small section it keeps going and of course it's a double helix it has its corresponding basis 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 AC 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 let's say 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 then it doesn't happen on its own it's facilitated by a bunch of proteins and enzymes but I'll talk about that the details of the microbiology in a future video so let me make separate so these guys separate from each other let me put it up here they separate from each other and let me do take the other guy now too big the guide looks something like that they separate from each other and then once they've separated each other what could happen let me delete some of that stuff over here delete that stuff right there so you have these this double helix they were all connected their 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 so now I can form a if this guy sitting by himself now a sudden a thymine base might come and join right here and so you could have 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 that was formerly 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 all right and I'll go more will do the microbiology of it in the future but this is just to get the idea this this is how the DNA makes copies of itself and especially when we talk about 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 in the DNA video is transcription 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 that it really is that easy to duplicate itself you just split the two strips the true HeLa C's 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 DNA to mRNA and then that mRNA leaves the nucleus of the cell and goes out to the ribosomes and I'll talk about in that I'll talk about that in a second so we could 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 but instead of and maybe actually maybe it makes more sense just to do one half of it so let me delete that so let's say just the green this is the quoting let's say that we're just going to transcribe the green side right here let me erase all of this stuff right now wrong color erase this stuff right here now what happens is instead of D instead of 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 magenta and so the RNA will pair up with it and so thymine on the DNA side will pair up with adenine guanine now with when we talk about RNA instead of thymine we have uracil uracil cytosine cytosine and it just keeps going this is mRNA M R na now what's this and then this separates that mRNA separates and it leaves the nucleus it leaves the nucleus and then you have and then you have translation translation translation that is going from the mRNA to you remember in the DNA video I little T in a tRNA the transfer RNA kind of were 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 RNA to the proteins to the proteins and we saw how that happened you have this guy let me make a copy here this guy separates let me actually copy the whole thing this guy separates leaves the nucleus and then I'll delete this stuff and then you had those little tRNA trucks that essentially drive up so you know maybe have some tRNA try an adenine guanine guanine this is tRNA that's a codon a codon is 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 it has a different amino acid this occurs in the rub and then the amino acids attached to each other and then they form this long chain of amino acids which is a protein so just to summarize and the proteins form these weird and complicated shapes so just just just to kind of make sure you understand also if we start with DNA and we're essentially making copies of DNA this is replication you're replicating the DNA replication now if you're starting with DNA and you are creating mRNA from the DNA template this is transcription transcription trans description you're you're transcribing the information from one form to another transcription now when the mRNA leaves the nucleus of the cell and I've talked to let me just draw a cell just to hit the point home you know if this is the whole cell and we'll do the we'll do the structure of a cell in the future that's the whole cell the nucleus is the center that's where all the DNA is sitting in there and all of the 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 to protein is translation you're translating from the genetic code so to speak to the protein code so this is translation translation so these are just good words to make sure you get clear and use them very make sure you 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 chromosome chromatin chromatin and chromatid chromatid so a chromosome is we already talked about you know you can have DNA you can have a strand of DNA it's a double helix maybe I'll you know this strand if I were to zoom in it's actually two different HeLa C's 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 that double helix gets wrapped around proteins that are called histones so let's say it's wrapped like there and it gets wrapped around like that and gets wrapped around like that and you have here these things called histones which are these proteins this these proteins now this structure when you talk about the DNA as it is wrapped or that when you talk about the DNA in combat 8 in 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 the the nucleic acid which is the DNA and you combine that with the proteins you're talking about the chromatin chromatin so this is DNA plus you can view the structural proteins that give the DNA its shape structural proteins and the idea chromatin was first used because when people look at a cell every time I've drawn the cell nucleus so far I've drawn these very well-defined I'll use words 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 we've two 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 really 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 it's homologous chromosome and remember in the variation video I talked about the homologous chromosome essentially codes for the same genes but it has a different version and it came from 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 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 chromosome now it would if we refer generally and I want to be clear here DNA only takes the shape at certain stages of its life when it's when it's actually when it's actually replicating itself when not when it's replicating when the cell is dividing DNA before the cell can divide DNA picks 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 this would be very hard for kind of the replication and the and the and the transcription met up kind of a 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 you can't even it's not even you can't even see it with a with a normal light microscope it's it's it's so thin that the DNA strand is just completely separated around the cell I mean I'm drawing it here so you can drive maybe the other one is like this all right and then you have that shorter strand that's like this and so it's all you can't even see it it's not in this well-defined structure this is the way it normally is and then you have the other short strand it's like this so you would just see this kind of big mess of a combination of DNA and proteins and this is what people essentially referred to as chromatin chromatin so the words it 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 the combination of the DNA and the proteins that give the structure or it can refer to this whole mess of multiple chromosomes and DNA which of which mido you have all this DNA for 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 does this chromatid thing what is this chromatid thing actually just in case I didn't I don't remember if I label these these proteins that give structure to the chromatin or that make up the chromatin or give structure to the chromosome they're called histones histones and they're multiple types that give structure at different levels and we'll do that in more detail so what's a chromatid so after when DNA replicates so let's say that let's say that this was my DNA before right when it's just in this 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 that 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 centromere now even though I have two strands here they're now attached and when I have these two strands that are contained the exact still 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 draw I can say this is one strand here and then I have another strand I have another strand here now I have two copies that can they they're coding for the exact same DNA they are identical I still call this a chromosome I still call this a chromosome chromosome this this whole thing is still called a chromosome but now each individual copy is called a chromatid so that's one chromatid chromatid and this is another chromatid sometimes I'll call them sister chromatids maybe they should call them twin chromatids because they have the same genetic information chromatid so this chromosome has two chromatids now before the replication occurred or the DNA replicated itself you could say that this chromosome right here this chromosome like your father has one chromatid you could call it a chromatid although that's 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 make that you know 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 it's like wait how did one chromosome become two chromosomes and you know how did a chromatid become a chromosome and it's all just revolves around the vocab and I would have picked different vocabulary then you know calling this a chromosome and then calling each of these individually chromosomes but that's the way we have decided to name them actually just so in case you're curious you probably think where does this word chromo come and if your old Kodak film it's called chroma color and chromo is essentially relates to color and I think it comes from the Greek word actually for color and then it got that word because when we people first started looking in the nucleus of cell they would apply dye and these things that we call chromosomes would take up the dye so that you could see it well with a light microscope so that's why and so them comes from soma for body so you could kind of view it as colored body so that's why they call it a chromosome so chromatids 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 well we're well equipped now to study mitosis and meiosis
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