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  • 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?