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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?
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
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染色体、染色分体、クロマチンなど (Chromosomes, Chromatids, Chromatin, etc.)

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keep seeing 2014 年 4 月 18 日 に公開

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