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How are chromatids, chromatin, and chromosomes related?
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
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
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.
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
You have an adenine, a guanine, thymine, thymine,
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
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
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
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
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
Before the cell can divide, DNA takes this very
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
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
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
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
Now, I have two copies.
They're coding for the exact same DNA.
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
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
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
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
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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