Chromosomes, Chromatids, Chromatin, etc.


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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