Chromosomes, Chromatin, Chromatids Video Lecture - NEET

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.