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