So we can synthesize that and make large quantities of it and
add that to the mixture as well.
So, DNA polymerase acts in the following, very straightforward way.
It looks for a place where the DNA is partly single stranded and partly
double stranded and it grabs on to the sequence right there and starts to copy.
So this DNA polymerase here will notice both of these two primers are now attached
to sequences that are single stranded, except where the primers have stuck.
So, the DNA polymerase will go and it will find those sites,
and it will start to fill in the missing sequence starting at the primer.
So that's the property of DNA polymerase that we really need,
and there are many polymerases.
Every living organism on the planet actually has one.
So, we don't actually use the human polymerase for this process,
but it doesn't matter.
You can use, in theory, any DNA polymerase to do the copying.
Although, we do need to specialized one for PCR.
So the result after doing this is that, if I show you here,
after one round of copying, I'll have completely filled in the sequence across
the top from my green primer.
As you see here, and then another polymerase, assuming I added lots of
polymerase molecules, another polymerase will have filled in the sequence
of the other strand across the bottom, starting with the blue primer.
Now, I also needed a mixture of As, Cs, Gs, and Ts, the raw material to make DNA.
So, I didn't say that yet, but in addition to adding my DNA polymerase,
I'm going to add raw As, Cs, Gs, and Ts in large quantities.
So that the DNA polymerase can incorporate them into the new double-stranded
DNA that it's creating.
So after one round of PCR, if we let things cool down, these two strands will
stick together, and we've now created two strands where we only had one before.
So we can just repeat that whole process.
And if we repeat the whole process, we get four strands.
And a very important property of PCR, the reason it's called a chain reaction,
is that with each round, we double the amount of DNA we had before.
So, very quickly, after just a few rounds, you go from just one molecule to many,
many molecules.
Or many, many copies.
So, typically we'd repeat this for 30 cycles or more.
And if you do the math, 2 to the 30th is about two billion,
so you can take one molecule and turn it into billions of molecules
after just a few dozen cycles of polymerase chain reaction.
So let's look at a little cartoon of how this works, just to drive home the point.
So we start with melting or denaturing the DNA at 94 degrees Celsius.
So here you see a double strand of DNA with some polymerase.
You'll notice some little arrows floating around.
And primers are shown as the little short fragments of DNA that are floating in
the solution here.
And as we heat it up,
you'll see the two strands of DNA in the middle start to denature.
They're falling apart.
So, you see they're melting apart there, and
eventually they're completely separated.
Now, an important part of PCR is you have to denature for long enough for
the two strands to fall apart.
But it doesn't take long at all.
We're talking only a few minutes to do that.
Then you cool it down to 54 degrees, and
at that temperature the primers will stick to the DNA, as you see here.
And the polymerase can then find these double-stranded pieces and
start to fill it in.
So here you see the two polymerases in this picture are starting to
fill in the existing strand and create double-stranded DNA.
So they'll just walk along until they get to the end of the molecule, and
then they'll fall off.
So now we've created two complete copies, and
we do that at a slightly higher temperature of 72 degrees.