- [Voiceover] In this video, we're going to talk
about something known as PCR, PCR.
PCR stands for Polymerase
Chain Reaction.
So let's look at the process of PCR.
And let's start by looking at the history of PCR.
PCR basically starts off with some DNA
that you're trying to amplify.
You're trying to make more copies of the DNA.
And you start off with a piece of DNA.
It's got two strands and in order to denature the strands,
you apply heat.
Now the problem is that a lot of enzymes
that would normally,
once you have the two strands separated,
a lot of the enzymes that would normally
then copy the strands and make double-stranded DNA,
would denature, they wouldn't work anymore,
because the heat would basically destroy them.
So scientists were able to isolate a bacterial species
that actually grows in the geysers
of Yellowstone National Park,
which is an extremely hot environment.
So the bacteria was something known
as Thermos aquaticus
and Thermos aquaticus was a species of bacteria
that basically had a DNA polymerase.
And we call it Taq for Thermos aquaticus Polymerase.
And this polymerase is able to withstand the high heat
needed to denature the DNA strands.
So if you apply Taq Polymerase
to the now denatured single-stranded DNA,
it's able to copy, make a complimentary copy of the DNA.
It would copy.
So basically this is the overview of PCR.
Now let's look at the specifics.
So you start off with some DNA and you apply heat
and denature the DNA, so now you've got two strands.
What you next need to do is you need to add a primer.
So you need to add a little primer
and the primer gives the Taq Polymerase a starting point.
So the Taq Polymerase comes in and basically is able
to make the complimentary strands.
So in order for the Taq Polymerase
to make the complimentary strand,
you also need to add in nucleotide.
So you need to add nucleotides, necleotides
and you also needed a primer.
So basically heat acts like helicase
which normally in human DNA
would split the double-stranded DNA up into two strands
and the Taq Polymerase basically acts
like our own DNA Polymerase
that normally would replicate the DNA.
So basically if you repeat these steps,
so you heat, you denature the DNA.
Then you reanneal the DNA.
Allow this elongation process to occur.
And if you just keep repeating that,
eventually you'll have a whole bunch of copies.
So here you start off with one copy of double-stranded DNA,
Here you have two copies.
If you repeat it, you're going to have four copies,
and so on and so forth.
So you basically get this exponential increase
in the copy number of the DNA.
And this is really useful in order
to create massive amounts of a particular DNA sequence.
So what I want to now talk about is,
I want to look at the exponential growth
that is a hallmark of the PCR reaction.
So after we do PCR one time.
So we start out with double-stranded DNA.
We add heat, it gets denatured.
And now we have two strands of double-stranded DNA.
So this is one round of PCR.
If we do another round, we're basically going to have
each one of these denatures and each one,
after we add Taq Polymerase
and after we add the primer,
we're going to have four strands.
So this is after round two, we have four strands.
And after round three, we're going to have eight.
So we go from one, two, four, eight.
After round four, we're going to have 16.
After five, we're going to have 32,
and so on and so forth.
And basically after just a few rounds of PCR,
we're able to have 1000s and 1000s
of strands of double-stranded DNA.
And that's the power of PCR is
that you get exponential growth.
So you start out here and then you really
just rapidly get an exponential amount of growth
and an exponentially larger amount of DNA
that you're interested in.