The world is awash in DNA.
It’s in all living things—and the things they leave behind,
like skin flakes, hair, secretions, and feces.
It moves along the single-stranded DNA, adding complementary nucleotides.
It goes until it falls off, reaches the end of the strand, or until the next temperature change.
Cycle 2 repeats the same 3 steps.
At 95 degrees, the DNA strands separate.
At 50 degrees, primers find their complementary sequences.
They bind not only to the original target DNA, but also to the products from cycle 1.
At 72 degrees, Polymerase extends the primers, adding complementary nucleotides as it goes.
In cycle 3 the steps repeat again.
After 30 cycles, there are over a billion copies of just the target sequence, and only 60 of the longer copies.
The target sequence overwhelmingly outnumbers the other products
That’s so you can design primers. These short pieces of DNA will attach on either end of the target.
To do PCR, you need to add many copies of the two primers. Plus a few more things:
DNA Polymerase is an enzyme that copies DNA.
Nucleotides are DNA building blocks.
Now you can analyze the PCR sample to get a test result, or use the DNA copies in an experiment
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or in any number of ways.
But an individual strand of DNA is so small you can’t see it, not even with a microscope.
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a strand of DNA
So how do scientists visualize it?
One way is with PCR.
It’s a technique that lets you focus in on one specific bit of DNA code.
It’s based on the natural process that cells use to copy DNA. (That’s the P, for Polymerase.)
But it adds a twist that lets you make copies by the billions. (That’s the Chain Reaction.)
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Since everything living has DNA, PCR has tons of uses . . .
from genetic engineering to forensic identification and medical testing.
Or, if the target sequence wasn’t present in your sample, you won’t have any products.
After all, PCR can only amplify something that’s already there.
The only catch is, you need to know the sequence of DNA letters around the bit that you want to target.
Click or tap to show the DNA in the sample.
Drag primer 2 to its complimentary sequence.
One primer binds to each strand through complementary base pairing.
The primers are what make PCR specific. They are custom made for each target.
Different targets need different primers.
And you need things like water and salts, to mimic the conditions inside a cell.
The rest is biochemistry. And you regulate the process through repeated cycles of temperature changes.
It disrupts complimentary base pairing, and the two DNA strands come apart.
Next, lower the temperature to 50 degrees.
Complementary DNA strands can re-join at this temperature.
But since the primers are present in much greater numbers,
they will lock on to their complementary sequences before the longer strands can come back together.
To begin cycle 1, raise the temperature to 95 degrees Celcius.
This temperature is close to boiling.
Now raise the temperature to 72 degrees
This temperature activates DNA Polymerase.
It attaches at the ends of the paired primers, and extends them.
There are just two copies now.
Click or tap the two desired products
Watch how the following cycles cause a chain reaction
that makes the desired product start increasing exponentially.
For the first time, the desired products appear.
These are shorter copies of just the target sequence.
At the end of cycle 4, there are eight copies of just the target sequence.
And now there are 22. The target copies outnumber the longer ones.
We'll fast forward through the rest of the cycles.