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The Polymerase Chain Reaction: No, It's Not The Latest Dance Craze!

DNA is the hottest topic in science today - but what do we actually do with it?

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These days, all the newest and most exciting research comes from studying the DNA that is contained within all of our body's cells.

DNA is a remarkable molecule. It is made up of two strands, which are connected to each other by chemical bonds as shown in the picture above. The strands are built out of a special biological code that, all together, spells out the entire "blueprint" for how our bodies are built. Reading along a strand of DNA is kind of like reading a book of instructions that tells our body how to function.

This instruction manual/blueprint is made up of little blocks of code called "genes." Thing of each gene as a chapter that determines one specific trait, like eye color, hair texture, blood type or susceptibility to breast cancer. Incredibly, scientists have been able to map out all the genes in the entire length of DNA for humans and many other animals.

But there's a catch...

Let's say a scientist wants to study one specific gene in the lab. This happens very often, because many genes are directly related to inheritance of diseases or other important characteristics that need to be studied. There are two challenges to working with DNA in the lab:

1) An entire DNA molecule is just one copy of the blueprint of the book, so it only contains one copy of each gene. Genes are relatively short, so one copy of a gene is definitely not enough to do any meaningful tests. Think of it this way – one atom of gold is invisible; you can only see a gold coin if hundreds of trillions of atoms are grouped together in a big hunk of metal. The same is true with DNA.

2) The one copy of the gene that we do have is jumbled up with alllllllllll of the other genes in the blueprint, so it's practically hidden and can't be analyzed until it's isolated from the rest of the DNA molecule.

So what do scientists do to solve these problems? Basically, they have created a DNA photocopy and isolation process called the Polymerase Chain Reaction (PCR for short). PCR quickly creates an insanely large amount copies of only the specific gene you want, ignoring the rest of the DNA. This quickly and easily solves both of the problems that limit lab work.

What goes on inside this crazy contraption?

Here's how it works. Take a look at a simplified picture of a small segment of DNA below, with the gene that you want to study marked in red.

Created by the author

The chemical bonds between the strands are shown in black, and the arrowheads indicate that the strands stretch on in both directions. You're only looking at a small segment of what is in actuality a much longer strand!

Remember, the gene you want is located in the middle of a long stretch of DNA that contains a bunch of other genes. The first thing you have to do is "highlight" the genetic code to indicate the gene of interest. This is done by adding primers to your DNA sample. Primers are "sticky" molecules created by the lab specifically to bind to the DNA area next to your gene. These primers tell the DNA photocopier exactly where to focus - just like a highlighter.

Next, you add some other materials into the mix: molecular "building blocks" of DNA, and molecular "machines" that use these building blocks to create new DNA where the primers tell them to.

Once everything is mixed together, you follow these steps:

1) Heat the DNA to separate the two strands.

2) Cool to body temperature to allow the molecular machinery to create new DNA copies, one from each strand. The primers tell the machines which area of the DNA to copy.

This results in two new DNA molecules, each of which contain only the gene you wanted to isolate!

If you repeat the heating and cooling cycles many times, each new DNA molecule will yield two more, until you have a very large amount of DNA as shown below.

Now that there are enough copies of the isolated gene, the scientist can finally run any experiment she wishes!

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