PCR: Extraction, Amplify, Repeat

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By Devin Walecka

Now that you're educated on how we get DNA out of cheek cells we collect from visitor samples (Don't know? Find out here), I'm going to tell you about a process called polymerase chain reaction (PCR). This technique is how scientists make lots… in fact trillions of copies of DNA from the small amount we get from the cheek cells of our research participants.

First thing you'll need to know before we proceed, that although DNA is double stranded, each strand has a different job. One strand of the DNA is called the coding strand; the "code" being the instructions telling the cell which amino acids to assemble in the right order to form a protein.

The other strand is called the non-coding strand.  The non-coding strand has a sequence that is the reverse compliment of the coding strand. This strand doesn't have the instructions to make proteins, even though it is a mirroring copy of the coding strand of DNA.  By binding to the coding strand, it makes the DNA take the famous double helix structure. This structure is what helps DNA be stable in our cells.

Back to PCR:

Step 1: Denaturation
We need to first "unzip" the coding and non-coding DNA strands. In order to do this, we heat the DNA to 95 degrees celcius (approxiamtely 203 degress farenheit). This allows us to access the two strands separately.


Step 2: Annealing
Now we're going to add primers. Primers are small pieces of DNA that match a small section of the gene we are interested in (in this case, the gene tas2r38).The complementary sequence allows the primers to bind to the gene.


Step 3: Extension
In this step, indivisual nucleotides and DNA polymerase get to work. Nucleotides are molecules that make up your DNA. There are four different nucleotides; adenine (A), thymine (T), guanine (G), and cytosine (C). DNA polymerase is an enzyme that builds the new DNA adding the correct nucleotides in the order determined by the strand the primer bound to. Think of it like this: if we're trying to build a DNA house, nucleotides are the bricks and DNA polymerase is the mason.


These three steps are repeated thirty to forty times to give us the trillions of copies we need for the next step in our DNA analysis.

Next step: cutting the gene sequence we investigate in our research!


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