PCR is the process of taking a specific piece or segment of DNA or RNA(cDNA) and making billions of copies. This segment can be as short as a single gene, or much longer, depending on what it is that you are studying. For the sake of consistency, lets focus on the amplification of a single gene.
In order to run a PCR, we must first define what goes into to the reaction. First, there are the primers. Primers are the specific complement that matches the section of DNA/cDNA that comes directly BEFORE the gene we want to amplify. They allow us to target the gene specifically and provide a starting point for amplification. Next, there are dNTPs. These are better known as the “free nucleotide bases”, or “building blocks” that are used to piece together the complementary strand of DNA/cDNA. Last are the enzymes–the things that actually DO the building and copying! There are several enzymes and all of which usually end in -ase. For the sake of simplicity I will avoid diving into the pond of complication for now.
So we have a specific gene and we want to study it, but the problem is that we don’t have enough of it to run multiple tests! This is where PCR comes in. If we combine the primers, dNTPs and enzymes together with the sample gene, we can run it in a thermocycler and exponentially increase our amount of the gene.
A thermocycler machine is a brilliant invention. This one machine can maintain a certain temperature for a specific amount of time and then change to different temperatures at a set time–all of which can be programmed. The PCR cycle has 3 steps- denaturing, annealing and elongation. The first activates an enzyme that peels apart the double-strands of DNA, allowing the dNTPs and other enzymes to access the bases of the sample. The second step matches the primers to now-available bases and the third attaches the dNTPs and creates the complementary strand.
There is a wonderful animation on YouTube that displays it quite well: PCR Animation