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If you wanted to make a copy of a page in a book, you would use a photocopier, but if you wanted to make a copy of a DNA sequence, what would you do?
The technique called Polymerase Chain Reaction (PCR) was invented for this exact purpose. Making copies of DNA is useful for many reasons such as performing DNA fingerprinting (for paternity tests and forensics investigations), making recombinant proteins (like insulin), identifying unknown organisms (using DNA barcoding) and many more.
Best of all, PCR is both fast and specific. Specific means that you can make copies of a certain DNA sequence even if other sequences are present. For example, if you think of the entire DNA present in a sample as a book, PCR would allow you to ‘photocopy’ just one particular paragraph.
PCR has three steps:
In the denaturation step, the DNA sample is heated to 94-96°C. This causes the two strands of the DNA molecule to separate from each other, which is necessary for them to be copied.
The second step (annealing) is when you specify which ‘paragraph’ of DNA you want to copy, by adding short pieces of DNA called primers into the PCR. When the temperature of the reaction is cooled to around 50-65°C, these primers will anneal (stick) to the complementary sequence present in your DNA sample, through the pairing of the nucleotides Adenine (A) with Thymine (T) and Guanine (G) with Cytosine (C).
In the third step (extension), the DNA copy is finally made. An enzyme called DNA polymerase adds the nucleotides (A, C, T, G) to the end of the primer, making a strand that is complementary to the one present in the sample. PCR uses a special heat-stable DNA polymerase, which is not destroyed when the sample is heated.
It’s that simple! By repeating the three steps over and over, you can exponentially increase the number of copies of the DNA sequence of interest.