PCR: A Love(ish) Story

Once upon a time, in a laboratory not so far away, there were two polynucleotide chains that coiled around each other to form a double helix.

They lived happily in a small test tube surrounded by molecules of Tris-HCL and EDTA and at a comfortable pH of 8 until one-day things started to get hot. Not left out of the fridge hot. Really hot. Like 94-95^oC (201-203^oF) hot! Suddenly, the hydrogen bonds between the two chains began breaking. Crack! Crash! Boom! And before the two strands of template DNA knew what was happening, they were separated.

Alone and heartbroken the single-stranded template DNA strands wandered through the TE buffer. First, they met tiny little molecules called MgCl₂ that flew to their shoulder and sang beautiful songs. Then they met a group of nucleotides, sweet sugar molecules attached to a phosphate group and a nitrogen-containing base. The nucleotides, who called themselves dNTPS, listened to template DNA’s tragic story and wanted to help so they took them to the primers.

Now the primers were longer (18-22 bp) single-stranded DNA sequences. To the little single-base dNTPs the primers seemed very big and wise. And they were (wise) as well as a little bit magic! The primers told the separated DNA strands that it was actually quite fun being single-stranded DNA.

However, when they saw how much the single-stranded template DNA still yearned for their lost loves they decided to take a more fairy godmotherish approach. The primers cooled down the surrounding temperatures to a mild 50-56^oC. Next, they found complementary DNA sequences on the template DNA strands. Finally, they attached themselves!

At first, the template DNA strands thought this was just a comforting hydrogen hug but then they saw magic glitters everywhere, heard in the distance the sound of galloping horses’ hooves, and felt the temperature go up to a very exciting 72^oC.

Just in time, with triumphant music playing in the background, a noble army of DNA polymerase enzymes arrived. These enzymes attached to the primers and template DNA. Then, with the help of all the little dNTPs, the primers extended the DNA sequence from 3’ of each primer to the end of the amplicon. Now all the template DNAs were double-stranded again with complementary DNA chains that were the perfect match for them.

These new double-stranded DNAs lived happily ever after.

(Until the entire process happened again. And then again and again for about 35 cycles.) Luckily around cycle 10 all the components got used to the ups and downs in the cycle of PCR love and adapted to it. So much so that even when the PCR experiment was over and the next chapter of the lab adventure began, all the double stranded DNA strands were ready for anything. But that’s an electrophoresis story for another day.

Happy almost Valentine’s Day from Edvotek! Check out our other popular Valentine’s Day blogs Heartfelt Science for Valentine’s Day and How Science Studies Love as well as our awesome Valentine’s Day Give Away!