WWLPT

"Fishing for Microsatellites"|

PCR Demo

Put your goggles on again and transfer the DNA sample through the entire cycle again, and then take your goggles off.

Discussion

First have your students remind you of the three processes of ‘Denature, Anneal, Extend.  Challenge them to hypotheses what the sample in the Eppendorf should look like by the end of these three processes.  In the image below, I focus only on the primer strands.  Notice that the primer strand is getting smaller and beginning to amplify only the stretch of DNA between the primers.

5’      CGCTTACGTCAA     3’         short primer strand
3’      GCGAATGCAGTT     5’         short primer strand

3’ TAGCGAATGCAGTT     5’         primer strand
5’      CGCTTACGTCAA     3’         primer strand

5’      CGCTTACGTCAACT 3’        primer strand
3’      GCGAATGCAGTT     5’         primer strand

3’      GCGAATGCAGTT     5’         short primer strand
5’      CGCTTACGTCAA     3’         short primer strand

At this time, you could choose to transfer the DNA sample from water bath to water bath completing the full cycle 27 to 30 more times.  Get the students to figure out that during each cycle the number of amplified fragments is growing exponential from an original 2, to 4, 8, 16, 32, 64, 128, 256, 512, 1024, 2048, 4096, 8192, 16,384, 32,768, 65, 536, 131,072, 262,144, 524,288, 1,049,576, etc…  You could have the students calculate the numbers of fragments on their own and construct a graph.  This way they may come to a better understanding of the fact that only small initial DNA samples are needed.

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