PCR, Electrophoresis and DNA Profiling
PCR copies a chosen stretch of DNA through repeated heating and cooling, and electrophoresis then sorts the fragments by length. A profile compares repeat lengths rather than reading whole genes.
What a learner can do afterwards
- States what each temperature step in a PCR cycle does.
- Explains why shorter fragments travel further through the gel.
- Says which part of the genome a profile compares and why it differs between people.
1 · Read
PCR copies one chosen stretch of DNA inside a test tube. A machine called a thermal cycler heats and cools the sample again and again. High heat splits the two strands apart. Cooling lets short primers dock at the exact spot where copying starts. Gentle warmth lets Taq polymerase build a fresh strand on each template. Each cycle doubles the DNA, so 30 cycles pass a billion copies.
Gel electrophoresis sorts fragments by length, not by gene. Every fragment carries the same negative charge, so the current pulls them all toward the positive end. The gel is a tangled mesh. Short pieces slip through fast and travel far, while long ones snag and stay near the wells. Each band holds pieces of one length.
A DNA profile compares short repeated sequences, not whole genes. Your repeat count at these stretches differs from mine, so our band patterns differ even though nearly all our DNA matches. That difference is what lets a profile link a sample to a person.
Run PCR before a gel or a profile, because both need far more DNA than a single strand supplies. Taq survives the heating because it comes from hot spring bacteria, while ordinary enzymes from your own cells would break down.
You copy the target with heat cycled PCR, sort the pieces by length on a gel, and match people by their repeat counts.
2 · Watch
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Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.