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.
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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.
More to read
Learn and try: Polymerase chain reaction (PCR)
PCR stands for polymerase chain reaction. It's a way to make millions of copies of one small piece of DNA using nothing but a test tube.
A machine called a thermal cycler heats and cools the sample over and over. Each cycle has three temperature steps.
First, high heat splits the DNA's two strands apart. Next, the sample cools so short DNA pieces called primers can stick to the exact spot where copying should start. Then the temperature rises again, just enough for a special enzyme, Taq polymerase, to build a fresh strand alongside each original one.
Taq polymerase comes from bacteria that live in hot springs, so it doesn't fall apart at the high temperatures PCR needs. Regular enzymes from our own cells would just break down.
Each cycle doubles the amount of target DNA. Run 30 cycles and one strand becomes over a billion copies, more than enough to see and study.
Scientists run PCR before putting DNA through a gel or building a DNA profile, since both need far more DNA than a single strand can give them.
Why Gel Bands Form
When scientists run DNA through a gel, they're sorting it by size, not by which gene it carries.
Every DNA fragment carries the same negative charge no matter how long it is, so an electric current pulls all of them toward the positive end of the gel at the same time. The gel itself is packed with a tangled mesh of fibers, kind of like a running track full of hurdles.
Short fragments slip through those gaps quickly and travel far. Long fragments snag on more of the mesh and barely move. Given enough time, the fragments settle into separate bands, and each band holds pieces of one particular length.
That's why the pattern of bands tells you about fragment length, not about which genes are in the DNA. Two people can have completely different band patterns even though almost all of their DNA is identical, because the number of times a short repeated sequence appears varies from person to person. That's the part a DNA profile actually compares.
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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.