A level Biology B · Edexcel 9BI0 · Topic 7A
DNA profiling and PCR
Key points
- PCR (polymerase chain reaction) needs DNA template, primers, free nucleotides and heat-stable Taq DNA polymerase.
- PCR cycle: 95 °C to separate strands, about 55 °C for primers to anneal, 72 °C for DNA polymerase to extend. Each cycle doubles the DNA, so after cycles there are copies.
- DNA profiling looks at short tandem repeats (non-coding regions that vary between people).
- DNA is cut with restriction enzymes or amplified with primers, then separated by gel electrophoresis: DNA is negatively charged and moves to the positive electrode; smaller fragments move further.
- Bands are made visible with stains or fluorescent/radioactive probes and compared with a DNA ladder.
- Uses: crime scenes, paternity tests and checking how closely related species are.
Topic notesPremium
Crime scenes and paternity cases often provide only a trace of DNA. PCR multiplies it into millions of copies, and gel electrophoresis separates the copies by size to give a DNA profile that is very nearly unique to each person.
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