A level Biology exam revision resources written by A level Examiners


DNA can be replicated artificially by the polymerase chain reaction. The enzyme DNA polymerase is used to make new double stranded DNA by synthesising a new complementary strand to a pre-existing strand, just as in natural replication. PCR makes it possible to synthesise large numbers of copies of very small samples of DNA. This is useful in activities such as forensics.

DNA fragments can be separated by gel electrophoresis. A voltage is applied to the gel, and the negatively charged DNA fragments move towards the positive electrode. Smaller fragments move faster than large ones.

The bands of DNA can be seen if radioactive nucleotides are used in the PCR. The pattern of banding in the gel can be made visible by placing the gel next to a sheet of unexposed photographic film overnight. The radioactive bands cause the film to turn black. A film that has been developed by radiation instead of light is called an autoradiograph.

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