Obtaining a genetic fingerprint begins with a sample of biological tissue.

At Guy’s Hospital in London, this research worker is using an automatic DNA extractor to produce a high purity DNA fraction from a blood sample. A restriction enzyme is added to the DNA. This enzyme moves along the DNA strand, cutting it at the so called, restriction sites. The restriction sites available to enzymes are located throughout the chromosome except in sections consisting of copies of the core systems. This process produces many pieces of DNA, which consists of repeats of the core system. The mixture of this DNA fragments is then placed unto an agarose gel.

A small voltage is applied across the gel. And the fragments move through the gel at a rate proportional to their size. Because gels are difficult to handle, the DNA band pattern on the gel is transferred to a nylon membrane by a technique known as southern blotting.

1. First, the nylon membrane is placed on the agarose gel.
2. Then absorbent papers layered on top of the membrane. It acts as a wick, drawing up the DNA bands unto the membrane where they stick.
3. Finally, the researcher adds a radioactive DNA probe which will bind specifically to the core sequence. Excess DNA probe is washed off, leaving only the radioactive probe that is bound to the DNA pattern on the membrane. This can be shown on a x-ray film, producing the genetic fingerprint.

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