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How do you obtain DNA for DNA fingerprinting?

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To obtain DNA for DNA fingerprinting, a sample of cells from skin, hair, blood, saliva or semen is collected. The white blood cells from the sample are broken open using detergent, and then the usable DNA is separated from all the other cellular material. Subsequently, the extracted DNA is cut into smaller pieces using restriction enzymes.

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Restriction enzymes cut the DNA at a specific sequence, which produces either blunt or sticky ends, resulting in many fragments of different length, called restriction fragments length polymorphisms, or RFLPs. The RFLPs are put into agarose gel and are then sorted according to size, using gel electrophoresis. When the current is turned on in the electric field, the negatively charged RFLPs move across the gel towards the positive end. The smaller fragments move further across the gel than the larger ones.

An alkali causes the hydrogen bonds to break and the DNA to become single-stranded, resulting in the nucleotides becoming free. These free nucleotides are used to pair up with probes. The agarose gel is then covered by a piece of nylon. Thin paper towels are used to absorb the moisture from the gel. The DNA fragments slowly get transferred to the surface of the nylon in a process called blotting.

Radioactive probes are then washed over the nylon surface. These probes join with DNA fragments that share the same composition. Finally, a photographic film is placed on the nylon. The probes mark on the film, at the places where they connected with the RFLPs. On developing the film, dark bands that represent the length of the hybridized RFLPs are found. The DNA fingerprints of two people can then be compared by placing the film on a light surface and analysing the difference in the lengths of the RFLPs.

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