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DNA Fingerprinting

The Mechanics Of Genetic Fingerprinting

The nucleus of every cell in the human body contains deoxyribonucleic acid or DNA, a biochemical molecule that is made up of nearly three-billion nucleotides. DNA consists of four different nucleotides, adenine (A), thymine (T), guanine (G), and cytosine (C), which are strung together in a sequence that is unique to every individual. The sequence of A, T, G, and C in human DNA can be found in more combinations or variations than there are humans. The technology of DNA fingerprinting is based on the assumption that no two people have the same DNA sequence.

The DNA from a small sample of human tissue can be extracted using biochemical techniques. Then the DNA can be digested using a series of enzymes known as restriction enzymes, or restriction endonucleases. These molecules can be thought of as chemical scissors, which cut the DNA into pieces. Different endonucleases cut DNA at different parts of the nucleotide sequence. For example, the endonuclease called SmaI cuts the sequence of nucleotides CCCGGG between the third cytosine (C) and the first guanine (G).

After being exposed to a group of different restriction enzymes, the digested DNA undergoes gel electrophoresis. In this biochemical analysis technique, test samples of digested DNA are placed in individual lanes on a sheet of an agarose gel that is made from seaweed. A separate lane contains control samples of DNA of known lengths. The loaded gel is then placed in a liquid bath and an electric current is passed through the system. The various fragments of DNA are of different sizes and different electrical charges. The pieces move according to their size and charge with the smaller and more polar ones traveling faster. As a result, the fragments migrate down the gel at different rates.

After a given amount of time, the electrical current in the gel electrophoresis instrumentation is shut off. The gel is removed from the bath and the DNA is blotted onto a piece of nitrocellulose paper. The DNA is then visualized by the application of radioactive probe that can be picked up on a piece of x-ray film. The result is a film that contains a series of lines showing where the fragments of DNA have migrated. Fragments of the same size in different lanes indicate the DNA has been broken into segments of the same size. This demonstrates a similarity between the sequences under test.

Different enzymes produce different banding patterns and normally several different endonucleases are used in conjunction to produce a high definition banding pattern on the gel. The greater the number of enzymes used in the digestion, the finer the resultant resolution.

In genetic or DNA fingerprinting, scientists focus on segments of DNA in which nucleotide sequences vary a great deal from one individual to another. For example, 5–10% of the DNA molecule contains regions that repeat the same nucleotide sequence many times, although the number of repeats varies from person to person. Jeffreys targeted these long repeats called variable number of tandem repeats (VNTRs) when he first developed DNA fingerprinting. The DNA of each person also has different restriction fragment sizes, called restriction fragment length polymorphisms (RFLPs), which can be used as markers of differences in DNA sequences between people. Today, technicians also use short tandem repeats (STRs) for DNA fingerprinting. STRs are analyzed using polymerase chain reaction or PCR, a technique for mass-producing sequences of DNA. PCR allows scientists to work with degraded DNA.

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