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Purpose of DNA Extraction

Commonly referred to as "genetic fingerprinting," DNA testing is one of the most accurate--and controversial--identification tools used. The role of DNA testing has expanded from the historical validation of human and animal remains to determining family and criminal court cases. Extraction of DNA is often an early step in many diagnostic processes used to detect bacteria and viruses in the environment as well as diagnosing disease and genetic disorders.

  1. The Facts

    • DNA or deoxyribonucleic acid is the hereditary material in humans and almost all other organisms. Scientists extract DNA from the nucleus of a cell. The DNA is stored as a code made up of four chemical bases: adenine (A), guanine (G), cytosine (C), and thymine (T). All humans have over 3 billion bases, and 99.9 percent of them have the same sequence. However, the different ones make up the uniqueness of one's self.

    Function

    • Extracting DNA is a three- to four-step operation, which involves:
      1. Breaking the cells open, commonly referred to as cell disruption or cell lysis, to expose the DNA within. This is commonly achieved by grinding or sonicating the sample.
      2. Removing membrane lipids by adding a detergent.
      3. Removing proteins by adding a protease (optional but almost always done).
      4. Precipitating the DNA with an alcohol, usually ice-cold ethanol or isopropanol.
      Instruments that are commonly used in DNA extractions are a gel box, bead beater and and a centrifuge. A gel box is used to separate DNA by sending charges through agarose gel. A bead beater is used to lyse or break apart cells to access DNA and a centrifuge spin at speeds of more than 15,000 rpm to help separate DNA in different phases of the extraction. Although forensic and medical labs are known to do this type of work, anyone can get DNA from any living organism by following protocol.

    Benefits

    • DNA extraction serves many different purposes. Evidence stemming from DNA tests has convicted or exonerated numerous people in rape, murder and other criminal cases. It's also the deciding factor in many paternity cases. Tests have also been able to unlock hereditary traits and ancestral paths. Testing has helped identify bodies found that can't be verified any other way.

    Misconceptions

    • As popular as DNA testing has become in the court of law and in public opinion, it isn't foolproof. There have been instances where DNA tests done correctly proved to be wrong. In Washington State, a mother's DNA didn't match her children's, even though she gave birth to them. Another example stems from a doctor who raped patients but foiling law enforcement officials by planting someone else's blood in his arm.

    Warning

    • Most DNA testing errors arise from human error. Testing labs have come under fire for flawed practices, blood sample contamination and careless documentation. There have been statistics adopting the law of averages theme that argues the chance of a DNA match against the odds of guilt or innocence. This defense in the legal arena is known as prosecutor's fallacy. This and other discrepancies have fueled court arguments against DNA tests. With such challenges, DNA testing will never stand alone as sole evidence in a criminal case.

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