DNA Extraction

DNA extraction is a routine procedure to collect DNA for subsequent molecular or forensic analysis. There are three basic steps in a DNA extraction:

  1. Breaking the cells open to expose the DNA within, such as by grinding or sonicating the sample.
  2. Removing membrane lipids by adding a detergent.
  3. Precipitating the DNA with an alcohol — usually ethanol or isopropanol. Since DNA is insoluble in these alcohols, it will aggregate together, giving a pellet on centrifugation. This step also removes alcohol-soluble salt.

Refinements of the technique include adding a chelating agent to sequester divalent cations such as Mg2+ and Ca2+. This stops dnase enzymes from degrading the DNA.

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Cellular and histone proteins bound to the DNA can be removed prior to its precipitation either by adding a protease or having prior to precipitation, precipitating with sodium or ammonium acetate, or extracting with a phenol-chloroform mixture.

If desired, the DNA can be resolubilized in a slightly alkaline buffer.

Detecting DNA

A diphenylamine (DPA) indicators will confirm the presence of DNA. This procedure involves chemical hydrolysis of DNA: when heated (e.g. ≥95oC) in acid, the reaction requires a deoxyribose sugar and therefore is specific for DNA. Under these conditions, the 2-deoxyribose is converted to w-hydroxylevulinyl aldehyde, which reacts with the compound, diphenylamine, to produce a blue-colored compound. DNA concentration can be determined measuring the intensity of absorbance of the solution at the 600 nm with a spectrophotometer and comparing to a standard curve of known DNA concentrations.

Measuring the intensity of absorbance of the DNA solution at wavelengths 260 nm and 280nm is used as a measure of DNA purity. DNA absorbs UV light at 260 and 280 nm, and aromatic proteins absorbs UV light at 280 nm; a pure sample of DNA has the 260/280 ratio at 1.8 and is relatively free from protein contamination. A DNA preparation that is contaminated with protein will have a 260/280 ratio lower than 1.8.

DNA can be quantified by cutting the DNA with a restriction enzyme, running it on an agarose gel, staining with ethidium bromide or a different stain and comparing the intensity of the DNA with a DNA marker of known concentration.

Using the Southern blot technique this quantified DNA can be isolated and examined further using PCR and RFLP analysis. These procedures allow differentiation of the repeated sequences within the genome. It is these techniques which forensic scientists use for comparison and identification.

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