How is DNA extracted from cells?

DNA extraction from cells involves breaking open the cell and nuclear membranes, separating the DNA from other cellular components, and then purifying the DNA. Several methods exist to accomplish this. DNA extraction techniques include organic extraction, nonorganic methods, and adsorption methods.

Common DNA Extraction Methods

Here's a breakdown of some common methods:

• Organic Extraction (Phenol-Chloroform Method):
  This method uses organic solvents, typically phenol and chloroform, to separate the DNA from proteins and lipids.

  • Process:
    1. Cells are lysed (broken open) to release their contents.
    2. Phenol and chloroform are added, causing the solution to separate into two phases: an aqueous (water-based) phase containing the DNA and an organic phase containing the proteins and lipids.
    3. The aqueous phase is carefully removed, and the DNA is precipitated (made insoluble) using alcohol (ethanol or isopropanol).
    4. The DNA is then washed and resuspended in a buffer solution.

• Nonorganic Method (Salting Out and Proteinase K Treatment):
  This method avoids the use of harmful organic solvents.

  • Salting Out: Uses high salt concentrations to precipitate proteins, leaving the DNA in solution.

    1. Cells are lysed, and RNase may be added to remove RNA.
    2. A concentrated salt solution (e.g., sodium chloride, ammonium acetate) is added, causing proteins to aggregate and precipitate.
    3. The mixture is centrifuged, and the supernatant (containing the DNA) is collected.
    4. The DNA is precipitated with alcohol, washed, and resuspended.

  • Proteinase K Treatment: Uses an enzyme, Proteinase K, to digest proteins.

    1. Cells are lysed.
    2. Proteinase K is added to digest proteins.
    3. The DNA is then purified by alcohol precipitation or another method.

• Adsorption Method (Silica-Gel Membrane):
  This method uses a silica-based membrane to selectively bind DNA.

  • Process:
    1. Cells are lysed.
    2. The lysate (cell contents) is passed through a silica-gel membrane in a spin column.
    3. DNA binds to the silica membrane, while other cellular components are washed away.
    4. A special elution buffer is used to release the purified DNA from the membrane.

Steps Generally Involved in DNA Extraction

Regardless of the specific method used, DNA extraction typically involves these steps:

1. Cell Lysis: Breaking open the cells to release the DNA. This can be achieved through physical methods (e.g., grinding, sonication) or chemical methods (e.g., using detergents).
2. Removal of Contaminants: Separating the DNA from proteins, RNA, lipids, and other cellular debris. This is the main differentiating step between the methods outlined above.
3. DNA Precipitation: Making the DNA insoluble so that it can be easily collected. This is commonly done using alcohol.
4. Washing: Removing any remaining contaminants from the precipitated DNA.
5. Resuspension: Dissolving the purified DNA in a suitable buffer solution for storage and further analysis.