Scientists can isolate large molecules of DNA from almost any cell type (bacteria, white blood cells, sperm, mammalian cells grown in culture, or a bit of minced tissue or organ from an adult or fetus). The process may last a few hours, however, the obtained DNA is in nearly pure form. To accomplish this task, scientists need to carefully follow some guidelines, usually called protocols, which can be summarized as follow:

• Cells are broken open with sodium dodecyl sulfate (SDS), a detergent that dissolves the lipids in cell membranes and lets the insides spill out;
  
  
• Proteins in the resultant cellular soup are coagulated by gentle shaking with phenol, an organic solvent that does not mix with water. This process leaves a separate aqueous layer containing primarily DNA. The aqueous solution is now very viscous and elastic because the extremely long, thin, stretchy threads of DNA intertwine with each other and
  
  
• DNA molecules are precipitated by adding ethyl alcohol. This DNA can be wound around the tip of a glass rod and lifted out from the liquid as a visible, glistening globule of nearly pure, high-molecular weight DNA.

*Picture credit: GROLIER, Multimedia Encyclopedia, 1996.

Comments to william.quintana@ibm.net / Go to NYU Web home page Last modified July 30th 1997