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.