Background Information to DNA extraction practical
(http://www.wetheteachers.com/files/0/53221225405059.doc)
DNA extraction is a fundamental procedure in scientific laboratories around the world. By extracting DNA and
studying just it alone (separated and purified from the other cell parts and substances), scientists can learn
how DNA encodes the instructions for all life processes.
DNA of course is important in the study of heredity (passing of traits through genetic material from generation
to generation) and the treatment of all genetic diseases. It can be used for identification of individuals through
DNA fingerprinting (matching patterns of the bases).
All life forms and even most viruses have DNA as their genetic material that they inherit from the previous
generation. In DNA can be found the information that controls what our cells become, how they function and
the cell’s life cycle. Multicellular organisms have different types of cells.
Each cell type only use the code from genes for their specific cell type even though every
body cell contains approximately 25,0000 genes located over 23 pairs of chromosomes in
their nucleus.
Changes in gene structure, called mutations, that have that allowed
organisms to survive in their environment is how evolution occurs.
In this practical you will be extracting DNA from wheat germ ( other material such as onions or bananas can
also be used)
Why is water temperature important?
The heat softens the phospholipids (fats) in the membranes that surround the cell and the nucleus. It also
denatures the deoxyribonuclease enzymes (DNase) which, if present, would cut the DNA into such small
fragments that it would not be visible. Denatured enzymes and DNA unravel, lose their shape, and thus
become inactive. Enzymes denature at 60° Celsius, and DNA denatures (falls apart) at 80° Celsius.
Detergent/Soap
Detergent contains sodium laurel sulfate, which
cleans dishes by removing fats and proteins. It acts
the same way in the DNA extraction, pulling apart
the fats (lipids) and proteins that make up the
membranes surrounding the cell and nucleus. Once
these membranes are broken apart, the DNA is
released from the cell.
Soap molecules and grease molecules are made
of
two parts:

Heads, which like water

Tails, which hate water.
Both soap and grease molecules organize
themselves in bubbles (spheres) with heads outside to face the water and tails inside to hide from the water.
When soap comes close to grease, it captures it, forming a greasy soapy ball.
A cell's membranes have two layers of phospholipids (fat) molecules with proteins going through them.
When detergent comes close to the cell,
it captures the lipids and proteins breaks
the cell and nuclear membranes and releases
the
DNA.
Why use concentrated alcohol/methylated spirits?
Watch carefully as the DNA precipitates or separates and moves through the alcohol layer with small bubbles
attached to the strands. The DNA released from the cell nucleus is dissolved in the water/detergent/wheat
germ solution and cannot be seen. DNA precipitates out of solution in alcohol, where it can be seen as white
mucous-like strands. Besides allowing us to see the DNA, the alcohol separates the DNA from the other parts
of the cell, which are left behind in the water solution. You should use 70-95% isopropyl, ethyl alcohol, or
methylated spirits –that is very concentrated (less water). Why cold ethanol? The cold temperature keeps the
DNA from breaking apart (dissolving) so easily. DNA is very thin and fragile.