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
through DNA fingerprinting (matching patterns of the bases). Scientists can genetically engineer
changes in the DNA sequence of the genes to modify (change) the traits of various plant, animal, or
bacterial species—any form of life, even viruses.
All life forms and even most viruses have DNA as their genetic material that they
inherit from the previous generation. In the DNA can be found the information
that controls what our cells become and how they function and the cell’s lifes
cycle. Multicellular organisms have different cell types. 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 removing 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.