As
you should know by now, DNA is found in every living cell, in fact
there is about 6 meters of the stuff in every single cell. It is
located in a compartment called the nucleus, and both the cell and
the nucleus contained within it are surrounded by a thin layer known
as a membrane.
A cell membrane is made up of two layers of lipids (fats) and
proteins. The lipids are long chains of carbon and hydrogen atoms
- there is a hydrophobic (water hating) end and a hydrophilic
(water loving) COO- group on the other end. This polarity means
that the lipid molecules will orientate themselves in water so
that the hydrophilic ends point inwards and the hydrophobic ends
point out. This leads to the formation of little spheres when
fats are added to water - try adding a drop of cooking oil to
a glass of water and you will see this happening.
Chewing up the DNA source in the blender separates
up all the cells to give you a nice thin soup. This isn't enough,
however, to break open the cells and we need to do this to be
able to get the DNA inside, out. This is why we add the detergent.
Think about why you use detergent to wash up your dishes at home
- without it, it is pretty hard to break up the fatty yuck that
lurks on your grill pan. Detergents are similar to lipids as they
have hydrophobic and hydrophilic ends that allow them to form
spheres in water (picture below). When you mix detergents and
fats, the spheres are formed out of both types of molecules and
this acts to separate all the fat up to let you get it off the
grill pan or in our case, to break up the membranes that surround
a cell.
The next step of the experiment was to add enzymes. The enzymes
act to chop up the proteins
found in the cell. The DNA that was liberated from the nucleus
by the detergents is wound around protein and we need to get rid
of these if we want to purify it. If you think about just how
much DNA there is in every cell its pretty clear that it needs
to be well packaged or you would end up with a big messy tangle.
This is why the DNA coils up and is then wrapped around proteins
in the nucleus.
Finally, we add alcohol to the mixture and, because it does not
mix with water, it forms a layer on top. The protein and fat stays
in the bottom but the DNA floats to the surface and this is the
white stringy stuff you should have seen in the experiment.
