|
Wednesday, June 20, 2007
When working with a crime scene, where do you get
your DNA evidence? Today in class, Dr. Carol Park taught us that when
working with blood, even though there are many components to the blood,
the red blood cells have no DNA because there is no nucleus. However,
DNA is found in white blood cells because there is a nucleus. Red blood
cells are not completely useless when trying to eliminate a suspect,
however, because they can be used to determine ABO and Rh blood type
using AB and Rh antibodies. If your suspect is AB+ and you find O- blood
at a crime scene, then perhaps you can eliminate that suspect.
PAST
Foundation students are extracting the DNA that we gathered from the
horse blood with assistance from Dr. Park
Since we learned that there is
no DNA to be obtained from red blood cells, the white blood cells have
to be separated out for analysis. To do this, we added whole blood to
the top of a density medium, being careful that the whole blood does not
mix with the medium. Then those two components are centrifuged to
separate the layers of blood components of red blood cells, white blood
cells, and plasma. As we are only interested in the white blood cells,
we used a pipette to siphon them out so that we can lysis the cells to
be able to obtain the DNA for the test. Once the white blood cells and
the membranes of the nucleus are lysised, we add it to ethanol to
precipitate DNA out of the solution. The DNA in the solution first
looked like a jellyfish and then compacted into a tiny white ball.
 |
 |
|
Team 1 is playing with coagulated horse blood. |
Dr. Park assists a student with loading the DNA into the wells of
the agarose gel. |
After lunch, Dr. Park gave us a
crash course on what DNA is and why we need to know the components of
it. DNA is made of sugar, which is deoxyribose, phosphate, and
nitrogenous bases pairs. The only variable in DNA is the base pairs. The
sugar and phosphate will always stay the same. There are four base
pairs, Adenine, Thymine, Cytosine, and Guanine. Adenine always pairs
with Thymine, and Cytosine always pairs with Guanine.
PAST
Foundation students are making their agarose solution for the agarose
gel.
The next thing that we learned
about was restriction enzymes. This is at the heart of telling one set
of genes from another. A restriction enzyme will cut the DNA at the
palindromes, which are different for everyone. This creates different
sizes of DNA pieces. In order to decipher the different sizes of DNA
that have been created by the restriction enzyme we have to have a
medium to size them. The following step was creating that medium, in
this case, agarose gel. This is a gelatin base created from super
purified seaweed. The class got to create this, and it was like making
Jell-O. The agarose is created with wells, or small holes, and this is
where the DNA was placed. At this point, the DNA has been dyed so that
we can see it easier and also to help weigh it in the buffer. Next, we
got a demonstration on how to process with electrophoresis. Once the DNA
is set in the wells with the buffer an electric field is added to the
apparatus. An important note is that DNA is negatively charged, and will
move toward the positive charge, leaving the DNA fingerprint with the
smaller pieces traveling further. This process is referred to as RFLP,
or Restriction Fragment Length Polymorphisms. This process is still used
in court cases today, but they also use Polymerase Chain Reaction (PCR),
with can make millions of copies of just the piece that you are
interested in.
At the conclusion of our day, we
understood how DNA can be processed and interpreted in a criminal case,
and then be presented to a jury. We would like to thank Dr. Park for
making DNA and Serology into such a fun and educational day!
The 2007 OSU/PAST Foundation Forensic
Archaeology Field School is sponsored by:
|
