DNA fingerprinting is a means by which to
identify a person based upon his or her genetic blueprint. One useful
application of this technology is the determination of guilt or innocence
among criminal suspects. A man is shown here breaking into a building and
leaving some of his blood behind, depicted as the red droplet. The police
find the blood at the crime scene and narrow the search for the criminal down
to three suspects, shown here as the stick figures. A blood sample is then
taken from each individual and is compared to the blood found at the scene of
the crime by DNA fingerprinting. The first step in this process involves the
extraction of DNA from each blood sample, including that found at the crime
scene. Next, each sample is cut into fragments by enzymes known as
restriction endonucleases, which cleave each person’s DNA in a unique pattern
that can be thought of as the individual’s genetic fingerprint. The
fragmented DNA of each suspect is then applied to separate lanes of a gel
matrix, usually agarose, which is a jelly-like
substance and is shown here as the grey rectangle. An electrical current is
run through the gel to separate the different sized fragments from each
another. The electricity causes the smaller pieces of DNA to move toward the
bottom of the gel while the largest fragments remain near the top. At this
stage, the DNA, which has been chemically stained, appears as white smears
down each gel lane because it has been cut into a large number of tiny pieces
that are too small to be seen individually. To visualize a pattern on the
gel, the DNA fragments from each suspect and from the blood at the crime
scene are first transferred to a thin membrane in a process known as a Southern
blot. The membrane is then treated with single-stranded DNA probe molecules
that are usually tagged with radioactivity or fluorescence. These probes
bind, or base pair, to sequences that they match among the DNA samples on the
membrane. Binding patterns of the probe to the DNA of each suspect can be
visualized by several different methods, such as placing a photographic film
over the membrane. The resulting image shows a series of bright bands where
the tagged probe molecules were able to bind to the membrane, with the DNA
found at the scene of the crime shown in the leftmost lane. If the probe
reveals that the restriction enzyme cut pattern of the DNA from the crime
scene exactly matches that of one of the suspects, the evidence against that
suspect is as strong as a real fingerprint and can be used against him or her
in a court of law.
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