To understand why we would use a DNA
fingerprint, it's helpful to think why we use
normal fingerprints to identify a person. Any two
people you pick are going to have an enormous
number of differences, pretty much everywhere if
you look close enough. However, the fingerprint is
one of the easiest parts of a person to look at
and compare to another person.
With DNA it's a similar concept. Any two
people (besides identical twins) will have
different sets of DNA, called genomes. But,
the genome is really, really large so for a long
time it wasn't possible to look at 2 whole genomes
and compare them to see if 2 people are different.
The idea of DNA fingerprinting is that you only
look at a very small part of the DNA that you
expect to be different to determine if 2 DNA
samples come from the same person.
The way this is done is that first a sample of
DNA from a person is found. That sample is usually
from human cells, blood, saliva or hair (with the
root attached). The DNA is then cut up using
molecules that cut DNA in a certain way. Then
"probes" are added which light up when they bind a
particular piece of DNA. In the case of DNA
fingerprinting, these particular pieces of DNA are
certain sequences that repeat over and over again,
but the number of times they repeat is different
in different people. This generates an image that
looks like a bunch of bars, kind of like a
barcode. In a real sense, the DNA
fingerprint achieved is a barcode for a person in
that it uniquely identifies people.
There are other ways to get a DNA fingerprint,
but they all rely on looking at certain places in
the genome that we expect to be different and
comparing how these places look in different
people. So the classic case is if a criminal
leaves behind blood, they can do a DNA fingerprint
on the blood of the criminal and the crime scene
blood and see if the "bars" on the fingerprint are
in the same places.
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