DNA Fingerprinting


Hi. It’s Mr. Andersen and in
this podcast I’m going to talk about DNA fingerprinting. We sometimes refer to this as DNA profiling
or your genetic fingerprint. And basically it started with this guy Alec Jeffries. Basically
in his lab he was working with x-ray and looking at DNA and what he figured out is he could
tell a lot about a person by looking at their DNA quote, unquote, fingerprint. In other
words he could see who they were related to and who they weren’t. He could tell paternity
for example. And so he was working at the University of Leicester and basically figured
out this whole idea of DNA fingerprinting. This was around 1984. And basically for the
next three years all DNA fingerprinting on the planet went through this university. And
so eventually it was privatized and this was everywhere. And it will probably eventually
be replaced by just DNA sequencing, sequencing all the letters in DNA. But to make it understandable
essentially what we have in a human is we have long linear segments of DNA. But within
that we have these genes. And so 99.9% of our DNA in everyone is going to be exactly
the same. The genes are going to be the same but again you’re going to have different copies
or alleles of those genes. That’s what makes you, you. But if we look into this area in
the middle, we used to call this junk DNA but now we know it’s really important in controlling
gene expression we find that there’s quite a bit of variability in here, which shouldn’t
surprise us because this, the gene, makes the protein and the protein makes the phenotype
and that’s really what natural selection is selecting for or against. But this in the
middle can go crazy. And so it does. And so an example of one that we use in DNA fingerprinting
is something called short tandem repeats. Originally we started with something called
VNTR, variable number tandem repeats and you’ll find in DNA sequencing that you have all kinds.
So we had STRs we have VNTRs. Before that we had restriction fragment length polymorphisms
and so there’s a bunch of different things that we could look at. But we’ve kind of moved
to this idea of these short tandem repeats. They work great. There’s quite a bit of variability
in individuals. And so what is it? You basically have letters of DNA that repeat over and over
and over and over and over and over and over and over. So sometimes, fifty times it repeats.
And so what does that look like? Well if we have these three individuals, we’ll call this
Mr. Blonde, Mrs. Red and then Mr. Mustache, and so if we look at these three people their
genes are going to be the same, but these STRs are going to be different. These single
or short tandem repeats are going to be different. You can see that Mr. Blonde has more than
Mr. Mustache and less than Mrs. Red. And so if I make that a little bit easier to grasp
onto, if I count them out and then remove everything else, what we get is variability
between all individuals. Everything else was the same but we see variability in here. And
we can cut these sections out using restriction enzymes and them we can amplify them using
polymerase chain reaction. And then we can separate them using gel electrophoresis. So
how does that work? Basically I’ll take the DNA and I’ll put them in a little well. And
so we’re looking down on this. This is an agarous gel. I could put Mrs. Red’s and then
Mr. Mustache, I could put those all in DNA. Basically I would then turn on the voltage.
So there’s going to be a positive charge here and a negative charge up here. DNA is a negative
charge and so it’s going to be pulled towards the positive and so what’s going to happen
is those little fragments of DNA are going to migrate. And so what does that allow me
to do? It allows me to tell the difference between each of these individuals. And so
this is their fingerprint. But you can tell this is a really bad fingerprint because we’ve
got some, these two are exactly the same here and so when they really do DNA profiling what
they do is they generally us thirteen different sections like this. And then those thirteen
section are each going to be highly variable. And so it’s a good way to tell who’s who.
When would we ever want to do this? Forensics is one reason and then also in paternity,
figuring out who’s dad. And so let’s talk about the murder. There was a murder that
was committed Somebody was brutally murdered by one of these three suspects, Mr. Blonde,
Mrs. Red or Mr. Mustache. But they left blood at the scene. And so what I can do is I can
grab samples of DNA from each of our suspects and then I could grab the blood itself and
then I could do DNA fingerprinting on them. So before we separate them you may think to
yourself which of these looks guilty? Who looks like they’re capable of murder? And
if we separate them then using that gel, what we can see is that Mr. Blonde is guilty. In
other words his blood matches up with the crime scene. And so what do I mean by matching
up? Well, those single or those short tandem repeats, if we look horizontally are going
to be exactly lined up. And if we were to look at Mr. Blonde’s son we’d find more similarities
than we would between the others. And so basically that’s DNA profiling, DNA fingerprinting.
It’s much more sophisticated than that but then again it’s kind of on its way out. We’ll
eventually replace this with DNA sequencing. In the US we, the FBI has started creating
this database of DNA, which is a little scary. And basically what they use are 13 different
areas within the chromosome or the genome and they they’re looking at those short tandem
repeats in there. Now why do I say that’s a little bit scary? I think you really want
to protect your DNA because as we learn more and more about genetics what’s going to be
found in your DNA, well predisposition to Alzheimer’s or breast cancer. Any of these
things your insurance company would love to get a hold of. And so it also doesn’t answer
the idea of Mr. Blonde, did he really do it? Did the police frame him and contaminate the
blood? So we don’t know that. All it does tell us is if we have two samples of DNA,
the odds of two people having the same DNA fingerprint are astronomical. Unless they’re
identical twins. And I hope that’s helpful.

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