Week 4 - 2 Techniques used in DNA Profiling

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Del curso dictado por Nanyang Technological University, Singapore

Introduction to Forensic Science

361 calificaciones

Nanyang Technological University, Singapore

Introduction to Forensic Science

361 calificaciones

We have all seen forensic scientists in TV shows, but how do they really work? What is the science behind their work? The course aims to explain the scientific principles and techniques behind the work of forensic scientists and will be illustrated with numerous case studies from Singapore and around the world. Some questions which we will attempt to address include: How did forensics come about? What is the role of forensics in police work? Can these methods be used in non-criminal areas? Blood. What is it? How can traces of blood be found and used in evidence? Is DNA chemistry really so powerful? What happens (biologically and chemically) if someone tries to poison me? What happens if I try to poison myself? How can we tell how long someone has been dead? What if they have been dead for a really long time? Can a little piece of a carpet fluff, or a single hair, convict someone? Was Emperor Napoleon murdered by the perfidious British, or killed by his wallpaper? *For Nanyang Technological University (NTU) students, please be noted that this course will no longer be eligible for credit transfer.

De la lección

DNA in Forensics

Opinion Poll 30:00

Week 4 - 1 Introduction to DNA16:15

Week 4 - 2 Techniques used in DNA Profiling8:16

Week 4 - 3 Polymerase Chain Reaction (PCR)4:29

Week 4 - 4 Short Tandem Repeats (STR)12:05

Week 4 - 5 Colin Pitchfork Case5:37

Week 4 - 6 Cold Cases I4:22

Week 4 - 7 Cold Cases II3:20

Week 4 - 8 Early Uses of DNA Profiling6:03

Week 4 - 9 Mitochondrial DNA9:19

Week 4 - 10 DNA Profiling of Other Species2:46

Week 4 - 11 Peter Falconio & Joanne Lees Case; Summary6:48

Conoce a los instructores

Roderick Bates
Associate Professor
Division of Chemistry & Biological Chemistry | School of Physical & Mathematical Sciences

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What about the use of DNA by Forensic Scientists?

DNA profiling sometimes called DNA typing and

sometimes referred to as DNA fingerprinting.

Well, this was discovered some decades ago

by Professor Sir Alec Jeffreys at the University of Leicester.

And he used a technique at that time,

called Restriction Fragment Length Polymorphism, RLFP.

This technique has since been superceded.

Back in those days, in order to do a deeper DNA profiling of someone,

it needed a reasonably large amount of material about the size of a dollar coin.

Now, the interesting point about this is

that the DNA used for DNA profiling is your junk DNA.

It's that non-coding DNA that we don't really understand.

It's not your coding region, it's not your genes.

Why is this?

And the reason is very simple.

If you look up the genes for human beings, between

any two human beings anywhere on the planet, it's virtually

identical because most of what makes us human beings is common to us all.

There are a few minor, minor differences such as hair color, eye color,

skin color and so forth.

That is just a tiny amount of your genetic material, so

you cannot use the coding region for

DNA profiling, because it's not highly individualized.

It's almost all identical from one person to another person to another person.

And, in fact, much of it is also shared by our related animals.

It's in the junk DNA that you get high variability from person to person.

And that's why DNA profiling uses the junk DNA.

Let's take a look at the technique used by Alec Jefferys.

So this is how it is done.

First of all, the DNA molecule has to be cut up into fragments,

and this is done by a special enzyme called a restriction enzyme.

And it's essentially a biochemical paralysis which chops up

the DNA into little pieces.

A technique called electrophoresis, which we'll look at in a little more detail,

is then used to separate out all those separate pieces.

Now, electrophoresis is carried out on a gel.

And it has to be transferred off the gel onto something more permanent, and

this is done by transferring onto a nylon membrane.

So on the nylon membrane, you have an exact image of what was on the gel.

Now, we have all these fragments of DNA all over the nylon membrane.

How do you find the ones that you're interested in looking at?

And how do you make them show up?

So in this older method, it was done using radioactive DNA probes.

Now, the probe is the complimentary strand of DNA,

complementary to the one that you're interested in.

And on the end of that strand of DNA, there is some radioactive atom.

So what will happen,

is that when you expose the nylon membrane to the DNA Probe, the Probe will

bind selectively to the DNA fragment of interest.

You can then, put this next to an X-ray film and

the radioactivity from that radioactive atom will then

show up on the X-ray film.

But of course, you will only see an image on the X-ray film,

where the DNA of interest was because of the specificity of the probe.

Now, one of the things about this is that you can use more than

a single DNA probe so you can use all the DNA probes of interest and

everything will show up on one X-ray film.

And in the case of the Bill Clinton, when he was accused of

having an affair with Monica Lewinsky, the FBI at that time used 7 probes.

Why do we use 7 probes?

It's for reasons of probability.

If you only use one probe, there is some possibility

that it is not that person, there could be another person with that DNA.

The more probes you use,

the better the probability you have that it is only that person.

And the FBI statistical people calculated that using those 7 probes,

there were odds of 1 in 8 trillion.

Let's take a look at the electrophoresis technique.

It's done in a gel.

The sample of interest is applied to the gel.

In addition, some standards are also applied for the gel.

And electrical potential is then applied across the gel and

this makes the DNA fragments migrate through the gel.

But the DNA fragments, of course, all migrate at different speeds and

that is why they separate out on the gel.

You can then compare the position of your fragment

of interest with your standard to identify it.

Now, if you think about this for a moment,

you'll realise that gel electrophoresis done like this is analagous to

thin layer chromatography that we talked about in an earlier lecture.

Well, in that earlier lecture, we talked about Thin Layer Chromatography and

then we talked about related methods Gas Chromatography and HPLC.

And again, there's an analogy because there is another technique

more widely used called capillary gel electrophoresis.

And this one you can think of as analogous to HPLC.

This time, the gel is not in the form of a block or

a plate, it is inside capillary tubing.

The farm pole is placed at one end of the tubing, and

electrical potential is applied across the capillary, so

the DNA fragments migrate through the capillary tubing, and

at the far end of the capillary tubing, there is a detector and

that detects the time at which they get to the end of the tubing.

So gel electrophoresis is analogous to TLC.

Capillary gel electrophoresis is analogous to HPLC.

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