Week 4 - 3 Polymerase Chain Reaction (PCR)

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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

[MUSIC]

Now, you remember Alec Jeffries.

When he started doing this work,

he required samples of material which were relatively large.

But many traces are actually extremely small, and

very small traces cannot be analysed using just those techniques.

But there is now a method whereby DNA can be amplified,

and this is called the polymerase chain reaction, or PCR.

And this is how it works.

Suppose you have length of double stranded DNA.

When that DNA is heated to a temperature of 94 degrees centigrade,

that is too hot for the hydrogen bonds.

And all the hydrogen bonds break, and the two strands of DNA separate.

Okay, now suppose you add primers,

which are short lengths of DNA which match,

they're complementary to the ends of these strands.

And this of course, has to be done at a lower temperature.

So the temperature's been brought down to 60 degrees,

otherwise the hydrogen bonds wouldn't form.

Okay, now at 72 degrees, an enzyme called

DNA polymerase is added as well as the four nucleotides.

The DNA polymerase will recognize the end of the primer, and

it will build up the DNA chain, so

that it is a complementary sequence to the original strand.

So if we go through this sequence with the different temperature changes,

at the end, we have double the amount of DNA that we started with.

In fact, what we've done, is copy it.

So if you do it once, you double the amount of DNA you have.

But there's no reason why you can't do it more times.

So you could go through the sequence once, twice, three times, four times, etc.

Each cycle turns out to take about five minutes.

And typically, it's done with 25 to 35 cycles, and

it's done in a machine like this, which is called a thermal cycler.

So what is the result of the PCR?

It allows you to repeatedly duplicate the part of the DNA

which you're interested in.

And those 25 to 35 cycles of PCR will give you about 1 million copies.

So if you have a very, very small trace from your crime scene, you can submit

it to the polymerase chain reaction, and you can convert it a million times.

This means very, very small amounts can provide DNA that can be analysed.

So if you have just 36 cells, you can do this.

And you can even go lower, maybe even 9 or so cells.

But now, this is really a small number, and

this is called low copy number, because 9 really is a very small number.

But with PCR, you can now analyse really small traces.

For instance, the saliva left on a cigarette end.

Okay, remember the case back in our introductory lecture.

You can analyze the saliva off the back of a stamp, or

you could analyse the DNA contained in a blood stain that's barely visible.

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