DNA profilingKey words: Polymerase chain reaction, introns,
exons, STR’s
Starter:
A big question “What is DNA profiling, and
why is it used in forensics?”
DNA Profiling
All pupils will:
Recall the uses of DNA profiling.
Most pupils will:
Describe the method used to prepare a DNA profile.
Some pupils will:
Discuss the benefits of genetic profiling Skills:
Communication, Problem Solving, WWO,
DNA is the single
most important
molecule in living
cells
DNA are large
molecules –
You have about 9
MILLION
KILOMETRES of
DNA in your body
If you were to
write down the
codes for all the
chemicals in your
DNA it would fill
about 200 phone
books –
about 3 billion
letters
DNA Profiling
• 1987 – Forensic scientist first used DNA profiling
(genetic profiling) to solve a rape case
• The chemical structure of everyone's DNA is the same
- the only difference is the order of the base pairs and
every person has a different sequence.
How it works
• Only 0.1% of DNA differs from one person to the next.
These differences are used to generate a DNA profile
(from blood, bone, hair, or other body tissues)
• Enzymes are used to cut out specific sequences of
DNA.
• Sequences are arranged in order of length using
electrophoresis.
• They are labelled so that they show up when
photographed.
DNA fingerprint
Forensic comparison
Which sample matches
the blood stain?
Sample A
Sample B
Activity 1
Suppose the following are DNA samples of a couple who
plan on having children.
Give a possible DNA coding for two of their children.
Mum
Dad
1
2
Mum
Dad
1
2
Mum
Dad
1
2
Mum
Dad
1
2
Mum
Dad
1
2
Mum
Dad
1
2
Mum
Dad
1
2
Activity 2
Here are six different DNA tests. Suppose these DNA
tests are of a couple who are married and their
possible children.
Which of the kids belong to both parents?
Which of the kids belong to only the mother?
Which of the kids belong to only the father?
Which of the kids belong to neither parent?
Mum
Dad
1
2
3
4
Mum
Mum
Dad
Dad
1
1
2
2
3
3
4
4
Mum
Mum
Dad
Dad
1
1
2
2
3
3
4
4
Gene Ownership – A National
Database
• Britain has the largest DNA database of its citizens
in the world. It holds details of over 4 million
people – 5.2% of the UK population
• These include people who have been found guilty
of a crime as well as those suspected of a crime but
eventually cleared.
• Should there be a database?
Activity
• In pairs think of as many arguments in favour of
having a National database of DNA profiles and
arguments against.
Arguments in favour
• Each person's DNA is unique so, DNA evidence
collected from a crime scene can implicate or
eliminate a suspect (like fingerprints)
• It also can analyse unidentified remains through
comparisons with DNA from relatives
• When evidence from one crime scenes are
compared, there may be a link to the same
perpetrator, so helping to solve crimes
• If biological evidence from crime scenes is collected
and stored properly, forensically valuable DNA, that
may be decades old, could help solve an unsolved
crime
Arguments against
• It is possible for an innocent person’s DNA to be
planted at a crime scene
• An innocent person’s DNA may also be at a crime
scene even though they weren’t involved in the crime
• High cost to maintain and develop the database
• It invades our right to privacy.
• The data might get into the hands of commercial
companies such as insurance, loan and employers. If
it were used to identify that you had a genetic risk for
a serious disease, could you find yourself refused life
insurance, a loan, or even a job?
http://www.bbc.co.uk/learningzone/clips/dna-database-legal-or-illegal/8463.html
Question
Explain what is meant by ‘genetic profiling’,
including its uses and the advantages and
disadvantages associated with it.
(6 Marks)
Answer….
• Genetic profiling is where DNA is analysed to show a DNA
‘pattern’
• That is specific to an individual
• DNA is cut into smaller pieces and separated by electrophoresis
• Pattern is compared to other samples
• Uses (any two): Paternity, criminal cases, familial connections
• Advantages (any one): Matching parents to children, genetic
screening, providing evidence for criminal cases
• Disadvantages: (any one): Insurance rejections, not 100%
accurate, invasion of privacy, affect chances of finding a job,
Keywords
• Specific
• DNA Profiles,
• Electrophoresis,
• Genetics
• Criminal
• Paternity
• Analysed
DNA – True or False?
http://www.teachers.tv/videos/dna
The big picture: “How is DNA
profiling used in forensics?”
Learning outcomes
To explain how a DNA profile is
made.
To analyse the stages in the
polymerase chain reaction (PCR).
To evaluate the benefit of using
PCR when producing a DNA profile.
What is DNA profiling?
• Also known as genetic or DNA
fingerprinting.
• Relies on the fact that everyone’s DNA is
different (except identical twins).
• Compares introns (non-coding regions) and
exons (coding regions) within the DNA.
• Also compares repeated sequences within
introns (known as short tandem repeats,
STR’s for short).
DNA profiling
https://www.youtube.com/watch?v=ZxWXCT9w
VoI
An overview of the process…
How is a DNA profile made?
A multi-stage process:
1: DNA sample extracted
2: DNA is amplified using PCR
3: Fragments of different lengths are
produced by cutting up the DNA
4: The fragments are separated and
visualised using gel electrophoresis.
DNA Profiling
1
2
3
DNA profiling is a technique that allows an individual’s genes to
be visualised. This allows someone's genetic makeup to be
compared to known genes to see if they too have them.
This technique can be used to identify
genetic disorders in individuals or match
DNA samples to individuals.
We usually sample regions of DNA called
introns, once called Junk DNA, since it
varies between individuals (but not
twins). As opposed to exons (genes in
the normal sense). These repetitive
regions of ‘Junk DNA’ are now called
STRs: Short Tandem Repeats.
Alex Jeffreys’ first use of DNA Profiling: http://www.youtube.com/watch?v=Fz6p8EgJZ3w
Sources of DNA for Genetic Profiling
How do we get the STRs from the DNA sample?
You’ll need to macerate the sample and
add enzymes to break down the
cellular membranes.
We’ll need to cut out the sequences of DNA we wish to compare, the STRs.
Enzymes are perfect for this, specifically restriction enzymes (restriction
endonucleases) which cut at designated DNA sequences.
Samples must be compared against possible sources
Suspects
Profile
Blood
sample from
crime scene
Victims
profile
Is this man the father of the child?
Mother
Child
Man
Famous Cases
• Colin Pitchfork was
arrested (1987) for the rape
and murder of two girls in
1986. He was only caught,
and then sentenced, in
1988 after DNA evidence
came to light.
Famous Cases
• In 1994 O.J Simpson was
cleared of murder charges
which relied heavily on DNA
evidence.
• The new technology, and the
jury’s lack of confidence with
it, formed a major part of the
case.
• This was later parodied in
SouthPark as the
Chewbacca Defence – it has
become a technical term.
https://www.youtube.com/watch?v=clKi92j6eLE
Old and modern forms of
gene profile presentation.
The big picture: “How can DNA
profiling be used to solve a crime?”
Learning outcomes
To recap the key features of the
polymerase chain reaction.
To analyse the stages in gel
electrophoresis and southern
blotting.
To evaluate the usefulness of this
technique.
DNA profiling
• Once fragments of DNA have been produced
(either through PCR or the uses of restriction
enzymes) the fragments then need to be
visualised.
• This is achieved using gel electrophoresis and
southern blotting.
• Gel electrophoresis separates the fragments
according to size on a agarose gel.
• The gel is fragile so the fragments are then
transferred to a nylon membrane in a process
known as southern blotting.
DNA profiling
• Fragments are added to the wells of an
agarose gel.
• A current is passed through the gel, as DNA is
negatively charged the DNA moves from
negative to positive down the gel.
• Smaller fragments travel faster, so will move
further down the gel in the same amount of
time.
• Southern blotting then makes the banding
pattern more stable- the DNA is transferred
to a nylon/ nitrocellulose membrane to be
viewed.
Tasks
All: Draw a flow chart to explain how the
fragments are visualised. Words to include:
agarose gel, gel electrophoresis, negative
charge, DNA probe, southern blotting
Most: Explain what a DNA probe is, and why it
is used in this process
Some: Analyse what DNA profiling is used for,
and how infallible it is (likely to give correct
results)
Exam question
Peer assessment
PCR
The polymerase chain reaction is used to copy
a DNA sample many times.
A visualisation of the process:
https://www.youtube.com/watch?v=iQsu3Kz9
NYo
Tasks
ALL: Definitions for these key terms- DNA
primers, STR’s, denaturation, annealing, taq
polymerase
Most: Produce a flow chart to explain the
stages of PCR. You should include the
temperatures of each stage.
Some: Why would PCR be needed when making
a DNA profile/ in forensics?
Exam question
Apply your knowledge to the exam question on
PCR
Time allowed: 10 minutes
Self-assessment