Post on 02-Jan-2016
DNA profiling-
Key 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
• 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.
DNA Profiling
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 1Suppose 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
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?
Activity 2
Mum Dad 1 2 3 4
Mum Dad 1 2 3 4
Mum Dad 1 2 3 4
Mum Dad 1 2 3 4
Mum Dad 1 2 3 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
The big picture: “How is DNA profiling used in forensics?”
Learning outcomesTo 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=ZxWXCT9wVoI
An overview of the process…
How is a DNA profile made?A multi-stage process:
1: DNA sample extracted2: DNA is amplified using PCR3: Fragments of different lengths are produced by cutting up the DNA4: The fragments are separated and visualised using gel electrophoresis.
DNA Profiling
1
2
3
Alex Jeffreys’ first use of DNA Profiling: http://www.youtube.com/watch?v=Fz6p8EgJZ3w
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.
Sources of DNA for Genetic Profiling
How do we get the STRs from the DNA sample?
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.
You’ll need to macerate the sample and add enzymes to break down the cellular membranes.
Suspects Profile
Blood sample from crime scene
Victims profile
Samples must be compared against possible sources
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 outcomesTo 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=iQsu3Kz9NYo
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