Electrophoresis & PCR: Applications
DNA profiling
- DNA profiling (genetic fingerprinting) enables scientists to identify suspects of a crime and identify corpses
- Every person (apart from identical twins) has repeating, short, non-coding regions of DNA (20 to 50 bases long) that are unique to them
- These regions of DNA are called VNTRs (Variable Number Tandem Repeats)
- DNA profiling involves using gel electrophoresis to separate VNTR fragments according to length to create a pattern of bands that is unique to every individual, sometimes called the genetic fingerprint
- To create a DNA profile from the DNA being tested scientists complete the following in sequence:
- Obtain the DNA, which can be extracted from the root of a hair, a spot of blood, semen or saliva
- Increase the quantity of DNA by using PCR to produce large quantities of the required fragment of DNA from very small samples (even just one molecule of DNA or RNA).
- Use restriction endonucleases to cut the amplified DNA molecules into fragments
- Separate the fragments using gel electrophoresis
- Add radioactive or fluorescent probes that are complementary and therefore bind to specific DNA sequences
- X-ray images are produced or UV light is used to produce images of the fluorescent labels glowing
- These images contain patterns of bars (the DNA profile) which are then analysed and compared
Use of DNA profiling in Paternity Investigations
- Every person inherits their DNA VNTRs from both their mother and their father
- A man may sometimes deny being the father of a child to evade parenting responsibilities
- A woman may not know for sure which of her recent sexual partners is the father of a child
- A child may wish to know definitively who his/her father is to be aware of possible inherited illnesses that might affect him/her in future
- DNA profiles of the mother and child are compared, along with the profile of the alleged father (all three are needed)
- Patterns of bands are compared on all three genetic profiles
- Any band that appears in the child's profile must show in either the mother's or father's profiles; if not, the alleged true father is a different man
Worked example
Who’s the Father? – Use the DNA profiles of all 6 people shown to work out who the child’s father is
DNA profiles of a child, a mother and 4 possible fathers
Remember, any band showing in the child's profile must be present in the mother OR father's profile, OR both. If not, that man is not the child's father.
Answer:
Step 1: Look at the child's first DNA band (labelled 1)
The mother possesses this same band, so the child could have inherited that DNA from its mother. It is therefore needless to look at whether any of the men possess that band
Step 2: Look at the child's second DNA band (labelled 2)
The mother does not possess this band, so the child must have inherited it from its father. Only men B and D possess this band, so men A and C are eliminated
Step 3: Look at the child's third DNA band (labelled 3)
As with band 1, the mother possesses this same band, so the child could have inherited that DNA from its mother. It is therefore needless to look at whether any of the men possess that band
Step 4: Look at the child's fourth DNA band (labelled 4)
The mother does not possess this band, so the child must have inherited it from its father. Only men A, B and C possess this band, but A and C have already been eliminated
Step 5: Conclude that B is the father
Step 6: Look for supporting evidence from band 6
The mother does not possess this band, and the only man who possesses it is B. This reinforces the conclusion that Man B is the child's father
Use of DNA profiling in Forensic Investigations
- DNA profiling has been used by forensic scientists to identify suspects of crimes
- Samples of body cells or fluids (e.g. blood, saliva, hair, semen) are taken from the crime scene or victims body (e.g. rape victims)
- DNA is removed and profiled
- The profile is compared to samples from the suspect (or criminal DNA database), victim and people with no connection to the crime (control samples)
- Care must be taken to avoid contamination of the samples
- DNA profiling can also be used in forensics to identify bodies or body parts that are unidentifiable (e.g. too badly decomposed or parts remaining after a severe fire)
- DNA profiling from a crime scene can also eliminate innocent people whose DNA may happen to appear there
Using DNA profiling in criminal investigations. Suspect 3 has the most fragments in common with the crime scene DNA so it is likely that Suspect 3 is the culprit.
Examiner Tip
In the exam, you will be expected to interpret the results of gel electrophoresis experiments used to separate DNA fragments. For example, you will be given a few different genetic fingerprints and will have to match the victim to the crime or determine the parents of children. In these questions, you need to look for the most bands in common or a combination of parents' fingerprints that covers all the child's bands.
NOS: Reliability is enhanced by increasing the number of measurements in an experiment or test
- Increasing the reliability of scientific conclusions is a very important aspect of all experimentation, but this is particularly important when evidence is being collected for use in a court of law
- It is almost impossible to be 100% certain that any evidence provided is absolutely correct
- In DNA profiling, reliability can be increased by increasing the number of VNTR markers being observed and compared
- The more VNTRs used, the lower the chance of a false match
