OCR A Level Biology

Revision Notes

6.3.7 DNA Profiling

Principles of DNA Profiling

  • DNA profiling (genetic fingerprinting) enables scientists to identify suspects for a crime and identify corpses because every person (apart from identical twins) has repeating short non-coding regions of DNA (20 to 50 bases) that are unique to them, they are called variable number tandem repeats (VNTRs)
  • The number of VNTR regions are inherited from your biological parents
    • The more closely related you are to a person the more likely the repeats have similar patterns
  • The length of the VNTR regions are unique to each individual (apart from identical twins)
  • When DNA testing occurs in forensic medicine and criminal investigations the image of these repeats in the DNA (indicated by a pattern of bars) creates a DNA profile or fingerprint. The profile is analysed to allow conclusions to be made (eg. who the suspects are)
  • To create a DNA profile from the DNA being tested scientists complete the following in sequence:
    1. Obtain the DNA, which can be extracted from the root of a hair, a spot of blood or semen or saliva
    2. 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).
    3. Use restriction endonucleases (different restriction endonucleases cut close to different VNTR sequences) to cut the amplified DNA molecules into fragments
    4. Separate the fragments using gel electrophoresis
    5. Add radioactive or fluorescent probes that are complementary and therefore bind to specific VNTR regions
    6. X-ray images are produced or UV light is used to produce images of the fluorescent labels glowing.
    7. These images contain patterns of bars (the DNA profile) which are then analysed

Gel electrophoresis (1), downloadable AS & A Level Biology revision notesGel electrophoresis (2), downloadable AS & A Level Biology revision notesGel electrophoresis (3), downloadable AS & A Level Biology revision notesGel electrophoresis (4), downloadable AS & A Level Biology revision notesGel electrophoresis (5), downloadable AS & A Level Biology revision notes

The separation of DNA fragments using gel electrophoresis. Gel electrophoresis can be used in DNA profiling where scientists separate the VNTRs (as these are unique to every person except identical twins)

Exam 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.

Uses of DNA Profiling

Forensic medicine / criminal investigations

  • DNA profiling has been used by forensic scientists to identify suspects of crimes
    • Samples of body cells or fluids (eg. blood, saliva, hair, semen) are taken from the crime scene or victims body (eg. 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 (eg. too badly decomposed or parts remaining after a bomb blast)

Gene technology_ Forensics, downloadable AS & A Level Biology revision notes

Using DNA profiling in criminal investigations. Suspect 3 has the most fragments in common with the crime scene DNA so it is likely that they are the culprit.

Other applications

  • DNA profiling (along with Next Generation DNA sequencing) can be used to identify individuals that are at risk of developing particular diseases, as research shows that certain VNTR sequences are associated with an increased incidence of particular diseases eg. cancers and heart disease
  • DNA profiling can be used to determine familial relationships for paternity cases (to suggest who the father is) or immigration cases (to determine if the family are related)
  • It can also be used in species conservation to help scientists with captive breeding programmes to reduce chances of inbreeding
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