CIE A Level Biology (9700) 2019-2021

Revision Notes

19.2.5 Use of Gene Technology in Forensic Science

Forensics Medicine & Criminal Investigations

  • Polymerase chain reaction (PCR) is a common molecular biology technique used in most applications of gene technology, for example, DNA profiling (eg. identification of criminals, determining paternity and species conservation) or genetic engineering
  • It is used to produce large quantities of specific fragments of DNA or RNA from very small quantities (even just one molecule of DNA or RNA). By using PCR scientists can have billions of identical copies of the DNA or RNA sample within a few hours
  • 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)
  • These repeats occur at the same position on chromosomes but the number of repeats varies between individuals (apart from identical twins) because the repeats are inherited from both parents. The more closely related you are to a person the more likely the repeats have similar patterns
  • 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:
    • Obtain the DNA, which can be extracted from the root of a hair, a spot of blood or semen or saliva
    • Increase the quantity of DNA by using PCR
    • Use restriction endonucleases (different restriction endonucleases cut close to different VNTR sequences) 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 VNTR regions
    • 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 is then analysed

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, salvia, 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)

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

Author: Catherine

Cate has over 20 years’ experience teaching Biology to IGCSE, IB and A-level students in seven different countries across Asia, Europe, North America and the Middle East. This has given her a fine appreciation of different cultures, places and teaching methods. Cate has a keen interest in producing Biology revision resources that will help students engage with the subject.
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