AQA A Level Biology

Revision Notes

8.4.12 Genetic Fingerprinting

Genetic Fingerprinting

  • DNA can be collected from almost anywhere on the body, e.g. the root of a hair or saliva from a cup. After collection DNA must be prepared for gel electrophoresis so that the DNA can be sequenced or analysed for genetic profiling (fingerprinting)
    • Fingerprinting can be used to determine genetic relationships and the genetic variability within a population
  • To prepare the fragments scientists must first increase (amplify) the number of DNA molecules by the polymerase chain reaction (PCR). Then restriction endonucleases (enzymes) are used to cut the DNA into fragments
  • Different restriction enzymes cut the DNA at different base sequences. Therefore scientists use enzymes that will cut close to the variable number tandem repeat (VNTR) regions
  • Variable number tandem repeats (VNTRs) are regions found in the non-coding part of DNA. They contain variable numbers of repeated DNA sequences and are known to vary between different people (except for identical twins). These VNTRs may be referred to as ‘satellite’ or ‘microsatellite’ DNA

DNA separation by gel electrophoresis

  • Gel electrophoresis is a technique used widely in the analysis of DNA, RNA and proteins. During electrophoresis the molecules are separated according to their size / mass and their net (overall) charge
  • The separation occurs because:
    • Of the electrical charge molecules carry – positively charged molecules will move towards the cathode (negative pole) whereas negatively charged molecules will move towards the anode (positive pole) eg. DNA is negatively charged due to the phosphate groups and thus when placed in an electric field the molecules move towards the anode
    • Different sized molecules move through the gel (agarose for DNA and polyacrylamide – PAG for proteins) at different rates. The tiny pores in the gel result in smaller molecules moving quickly, whereas larger molecules move slowly
    • Of the type of gel – different gels have different sized pores which affects the speed the molecules can move through them

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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.

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