19.2.6

DNA Profiling & Forensics

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DNA Profiling

DNA profiling is a technique for analysing samples of DNA and has multiple applications from police forensics to screening for heritable disease. DNA ‘profiles’ are specific DNA patterns that can be used to identify people, families and diseases.

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DNA contains polymorphisms

  • Humans share 99.9% of DNA with one another.
  • Certain regions of our genomes do vary greatly, these varying regions are known as polymorphisms. Polymorphic DNA can be used to distinguish between target individuals and groups.
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Obtaining DNA for analysis

  • DNA samples are obtained from hairs, skin, semen, saliva or any other sample that contains body cells.
  • DNA is extracted from the nucleus of the cells and isolated from other cellular matter using chemicals such as detergents.
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Polymerase chain reaction

  • DNA is replicated using polymerisation chain reaction (PCR) to increase the sample size:
    • The DNA is heated to 96oC to denature the strands and provide single-stranded templates for replication.
    • The reaction is cooled to 60oC to allow primers (short lengths of DNA to which free nucleotides can attach) to anneal to the single strands of DNA.
    • The reaction is heated to 72oC so that Taq polymerase (the DNA polymerase enzyme used) can work at optimum conditions and rapidly extend the nucleotide chains from the primers.
  • This process is repeated 25-35 times and the amount of DNA increases exponentially with each repeat.
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Gel electrophoresis

  • Gel electrophoresis is a technique for separating the DNA fragments out according to size (number of base pairs).
  • The DNA is loaded into wells at one end of a slab of agarose gel and an electric current is passed through.
  • The negatively charged DNA moves through the gel towards the positive electrode, with the smaller fragments moving faster and further.
  • The DNA ends up arranged in bands, with similar length strands grouping together.
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Applications of DNA profiling

  • Once sequenced, and the exact series of nucleotides in the sample has been established, forensic scientists can match the sample up with samples of DNA from known sources.
  • DNA profiling is used at crime scenes to identify potential criminals, to identify whether somebody is at risk of a genetic disease or simply to find out the degree of relatedness between people.

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2Biological Molecules

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4Cell Membranes & Transport

5The Mitotic Cell Cycle

6Nucleic Acids & Protein Synthesis

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17Selection & Evolution (A2 Only)

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19Genetic Technology (A2 Only)

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