The Process Used to Analyse DNA

Describe the process used to analyse DNA and account for its use in: identifying relationships between people, identifying individuals

DNA analysis is generally done by:

  • Collection, Storage, Extraction and Fragmentation
    • Samples containing DNA such as tissue, blood, semen and saliva are collected at the crime scenes.
      Note: A chain of custody is established to prevent contamination
    • Retrieval of DNA (in hair, saliva, etc) from relatives of involved person for comparison purposes
    • Samples are purified to remove dirt and other extraneous materials
    • Restriction enzymes are used to cut the long DNA strands into segments in which the cuts are made at loci, the part where the tandem repeated sections of the DNA occur
  • DNA amplification
    • Polymerase chain reaction (PCR) is used to amplify or replicate many copies of the DNA segments to obtain relatively high DNA sample concentration for better analysis.
  • Characterization using Gel electrophoresis.
    • These DNA fragments are separated by electrophoresis using polyacrylamide gel.
      • DNA molecules are negatively charged overall so they move towards the positive terminal at different speeds depending on their chain length.
      • Fragments with less tandem repeats move faster, thus separating the DNA fragments into narrow bands.
      • A standard sample of polynucleotides of known chain length is used to compare the position of the DNA fragments in the sample to determine their length.
      • The gel is irradiated with UV light to differentiate which bands come from which primer pairs
      • Each locus on the DNA will produce two bands as there are two fragments, one from the chromosome of each parent.

DNA Analysis in identifying relationships between people:

  • DNA analysis allows the identification of paternity relationships wherein the genetic fingerprint of a child will show bands from both parents in the gel electrophoresis profile as indication of their paternal relationship.

DNA Analysis in identifying individuals:

  • Allows the identification of convicts and/or exclude suspects such that a victim’s sample blood, for example, can be matched to the sample obtained from cloth of the suspect.