DNA profiling

Satellite DNA

  • some sections of DNA can be repeated multiple times

    • Particularly in introns, telomeres, and centromeres

  • Minisatellites

    • 20-50bp section repeated from 50-100s of times

    • VNTR (variable number tandem repeat)

  • Microsatellites

    • 2-4 bases repeated 5-15 times

    • STRs (short tandem repeats)

Variation in satellites

  • Both types of repeat unit occur throughout the genome

    • The locus for a particular repeat is always the same but the number of repeats can vary in individuals

    • They are inherited from both parents so the pattern of satellite DNA is unique to each person

    • The more closely related you are the more similar the patterns will be

  • Different people can have different numbers of repeats at each loci

DNA profiling stages

  1. Extracting DNA

  2. Digesting the sample

  3. Separating fragments

  4. Hybridisation

  5. Viewing the sample

Extracting the DNA

  • DNA is extracted from a tissue sample

    • Eg blood

  • PCR (polymerase chain reaction) is carried out to increase the amount of DNA available

Digesting the sample

  • restriction endonuclease (enzymes that cut DNA at specific base sequences or recognition sites) are used to cut the DNA into smaller pieces

  • The restriction enzymes are choses so that the satellite repeating regions are left intact

Separating the fragments

  • the DNA fragments are separated according to size using electrophoresis and southern blotting

  • Electrophoresis

    • DNA molecules are loaded onto a gel

    • An electric current is passed through the gel

    • DNA molecules are negatively charged and so move towards the positive electrode

    • Small molecules move faster than larger molecules

  • Southern blotting

    • The DNA molecules in the gel are transferred to a nylon membrane

    • The gel is soaked into alkali to separate the double stranded DNA into single strands

    • Capillary action is used to transfer DNA to the membrane in the same positions as it was on the gel

    • The DNA is fixed in place by heating or exposing the membrane to UV light

Hybridisation

  • to identify the satellites present probes must be used to visualise the DNA

  • A probe is a short single stranded piece of DNA with the complementary sequence to the piece of DNA you want to find

  • Because the DNA on the membrane is single stranded they can bind together

  • The DNA probe has a radioactive or fluorescent label attached so can be viewed using an x-ray image or UV light respectively