• Question: How do you analyze someones DNA?

    Asked by dannyboy1997 to Jamie, Jodie, Kat, Mark, Niamh on 23 Mar 2011 in Categories: .
    • Photo: Mark Hill

      Mark Hill answered on 21 Mar 2011:

      Hello Dannyboy1997,

      I am sorry, but this is not my field of expertise. I think that it falls more within the expertise of Kat, Niamh and Jodie. However, I will try and help.

      I understand that DNA samples are compared with known held records, from people convicted of a crime. The DNA is made up of long chain combinations of paired molecules of one of four different nucleotide acids – Adenine, Guanine, Cytosine and Thymine. The unique way in which they pair up and the orders of those pairs give us an opportunity to compare different samples.

      Initially, just after DNA evidence entered forensic science, single locus probe (SLP) comparison, at one point in a DNA string. This progressed into short tandem repeat (STR) comparison, which uses several preselected sites on a DNA string, for comparison.

      The result of a comparison test is never conclusive, but is expressed as a probability of the test sample NOT being that of the suspect – usually in being 1 in 3 billion chance of the sample not being that of the suspect.

      I hope that this helps answer your question.

    • Photo: Jamie Pringle

      Jamie Pringle answered on 21 Mar 2011:

      Hello dannyboy1997,

      I believe you take a blood sample and run it thorugh some sort of machine, but my colleagues are the experts so I will leave them to give you a comprehensive answer, my apologies as it is not my area.

    • Photo: Niamh Nic Daeid

      Niamh Nic Daeid answered on 21 Mar 2011:

      HI danny boy

      DNA analysis is not that complex a process, Firstly you have to extract the DNA from cellular material. DNA is normally wrapped within cellular proteins within the cell nucleus so it has to be removed from these. Next the DNA has to be amplified and this is done using a technique called polymerase chain reaction or PCR. Essentially this means breaking the double helix structure of the DNA (called denaturing the DNA). Each individual DNA strand is then used as a template to copy the DNA sequence in the strand. Special chemicals called primers are added so that only targeted portions of the DNA called short tandem repeats (STR’s) are copied. In the system of DNA profiling used in the UK we pick 10 regions of DNA or 10 STR’s) to analyse.

      Once the DNA has been amplified it is analysed using a process called capillary electrophoresis which basically passes and electric current through the DNA sample and separates the STR fragments according to their size so you can compare them from person to person.

    • Photo: Katherine Davies

      Katherine Davies answered on 23 Mar 2011:

      Hi

      Firstly you need a sample; the usual ones are blood, urine or cheek cell.
      1)The cells which contain the DNA are then exploded open using strong SDS (a component of shampoo) and salt solutions to release the DNA.
      2) The cell debris, i.e. membranes, sugars and proteins are removed by centrifuging/spinning them through a silica filter, upon which the DNA gets stuck.
      3) The DNA can then be washed off the membrane with ordinary water

      Once you have the DNA sample, you can then amplify specific regions called STRs (single tandem repeats) which are unique between people using a method called PCR (polymerase chain reaction). The STRs are analysed using fluroescence and the ‘peaks’ examined to determine who that person is (if their DNA is in the database, it can be matched up). From the DNA alone, we can tell a person’s sex and likely race, if we dont have a comparative sample.

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