Dna Evidence


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DNA Evidence

Forensic molecular Bioogy                                                                                    

DNA evidence is crucial for determining innocence or guilt in a crime, its collection at a crime scene must be performed to scrupulously high standards.

 Human DNA sources


*Taken direct from suspects or in mass screens for elimination or from a known source.

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Dna Sources




*Red blood cells anucleate .*All white blood cells contain nuclear DNA.*Invasive  


   *Sloughed off epithelial cells from the inside of the buccalcavity.                       * Retrieved with swab *Non-invasive     

 Hair roots

 *Many follicle cells when pulled  *Shed hairs less useful                                                  *Good reference for missing or unidentified person-sample from brush or home environment

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Dna Collection

  DNA evidence collection                                                                                

Prevention of contamination is key,particular care must be taken : 

  • From collecting officers
  • Cross-contamination
  • Full protective SOCO suits essential
  • Elimination of officers DNA profiles
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Presumptive testing

 Presumptive testing for evidence                                                                       



Sloughed cells

 •  Wherever fingerprints may be found  •  Wherever cells can collect 

*   Under fingernails; on clothing and footwear; plasters; toothbrushes; envelopes;


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Preservation of evidence

 Preserving                  evidence                                                                                                                                                                          

Prevention of microbial degradation

Dry and at low temperature

 * 20°C to -70°C for long term

* Dry on proprietary cards…   

-Proprietary cards  such as FTA cards  

* FTA Cards contain chemicals that lyse cells, denature proteins and protect nucleic acids from nucleases, oxidation and UV damage.  

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Quantifying DNA


          DNA is separated from other cellular components

          The amount extracted is measured

        To see if sufficient is present for typing

        To determine how much should be used

        Quality can also be determined



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Testing Dna Extract


         Agarose gels         Quantity and how intact?

         UV spectrophotometry     260nm; 260/280 ratio; 260/230 ratio

       Pico green fluorescence spectrophotometry     Detects down to 25pg/ml DNA

         Real time PCR


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