Forensic DNA extraction

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  • DNA Extraction
    • The process of separating DNA from the rest of the cells
      • To release the DNA contained within the cells of the recovered biological sample and to isolate it so that it can be subject to further analysis
    • Aims
      • Quality
        • To maximise the yield DNA from a sample and to permit a full DNA profile
        • DNA is not usually in pristine condition
      • Quantity
        • To extract DNA that is pure enough for subsequent analysis. Once DNA is extracted, quantifying the DNA is important for analysis
    • Hard Tissues
      • If the time between death and recovery is short, then muscle tissue will provide a rich source of DNA which can be extracted
        • If soft tissues are decomposed, they will not provide any DNA suitable for analysis
    • Hair Shafts
      • Hair shafts that have been pulled out often possess a root that is rich in cellular materials and DNA.
        • In many cases it is only possible to obtain a DNA profile from mitochondrial DNA
    • Semen
      • DNA is found in the head of spermatozoa
      • Vasectomised males has no spermatozoa
      • DNA is effienctly digested with restriction enzymes and amplified by PCR
    • Quantification of DNA
      • Yield gels
        • A quick and easy method for assessing both quality and quantity of extracted DNA.
      • UV absorbance
        • DNA absorbs light, and this can be used to estimate the amount of DNA in an extract by measuring a range of wavelengths.
      • Hybridization
        • A molecular biology technique that measures the degree of genetic similarity between pools of DNA sequences
          • Extracted DNA is applied to a positively charged nylon membrane using slots or dot blood process. The membrane is the exposed probe, that is specific to human DNA.
      • Real-time PCR
        • Newly made DNA is tagged with florescent dye and the levels of florescence can be measured after every PCR cycle.
    • Methods of extraction
      • Chelex 100 Beads
        • Leaves DNA in solution
      • Dependant on: Sample type, amount of sample, speed, success rates, cost, staff experience
      • Silica-based DNA extraction
        • DNA binds to silica beads
      • Phenol-chloroform based DNA extraction
        • Use of FTA papers
        • Used for challenging samples
    • Stages of DNA extraction
      • 1) Cell Lysis
      • 2) Denaturation of the proteins binding the DNA molecule
      • 3) Seperation of DNA from the denaturated protein and other components of the cells


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