Introduction to forensic DNA

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  • Forensic DNA Evidence
    • DNA Structure
      • DNA exists as a double-stranded molecule, that adopts a helical arrangement
        • First described by Watson and Crick in 1953
        • DNA has a sugar phosphate backbone
          • Consists of 5-carbon deoxyribose sugars and phosphate groups
            • Linked by a phosphodieser bond
        • There are 4 different nitrogenous bases: A, C, G and T
          • A - T and G - C
            • Joined by hydrogen bonds
      • DNA is made from genes
      • Chromosomes
        • 46 Chromosomes
          • 23 pairs
        • Genes are present on the chromosomes
        • Sex cells
          • Women = **
          • Men = XY
    • DNA Replication
      • DNA duplicates itself during cell division
        • This process is formed at the beginning of every cell division so that when a cell divides each daughter cell will inherit an identical copy of DNA
      • Semi-conservative
      • Semi discontinuous
      • Steps: 1) DNA unwinds the parental double helix. 2) DNA polymerase will add the free DNA nucleotide using complementary base pairing (A-T and C-G) to the end of the primer to allow a new DNA strand to form.         3) The two new strands twist to form a double helix - each identical to the original strand.
      • Mitosis
    • Genetics
      • The aim of genetic analysis for forensic casework is to produce a DNA profile that is highly discriminating
        • One that is potentially unique to each individual
        • Humans have 99.9% of similar DNA to one another
      • It is now possible to analyse any region within the 3.2 billion bases that make up the human genome
      • Genetic differenences
        • Phenotype
          • The observable characteristics of an individual
        • Genotype
          • The genetic constitution of an individual
      • Human gnome - The total DNA content found within the nucleus of a human cell
      • Mendelian Inheritance
        • A type of biological inheritance
        • Law of segregation
          • Each trait from the parents genes (alleles) split and one gene passes from each parent to the offspring.
        • Law of independent assortment
          • Different genes (alleles) are passed onto the offspring independently
          • Exception of genetic linkage
            • When genes are very close to each other on the same chromosomesmeaning that they move together in recombination
      • Punnet square - used to show the combination of alleles in a gene
    • Why is DNA important in forensics?
      • Each persons genome contains a large amount of DNA that is potential target for DNA profiling
        • Within an individuals DNA, there are detectable patterns that are repeated a number of times showing different characteristics
      • Successful DNA analysis produces a profile that has potential to identify a possible source of the evidence left at a crime scene
      • DNA is a relatively robust molecule that will usually survive under a range (but not all) environmental conditions
      • DNA can be isolated from a wide range of biological samples
      • Cheap techniques
      • DNA profiles are assembled into databases, which can be searched against by samples found at a crime scene
    • DNA Profiling
      • The examination of an individuals DNA to produce a unique, unalterable pattern that will be a characteristic of any tissue of body fluid that originates from him/her.
      • Examples: Colin Pitchfork, OJ Simpson and Leanne Teirnan
      • Short Tandem Repeats (STRs)
        • Current technique for DNA profiling uses STRs
        • Polymorphisms
        • Non-coding DNA that contains repeats of the same nucleotide sequences
        • Found at different places or genetic loci in a persons DNA
    • Cell division
      • Meiosis
        • Produces 4 non-identical haploid (sex) cells.
          • 1 replication but 2 successive divisions
      • Mitosis
        • Produces two identical diploid cells for growth and repair
          • No reshuffling of genetic material
    • Polymerase Chain Reaction (PCR)
      • A technique for amplifying DNA
        • Uses  complimentary primers for specific target DNA sequences
      • 1) Denaturation (94 degrees) Breaks hydrogen bonds in between bases to create 2 single stranded molecules
        • 2) Annealing (50-65 degrees) Primers anneal to the template DNA and bind to complementary sequences
          • 3) Extension (70-80 degrees) Taq polymerase is added and synthesises to a new DNA strand to the complementary template DNA
      • PCR errors
        • Taq polymerase does not have the ability to 'proof read' resulting in errors in amplification
          • The greater the amplification, the more likely it is that such errors will occur.
    • Electrophoresis
      • During DNA analysis the individual fragments of DNA can be separated using electrophoresis to produce the distinct DNA fingerprint
        • A method of separating molecules by their size through application of an electric field, causing molecules to migrate at a rate and distance dependant on their size.
          • An electric current is added
            • Negatively charged DNA fragments begin moving through the gel towards the positively charged anode
              • The gel acts as a molecular sieve, allowing smaller molecules to travel faster than larger fragments
          • Samples are put into wells at the top of the gel using pipettes
          • Fluorescent  probes or dyes can be added from visualisation
          • Allows fragments to be measured

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