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FORENSIC SCIENCE. The purpose of crime scene investigation is: to help establish what happened (crime scene reconstruction), and to identify the responsible person. Collect evidence such as: Blood, Seminal, Saliva Stains, Skin Hair, Fibers and Threads Glass, Paint, Flammable Liquids
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FORENSIC SCIENCE • The purpose of crime scene investigation is: • to help establish what happened (crime scene reconstruction), and • to identify the responsible person. • Collect evidence such as: • Blood, Seminal, Saliva Stains, Skin • Hair, Fibers and Threads • Glass, Paint, Flammable Liquids • Firearms Evidence • Tool Marks • Controlled Substances and Medicinal Preparations • Questioned Documents • Latent Fingerprints
Reading DNA“bar codes” or “fingerprints” • The knowledge of DNA is very valuable because it can: • identify people, sex of an unborn baby, parents of a child, risk for genetically-linked diseases, etc. • genetically engineer crops that are resistant to insects and harsh environments or climates • save information about endangered wildlife species • genetically engineer stem cells to cure diseases • DNA fingerprinting in the field of forensic science can determine whether a suspect is innocent or guilty in a crime.
150 decimal Step 1: Prepare a DNA Sample • Wearing gloves, DNA is extracted from samples with blood, saliva, semen and hair roots. Keep DNA on ice!!! • CHELEX (a chelating agent that extracts DNA, for example from the follicle of an eyebrow hair, and removes contaminants) may be added to the sample. • Restriction enzymes (viral or bacterial digestive enzymes) are used to cut DNA in specific spots into precise different sized smaller pieces called restriction fragments.
mineral oil (prevents evaporation) DNA supernatant chelex (sediments) Step 2: Amplify DNA Sample • Using a PCR (polymerase chain reaction) machine or thermal sequencing vector cycler, Taq polymerase, primers, deoxynucleotides, dideoxynucleotides and buffer are added to the DNA sample. • This copies a DNA sequence that may be less than 1/million of a total DNA sample (size of a gene or even smaller). • http://www.dnalc.org/shockwave/cycseq.html • http://www.dnalc.org/shockwave/pcranwhole.html
DNA Fingerprinting:http://www.biotech.iastate.edu/publications/ppt_presentations/html/Fingerprinting/default.html • A dye is added to the sample and put into one well of an agarose electrophoresis gel which acts like a molecular sieve. • Since DNA is negatively charged due to the phosphate molecules, thus the smaller/shorter fragments will move faster to the positive terminal when an electric current is applied. • The DNA will be separated based on the size of the fragment.
Apparatus: Transfer Buffer Sponge Agarose Gel Nylon Membrane or Nitrocellulose Filter Paper Paper Towels Weight Double-stranded DNA fragments are treated with an alkaline solution which separates the strands into single-stranded DNA which are transferred onto a nylon membrane (nitrocellulose filter paper). The nylon membrane is then incubated with a radioactive single-strand DNA probe which binds to the complementary parts of the DNA on the nylon membrane. The nylon membrane is then washed to remove excess probe and then sandwiched to an x-ray film. The probe attached to the DNA will emit radioactivity which shows up as dark bands on the x-ray film. Additional probes may be added to detect other DNA parts. Southern Blot:http://www.dnalc.org/shockwave/southan.html
Limits: • Using certain rare VNTRs to create the VNTR pattern increases the probability that the two DNA samples will appear to match, even though they did not actually come from the same person. • VNTRs are results of genetic inheritance which are not distributed evenly across all of human population. In the field of population genetics, there is controversy that identifying people through genetic anomalies along racial lines comes alarmingly close to the eugenics (selective breeding of humans to improve species), ethnic purification, and racial discrimination. • Errors in the hybridization and probing process must can occur. Only until recently, the standards for determining DNA fingerprinting matches and for laboratory security and accuracy, have been stringently and universally codified due public outcry.
Crime Scene Investigation Laboratory: • Sample A was from the crime scene. • Sample B was from Suspect #1. • Sample C was from Suspect #2. • Sample D was from Suspect #3. • Who committed the murder?
