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1、DNA Fingerprinting and Forensic AnalysisWhat is a DNA fingerprint?Every cell of an individual carries a copy of the DNAa cell collected from a persons skin or hair follicle contains the same DNA as from that persons heart tissue or white blood cellsOrder of base pairs in the DNA of every individual
2、is different except identical twinsHow do we distinguish one persons DNA from another?Dont need to sequence all 3 billion base pairs of a persons DNA to distinguish it from another persons DNAIntron regions of DNA (junk DNA) contain sequences that are 20-100 bp in length that are repeated at differe
3、nt locations (loci) along the chromosome. CGGCTACGGCTACGGCTA (repeated 3 times at this location; at another location, it may be repeated 9 times)These sequences are called Short Tandem Repeats (STRs) or VNTRsSTRsEach person has some STRs that were inherited from mother and some from fatherNo person
4、has STRs that are identical to those of either parentThe number of repeats at each loci on chromosome is highly variable in the population, ranging from 4 to 40.The length of the DNA after cutting the chromosome with a restriction enzyme, and its position after electrophoresis will depend on the exa
5、ct number of repeats at the locusThe uniqueness of an individuals STRs provides the scientific marker of identity known as a DNA fingerprint.In the United States the FBI has standardized a set of 13 STR assays (13 different locations on the chromosomes) for DNA typing, and has organized the CODIS da
6、tabase for forensic identification in criminal cases. The United States maintains the largest DNA database in the world: The Combined DNA Index System, with over 60 million records as of 2007. Preparation of a DNA fingerprintStep 1Specimen collectionblood, semen, etceasy to contaminate a DNA sample
7、with DNA from other sources (bacteria, DNA of person collecting sample)DNA is not stable for very long-it degradessunlightheatmoistureDNA fingerprinting is a comparative process:DNA from crime scene is compared with DNA of a suspectSo minimum of two samples must be preparedStep 2DNA extractionstanda
8、rdized methods have been developedneed to separate DNA from other cell material and debris from crime scene.Step 3PCR using primers targeting STRs at different lociPCR amplify STRs using target sites on chromosomeStep 3 PCR amplification of DNA1 strandof DNAHeat todenaturedouble-strandedDNADesign pr
9、imers that anneal to STR locusAmplify all the regions of the chromosomewhere the STRs exist.STR locusSTR locusPCR allows you to make millions of copies of the STR region from a single copy of DNA you recovered from crime scene.Since the # of times sequence is repeated is different for each person, f
10、ragment size will be different. This is done for 13 different STR sequences at this one locusDifferences occur among individuals at each of the 13 loci on the chromosome where the STRs occurThis allows for a lot of variationRestriction Fragment Length PolymorphismG-G-C-C-X-X-X-G-G-C-C-X-X. G-G-G-C-C
11、-X-X-G-G-C-C-X-X.STRC-C-X-X-X-G-GC-C-X-X-G-GPCR amplifySTR regionSTRwellwellGelelectrophoresisPerson AForensic sampleFor 1 STR sequence at 1 locusIf you do this for 13 different repeat sequences at 13 different loci on the chromosome, each person produces a different band pattern when the fragments are separated by gel electrophoresisDifferent STRs atother lociSTR1STR2STR3Do any of the individuals compare with forensic sample?
