2 What is a DNA fingerprint? Every cell of an individual carries a copy of the DNAa cell collected from a person’s 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 is different except identical twins
3 How do we distinguish one person’s DNA from another? Don’t need to sequence all 3 billion base pairs of a person’s DNA to distinguish it from another person’s DNAIntron regions of DNA (junk DNA) contain sequences that are bp in length that are repeated at different 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 VNTRs
4 STRsEach person has some STRs that were inherited from mother and some from fatherNo person 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 exact number of repeats at the locus
5 The uniqueness of an individual’s 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 database 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.
6 Preparation of a DNA fingerprint Step 1 Specimen collectionblood, semen, etceasy to contaminate a DNA sample with DNA from other sources (bacteria, DNA of person collecting sample)DNA is not stable for very long-it degradessunlightheatmoisture
7 Step 2 DNA fingerprinting is a comparative process: DNA extraction DNA from crime scene is compared with DNA of a suspectSo minimum of two samples must be preparedStep 2DNA extractionstandardized methods have been developedneed to separate DNA from other cell material and debris from crime scene.
8 Step 3 PCR using primers targeting STRs at different loci PCR amplify STRs using target sites on chromosome
9 Step 3 PCR amplification of DNA 1 strandof DNAHeat todenaturedouble-strandedDNASTR locusDesign primers that anneal to STR locusAmplify all the regions of the chromosomewhere the STRs exist.STR locus
10 PCR allows you to make millions of copies of the STR region from a single copy of DNA you recovered from crime scene.
11 Since the # of times sequence is repeated is different for each person, fragment 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 variation
12 Restriction Fragment Length Polymorphism For 1 STR sequence at 1 locusG-G-C-C-X-X-X-G-G-C-C-X-X..G-G-G-C-C-X-X-G-G-C-C-X-X…..STRC-C-X-X-X-G-GC-C-X-X-G-GPCR amplifySTR regionwellGelelectrophoresisPerson AForensic sample
13 If 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 electrophoresis
14 Different STRs at other loci Do any of the individuals compare with forensic sample?