DNA Fingerprinting and Forensic Analysis

What is a DNA fingerprint? Every cell of an individual carries a copy of the DNA a cell collected from a person’s skin or hair follicle contains the same DNA as from that persons heart tissue or white blood cells Order of base pairs in the DNA of every individual is different except identical twins

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 DNA Intron regions of DNA (junk DNA) contain sequences that are 20-100 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

STRs Each person has some STRs that were inherited from mother and some from father No person has STRs that are identical to those of either parent The 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

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.

Preparation of a DNA fingerprint Step 1 Specimen collection blood, semen, etc easy to contaminate a DNA sample with DNA from other sources (bacteria, DNA of person collecting sample) DNA is not stable for very long-it degrades sunlight heat moisture

Step 2 DNA fingerprinting is a comparative process: DNA extraction DNA from crime scene is compared with DNA of a suspect So minimum of two samples must be prepared Step 2 DNA extraction standardized methods have been developed need to separate DNA from other cell material and debris from crime scene.

Step 3 PCR using primers targeting STRs at different loci PCR amplify STRs using target sites on chromosome

Step 3 PCR amplification of DNA 1 strand of DNA Heat to denature double- stranded DNA STR locus Design primers that anneal to STR locus Amplify all the regions of the chromosome where the STRs exist. STR locus

PCR 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, fragment size will be different. This is done for 13 different STR sequences at this one locus Differences occur among individuals at each of the 13 loci on the chromosome where the STRs occur This allows for a lot of variation

Restriction Fragment Length Polymorphism For 1 STR sequence at 1 locus G-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….. STR C-C-X-X-X-G-G C-C-X-X-G-G PCR amplify STR region well Gel electrophoresis Person A Forensic sample

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

Different STRs at other loci Do any of the individuals compare with forensic sample?