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 fingerprintStep 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

• DNA fingerprinting is a comparative process:– 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 3PCR using primers targeting STRs at

different loci

• PCR amplify STRs using target sites on chromosome

Step 3 PCR amplification of DNA

1 strandof DNA

Heat todenaturedouble-strandedDNA

Design primers that anneal to STR locus

Amplify all the regions of the chromosomewhere the STRs exist.

STR locus

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

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 amplifySTR region

STR

well well

Gelelectrophoresis

Person A Forensic sample

For 1 STR sequence at 1 locus

• 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

STR1STR2STR3

Do any of the individuals compare with forensic sample?