Download Biotechnology and Genetic Engineering

Chapter 11-DNA Profiling (or DNA
Fingerprinting) & Forensics
• History
• Uses of DNA Profiling
• Hypervariable DNA sequences examined (RFLPs,
VNTRs, STRs, SNPs, mitochondrial DNA, Y
chromosomal DNA)
• Methods (Southerns & PCR)
• Statistical considerations
• Technical considerations
• Databases and Privacy
DNA Fingerprinting
• You're 99.9% identical
• But of course, you are unique--in a genome of
three billion letters, even a 0.1 % difference
translates into three million differences.
• These differences (or polymorphisms) reside in
several places in the genome, often in
microsatellites
• Examples of such polymorphisms include
VNTRs, STRs, RFLPs and SNPs
DNA Fingerprinting
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Focuses on the 0.1-1.0% of human DNA that is unique
First described in 1985 by Dr. Alec Jeffreys in England
DNA evidence is admissible in courts
Labs such as Cellmark Diagnostics and Lifecodes
Corporation are examples of companies which provide
such DNA evidence to courts, but states and many
U.S. cities have labs for DNA fingerprinting
• Have any of you worked in a crime lab?
Uses of DNA fingerprinting
• Paternity testing
• Identification of criminals (e.g. murderers, rapists,
letter bombers)
• Immigration disputes (family relationships)
• Identification of deceased individuals with mutilated or
decomposed bodies (e.g., the military, 9/11 victims)
• Identifying the sperm donor who “decorated” Monica
Lewinsky’s blue dress
How is DNA fingerprinting done?
• DNA obtained from hair, semen, blood, sweat,
saliva, bone or any other tissue (often found at a
crime scene)
• Can be done by southern blotting with an
appropriate probe or by a PCR method using
appropriate primers
• Can use single locus probes/primers or multilocus
probes/primers
• DNA can be resolved on a gel or by a capillary
electrophoresis system
Sequences examined in DNA fingerprinting
• VNTRs-variable number tandem repeats; composed of
8-80 bp repeat units (e.g., [GCGCAATG]n) which are
tandemly repeated so that the overall length is 1-30 kb
• STRs-short tandem repeats; composed of 2-7 bp
repeat units (e.g., [AC]n) which are tandemly repeated
so that the overall length is less than 1 kb
• RFLPs-restriction fragment length polymorphisms
• SNPs-single nucleotide polymorphisms
• Mitochondrial DNA-maternal inheritance, tends to be
more stable than nuclear DNA
• Y chromosome DNA- passed from father to son
DNA fingerprinting: an example
• D1S80, a VNTR located on human chromosome 1,
contains a 16 bp repeat unit
• The number of repeats varies from one individual to
the next, and is known to range from 14-41
Some examples of DNA fingerprinting
• Paternity cases
• Crime scenes
Determining the probability of a match
• Relies on statistics
• Analysis depends upon your ethic
background (i.e. African American,
Caucasian, Hispanic Asian, etc.)
Technical Considerations
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Preserve the integrity of DNA sample
Avoid DNA contamination & degradation
Avoid incomplete digestions if REs are used
Use standard hybridization conditions
Use standard PCR primers and procedures
Gel analysis is less reproducible than capillary
electrophoresis of PCR products
• Difficulties in interpreting bands on a gel or X-ray film
DNA databases
• Already in place in the FBI for convicted felons
(i.e., CODIS-COmbined DNA Index System,
involves 13 STR loci) and the Dept. of Defense
for armed service personnel and the Virginia
saliva and blood bank of convicted felons
• A national DNA database has been suggested.
What do you think?
• Could current or potential employers or
insurance companies base decisions they make
on this kind of data?