DNA Fingerprinting • A method of developing a person’s DNA “profile,” similar to a fingerprint. • Pioneered in England in 1984 by Dr. Alec Jeffreys Dr. Alec Jeffreys
Feb 23, 2016
DNA Fingerprinting
• A method of developing a person’s DNA “profile,” similar to a fingerprint.
• Pioneered in England in 1984 by Dr. Alec Jeffreys
Dr. Alec Jeffreys
First Forensic Use
• First used by law enforcement in England in the mid-1980’s.
• DNA evidence exonerated one man, and convicted another.
• Described in The Blooding, by Joseph Wambaugh
How does it work?
• 99.9% of your DNA is the same as everyone else’s.
• The 0.1% that differs are a combination of:– Gene differences (Differences in the genes
themselves)– Differences in “polymorphic regions” between the
genes on the DNA.
How does it work?
• Certain points between the genes on the DNA have repeating base sequences.– For example:
ATTACGCGCGCGCGCGCGCTAGC– These are called short tandem repeats (STRs for
short)
How does it work?
• Everyone has STRs at the same place in their DNA, but they are different lengths for different people.– For example:
Person 1: ATTACGCGCGCGCGCGCGTAGC(7 repeats)
Person 2: ATTACGCGCGCGCGTAGC(5 repeats)
To Make a DNA Fingerprint…
• First, we use restriction enzymes to chop the DNA up into millions of fragments of various lengths.– Some of the fragments contain STRs; some do
not. The ones that do are different lengths for different people.
Restriction Fragment Length Polymorphisms (RFLPs)
• Polymorphisms are slight differences in DNA sequences as seen in individuals of the same species
To Make a DNA Fingerprint…
• Next, we use gel electrophoresis to sort the DNA fragments by size.
Gel Electrophoresis
• Method for sorting proteins or nucleic acids on the basis of their electric charge and size
Gel Electrophoresis• Electrical current carries
negatively-charged DNA through gel towards positive electrode
• Agarose gel sieves DNA fragments according to size– Small fragments move
farther than large fragments
Gel Electrophoresis
To Make a DNA Fingerprint…
• Finally, a radioactive probe attaches to our STRs. Only the fragments with our STRs will show up on the gel.
Figure 12.11C
Restriction fragmentpreparation
1
Restrictionfragments
Gel electrophoresis2
Blotting3
Probe
Radioactive probe4
Detection of radioactivity(autoradiography)
5
Film
To Make a DNA Fingerprint…
• Since STRS are different lengths in different people, this creates a DNA Fingerprint.
Two uses for DNA Fingerprints...
• ForensicsDNA taken from crime scenes (blood, semen, hair, etc.) can be compared to the DNA of suspects.
Real-life CSI!
Two uses for DNA Fingerprints...
• ForensicsThis is an example of a gel that might be used to convict a rape suspect. Compare the “Sperm DNA” to the “Suspect DNA.” Which suspect committed the rape?
Two uses for DNA Fingerprints...• Paternity Testing
Since all of our DNA markers came from either mommy or daddy, we can use DNA fingerprints to determine whether a child and alleged father are related…just like on Maury Povich!
Two uses for DNA Fingerprints...
• Look at the two “Child” markers on this gel. Can they both be matched up to either the mother or the “alleged father?”
• Yes. This is a “positive” test for paternity.
Two uses for DNA Fingerprints...
• How about this gel? Do both of the child’s markers match either the mother or the “alleged father.”
• No! The “alleged father” is not this child’s biological parent.
Interpreting DNA Fingerprints• Which child is not
related to the mother?
• Son 2
• Which children are not related to the father?
• Daughter 2 and Son 2
Interpreting DNA Fingerprints
• A blood stain was found at a murder scene. The blood belongs to which of the seven possible suspects?
Suspect 3
Interpreting DNA Fingerprints
• These DNA fingerprints are from a mother, a child, and two possible biological fathers. Which one is the daddy?
2nd alleged father
The Polymerase Chain Reaction (PCR)
• The polymerase chain reaction, PCR, can produce many copies of a specific target segment of DNA
• A three-step cycle—heating, cooling, and replication—brings about a chain reaction that produces an exponentially growing population of identical DNA molecules
Genomic DNA
Targetsequence
5
3
3
5
5
3
3
5
Primers
Denaturation:Heat brieflyto separate DNAstrands
Annealing:Cool to allowprimers to formhydrogen bondswith ends oftarget sequence
Extension:DNA polymeraseadds nucleotides tothe 3 end of eachprimer
Cycle 1yields
2molecules
Newnucleo-
tides
Cycle 2yields
4molecules
Cycle 3yields 8
molecules;2 molecules
(in white boxes)match target
sequence
Genomic DNA
Targetsequence
5
3
3
5
5
3
3
5
Primers
Denaturation:Heat brieflyto separate DNAstrands
Annealing:Cool to allowprimers to formhydrogen bondswith ends oftarget sequence
Extension:DNA polymeraseadds nucleotides tothe 3 end of eachprimer
Cycle 1yields
2molecules
Newnucleo-
tides
Cycle 2yields
4molecules
Cycle 3yields 8
molecules;2 molecules
(in white boxes)match target
sequence