DNA Fingerprinting

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