Transcript DNA Fingerprinting

DNA Fingerprinting
“Elementary, my dear Watson.”
-Sherlock Holmes
California Science Standards
#5c,d,e
What is it?
 DNA fingerprint =
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A pattern of bands; each band is a fragment
of DNA of a particular size.
Each person has a unique DNA fingerprint
(except identical twins). This is similar to each
person’s actual fingerprint being different.
DNA fingerprinting has many applications.
What does it look like?
DNA
samples
taken from 2
suspects
DNA
collected as
evidence
from crime
scene
What’s it good for?
 1. Determine whether two individuals are related
(“Who’s Yer Daddy?”)
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Compare banding patterns from two or more individuals
May be used in paternity cases, or to determine if
someone is a long lost relative, etc.
The military compares DNA of deceased soldiers to a
family member or to a database to confirm identity
What’s it good for?
 2. Help solve a crime
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Compare samples of blood or tissue found at a
crime scene with a suspect’s blood sample
May be used to identify a victim
Has also been used to exonerate (clear)
individuals wrongly convicted, many whom
were serving time on death row.
What’s it good for?
 3. Determine how closely two species are
related (evolution research)
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Compare banding patterns from members of two or
more different species
 4. Monitor populations:
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Tracking grizzly bears in Glacier Nat’l Park.
Determining if farm-raised salmon mate with native
salmon
Determine amount of genetic diversity that exists in
a small population, such as an endangered species.
How is DNA fingerprinting done?
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Just four “easy” steps:
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Isolate DNA from a cell and cut DNA into many
fragments by restriction enzymes (“digestion”)
Separate DNA fragments by gel electrophoresis
Bind Radioactive probes to selected fragments*
Photograph radioactive probes, producing the actual
“DNA fingerprint”*
* Note: our lab will involve a slightly simpler procedure. The number
of fragments is small, so no probes are needed. Also, the “DNA”
can be visualized directly without radioactive probes.
1. “Digestion”
a) Extract DNA from blood or other tissue (ex: cheek cells, hair
follicles, skin, etc.) Remember, all of these cells, taken from
the same person have the exact same DNA.
b) Cut it into fragments using restriction enzymes.
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These enzymes were first discovered in bacteria. Again, we see
bacteria are beneficial!
Each restriction enzyme recognizes a specific nucleotide sequence.
Result: the number of fragments and the lengths of the
fragments vary from person to person. Recall that each person’s
DNA sequence is unique (except identical twins), especially in
“noncoding” regions.
Example of Digestion
• The enzyme “EcoR1
recognizes the DNA
sequence on the left.
• In 3 different people, this
sequence will occur a
different number of times
and/or at different
locations along a stretch
of DNA.
• After digestion, Bob’s
DNA would be in 2
fragments, Larry’s would
be in 3 fragments, and
Mary’s DNA would not
produce fragments.
2. Gel Electrophoresis
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From the Latin electrocus
(“electricity”) and the
Greek phoresis (“to carry”)
This procedure will use
electricity to separate the
DNA fragments based on
their size.
DNA is negatively charged
which will cause it to be
attracted to (move toward)
the positive electrode.
2. Electrophoresis (continued)
a) Make a gel (kinda like Jello)
b) Make wells in gel
c) Inject sample containing DNA fragments
into wells
d) Run electric current through the gel
e) (-) DNA fragments’s move to (+) end of
gel
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but at different speeds:
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Smaller DNA fragments migrate faster and further
than longer fragments
Result of Electrophoresis
 Diagram of an Example:
Actual DNA Fingerprint
Each “lane” (column)
represents a different DNA
sample.
3. Bind Radioactive Probes
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Goal of this step: make some bands visible (we
only care about certain fragments)
a) Split the migrated DNA fragments into single chains
b) Blot onto filter paper
c) Add complementary segments of DNA to paper
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These probes are “hot” -- radioactively labeled
The probes bind to their complements on paper (they will only
bind to some of the fragments, thus not all fragments will be
visualized.)
Probes
http://www.accessexcellence.org/AE/AEPC/NIH/images/probe.gif
4. Photograph
 Take a picture:
a) Expose photographic film to blot
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Since radioactive substances show up on film, the radioactive
probes show up in the picture when the film is developed
b) Develop film
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The dark spots are the locations of each of the “tagged”
DNA fragments (fragments to which the probes attached)
Or, if the background is black, the fragments appear as
white lines (kinda like an x-ray).
c) Analyze DNA fingerprint
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By comparing the dark bands, we are actually comparing
the DNA of the different samples
Well, Watson? Who Dunnit?
A Tool for C.S.I.
 PCR
 Polymerase Chain Reaction
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Often, there are only tiny amounts of DNA
at a crime scene. But it’s still enough:
PCR “amplifies” the DNA such that
enough copies are produced to allow for
analysis.