Download DNA - Madison Public Schools

DNA: The Indispensable
Forensic Science Tool
Chapter 9
Introduction




1985-Sir Alex Jeffreys recognized that portions
of DNA structure of certain genes are unique to
each individual.
The process of identifying these markers is
known as DNA fingerpringting, DNA typing, or
DNA profiling
Forensic Scientists now had a way of linking
biological samples like blood, semen, hair, and
tissue to a single individual
US courts have accepted the reliability of DNA
evidence
DNA = Deoxyribonucleic Acid



DNA is a polymer
Polymers are very
large molecules put
together by linking
units called
monomers
The monomers of
DNA are called
nucleotides.
Nucleotides

DNA nucleotides
consist of :



A sugar molecule that
forms the backbone
A phosphate group
which links the sugars
A nitrogen containing
base (adenine, cystine,
guanine, thymine)
Nucleotides are linked
together to form a
double helix
What is DNA?

Complementary Base Pairing



Adenine always pairs
with Thymine
Guanine always pairs
with Cytosine
Bases are joined
together by hydrogen
bonding.
How DNA works




DNA directs the production of proteins
which play a basic role in the structure
and function of living organisms
Proteins are built by linking amino acids
The are 20 amino acids that make up
proteins
The sequence of amino acids determines
the shape and function of the protein
How DNA works

Each amino acid is
coded by a sequence
of 3 bases
Replication of DNA



Replication – synthesis
of new DNA from
existing DNA
The double helix is
unwound and a copy
is made.
DNA polymerase –


Assemble new DNA
strands in the proper
base sequence
Correct mismatched
base pairs
Polymerase Chain Reaction (PCR)



A technique for replicating a portion of
DNA outside of a living cell
DNA polymerases are used to make copies
of DNA material
This is useful to forensic scientists because
small samples could be multiplied.
DNA Typing with Tandem Repeats




Tandem repeat –sequence of base pairs
repeated numerous times.
~30 % of human genome is made up of
tandem repeats.
They act as spacers in between coding
regions of the DNA
Used by forensic scientists to distinguish
individuals.
DNA Typing with Tandem Repeats


Humans have the same type of repeats
but a wide variety in the number of
repeats.
Restriction fragment length
polymorphisms (RFLP’s) – repeat
segments cut out of DNA by restriction
enzymes.
DNA Typing with Tandem Repeats



Length differences associated with RFLP’s
allow forensic scientists to identify
individuals.
Electrophoresis is used to separate
different sized fragments.
Gel Electrophoresis
DNA Typing with Tandem Repeats
Polymerase Chain Reaction (PCR)
PCR replaced RFLP techniques in the mid1990’s
The Basics:
 Short DNA sequences are identified.
 DNA strands are unwound and replicated
yielding two copies of the original sample
 28-32 cycles are carried out yielding over
a billion copies of the DNA strand.
Polymerase Chain Reaction

Polymerase Chain Reaction (PCR)
Advantages of PCR over RFLP typing:
1. PCR uses much smaller strands of DNA
2. Smaller strands are more stable and less
subject to degradation
3. Smaller strands overcome the samplesize problem associated with crime-scene
evidence
Short Tandem Repeats (STR’s)




STR’s – DNA profiling procedure used
currently
Short Tandem Repeats – regions of DNA
molecules containing short segments of 3
to 7 repeating base pairs.
STR’s are less susceptible to degradation
and can be amplified by PCR
Multiplexing: A technique that
simultaneously detects more than one
DNA marker in a single analysis.
Short Tandem Repeats (STR’s)





In the US, the forensic science community
has standardized 13 STR’s for entry into
the CODIS database
FBI Core STR Loci
Calculating the frequency of occurence of
a DNA profile
Separation of STR's by capillary
electrophoresis
Results of electrophoresis on STR's
Sex Identification Using STR’s
The sex of a DNA contributor can be
determined by looking at the
amelogenin gene. The gene is 6
bases shorter on the X chromosome
than it is on the Y chromosome.
 Y-STR’s – STR’s found on the Y
chromosome; useful when there is a
mix of DNA from more than one
male.

Mitochondrial DNA






Mitochondrial DNA (mtDNA) – found in the
mitochondria (outside of the cell nucleus)
mtDNA is inherited solely from the mother
Forensic Scientists use mtDNA to identify
subjects when nuclear DNA is not
available.
Frequencies of sequences in the FBI
database are about 1 %
inheritance of mtDNA
structure of mtDNA
The Combined DNA Index System (CODIS)


CODIS – software program and database
developed by the FBI
Contains DNA profiles of convicted
offenders, unsolved crime-scene evidence,
and profiles of missing people.
Collection and Preservation of
DNA Evidence




STR profiling requires only 125 picograms
of DNA
One human cell contains ~ 7 pg of DNA
Only 18 cells are necessary
As a result, DNA has been successfully
analyzed from stamps, envelopes, cups,
cigarette butts, chewing gum, skin cells,
etc.
Collection and Preservation of
DNA Evidence




Biological evidence should be documented
by notes, sketches, and photographs first
The shape and position of bloodstains
should be investigated prior to collection.
Latex gloves will protect investigators from
infection AND avoid contaminating the
evidence.
Items like clothing should be collected
from suspects and victims even if blood
isn’t immediately visible.
Collection and Preservation of
DNA Evidence
Packaging of Evidence:
 Biological evidence should not be
packaged in an airtight container; paper
bags, envelopes, or cardboard boxes can
be used. Samples should be refrigerated
and kept out of direct sunlight
 Blood stains can be removed from a
surface using a cotton swab soaked in
distilled water.
 Swabs must be dried prior to packaging
Collection and Preservation of
DNA Evidence
Obtaining DNA Reference Samples:
 Reference samples can be obtained by
collecting blood or a buccal swab from an
individual
 If the individual is not available:





Tootbrush
Combs and hairbrushes
Razor
Dirty laundry
Cigarette butts
Collection and Preservation of
DNA Evidence
To minimize contamination:
 Change gloves before handling each new
piece of evidence
 Collect a substrate control.
 Use forceps to pick up small items
 Package items is individual well-ventilated
containers.
 Luminol does not destroy DNA