Title: DNA%20Profiling%20and%20Forensic%20Analysis
1DNA Profiling and Forensic Analysis
2- Forensic science is the application of science to
law - previous technologies used
- photography, video cameras
- fingerprinting
- new technologies
- DNA fingerprints
3What is a DNA fingerprint?
- Every cell of an individual carries a copy of the
DNA - a cell collected from a persons skin or hair
folicle contains the same DNA as from that
persons heart tissue or white blood cells - Order of base pairs in the DNA of every
individual is different except identical twins
4How do we distinguish one persons DNA from
another?
- We do not need to sequence the entire 3 billion
base pairs of a persons DNA to distinguish it
from another persons DNA - Intron regions of DNA contain sequences that are
20-100 bp in length that are repeated at
different locations (loci) along the chromosome.
CGGCTACGGCTACGGCTA (repeated 3 times at this
location at another location, it may be repeated
9 times) - These sequences are called Short Tandem Repeats
(STRs) or VNTRs
5STRs
- Each person has some STRs that were inherited
from mother and some from father - No person has STRs that are identical to those of
either parent - The number of repeats at each loci on chromosome
is highly variable in the population, ranging
from 4 to 40. - The length of the DNA after cutting the
chromosome with a restriction enzyme, and its
position after electrophoresis will depend on the
exact number of repeats at the locus
6- The uniqueness of an individuals STRs provides
the scientific marker of identity known as a DNA
fingerprint. - In the United States the FBI has standardized a
set of 13 STR assays (13 different locations on
the chromosomes) for DNA typing, and has
organized the CODIS database for forensic
identification in criminal cases. - The United States maintains the largest DNA
database in the world The Combined DNA Index
System, with over 60 million records as of 2007.
7Preparation of a DNA profileStep 1
- Specimen collection
- blood, semen, etc
- easy to contaminate a DNA sample with DNA from
other sources (bacteria, DNA of person collecting
sample) - DNA is not stable for very long-it degrades
- sunlight
- heat
- moisture
8- DNA fingerprinting is a comparative process
- DNA from crime scene is compared with DNA of a
suspect - So minimum of two samples must be prepared
- Step 2
- DNA extraction
- standardized methods have been developed
- need to separate DNA from other cell material and
debris from crime scene.
9Step 3PCR using primers targeting STRs at
different loci
- PCR amplify STRs using target sites on chromosome
10Step 3 PCR amplification of DNA
STR locus
Design primers that anneal to STR locus Amplify
all the regions of the chromosome where the STRs
exist.
STR locus
11PCR allows you to make millions of copies of the
STR region from a single copy of DNA you
recovered from crime scene.
12- Since the of times sequence is repeated is
different for each person, fragment size will be
different. - This is done for 13 different STR sequences at
this one locus - Differences occur among individuals at each of
the 13 loci on the chromosome where the STRs
occur - This allows for a lot of variation
13Restriction Fragment Length Polymorphism
For 1 STR sequence at 1 locus
14- If you do this for 13 different repeat sequences
at 13 different loci on the chromosome, each
person produces a different band pattern when the
fragments are separated by gel electrophoresis
15Different STRs atother loci
STR1 STR2 STR3
Do any of the individuals compare with forensic
sample?
16Other approaches to DNA fingerprinting
- Dot Blotting
- Genes that display high sequence variability can
be substituted for STRs - An example of a gene with this property is the
human leukocyte antigen (HLA). - The gene encoding this protein has lots of
sequence variability across the human population. - Since this gene is not present in other life
forms, it reduces the interference that could
otherwise be contributed by bacteria, fungi, dog,
or cat DNA picked up in the sample at crime
scene.
17- Dot blotting (continued)
- Because every gene amplified by PCR has the same
length, we dont need to use electrophoresis to
sort and separate the fragments. - Instead, we use blot strip or dot blot which
contains a different DNA probe that is sensitive
to the sequence variability.
18Dot Blot
Probe made from sequence obtained from forensic
sample
Single strand of HLA gene amplified DNA from
sample
19Binding of probe to complementary DNA
Binding takes place
No binding takes place
20Wash away unreacted probeand add biotin-reactive
enzyme
Colorless substrate
Strepavidin (colorless enzyme)
Colored product (spot lights up)
21Dot Blot
- A visual signal is produced when the different
probes anneal (bind) to the complementary
sequence in the DNA sample
Probe 1
Crime scene PCR amplified DNA on each spot
22What do we end up with?
- Blot strips show a pattern of spots that either
light up or remain dark - Compare pattern produced from crime scene DNA to
pattern produced from suspect DNA
Suspect DNA
Scene DNA
23Types of Blot Tests
24Characterization Southern blot hybridization
-transfer of DNA from a gel to a membrane (e.g.,
nitrocellulose, nylon) -developed by Edwin
Southern
25Characterization Northern blot hybridization
X RNA
X
x salt
X RNA
-transfer of RNA from a gel to a membrane (e.g.,
nitrocellulose, nylon) -reveals mRNA size (and
approximate protein size), tissue- and
organ- specific expression, and kinetic patterns
of expression
26Characterization Western blotting
X Protein
Enzyme reaction or
X
React with Antibody X
x Buffer requires electric current
X
-transfer of protein from a gel to a membrane
(e.g., nitrocellulose, nylon) -requires the use
of an electric current to facilitate transfer
27Characterization Eastern blotting
-biochemical technique used to analyze protein
post transitional modifications (PTM) such as
lipids, phosphomoieties and glycoconjugates. It
is most often used to detect carbohydrate
epitopes -can be considered an extension of the
biochemical technique of Western blotting
28DNA fingerprinting in practice
- Rape cases often sample a victims vagina for
sperm in order to get a fingerprint of the rapist - Victims vaginal cells that are mixed in with
rapists sperm cells make it difficult to get a
fingerprint of the rapists DNA - Scientists studying sperm cells discovered that
they resist lysis in certain solutions that
induce lysis of vaginal cells. - They used this knowledge to separate DNA from
sperm and vaginal cells
29Transfer cells on swab to lysis buffer 1
Lysis buffer causes vagina cells to lyse and
release DNA into solution
Centrifuge tube to sediment sperm cells, then
decant supernatant to remove vagina DNA
Add lysis buffer 2
Lysis buffer causes remaining sperm cells to
lyse and release DNA into solution
30Always need suspect DNA
- When suspects DNA doesnt match sperm DNA from
victim, investigation hits a barrier. - Need to find more suspects
- Computer searchable DNA databases are now
authorized by all 50 states - in some cases courts have agreed that collecting
blood sample from someone without probable
cause violates state and federal laws
prohibiting unreasonable search and seizure.
stopped
31- Need lucky break or a lead that gets you to the
perpetrator or the crime.
32DNA profile database
- CODIS Combined DNA Index System
- run by FBI
- contains profiles of convicted offenders
- contains unidentified DNA taken from crime scenes
- visit CODIS website to see how it works
- www.fbi.gov/hq/lab/codis/index1.htm
- CODIS allows identifying possible suspects when
no prior suspect exists
33Invasion of privacy
- Some groups are worried that DNA samples will get
in hands of insurance companies or potential
employers - use to identify genetic defects that might cost
them - Why is this concern invalid?
- What do you need to identify a genetic defect?
- What does the STR analysis yield in the way of
data that can provide information on genetic
disorders? - Some groups are demanding that DNA samples be
destroyed after investigation is complete. - Is this a good idea?
34- DNA fingerprint can be used to refute erroneous
evidence that would otherwise convict an innocent
suspect - DNA evidence can expose a faulty eye-witness
- Forest Hills Rapist
- 3 separate victims identified assailant as Black
to the police - Suspect on trial for crimes was White
- DNA of white suspect was tested and matched DNA
from sperm left at scene of each crime
35Meeting Legal Standards
- Court uses 5 different standards to determine
whether evidence should be allowed in court - Any new technique must meet one or several of the
standards before evidence using new technique can
be introduced.
365 Standards
- Relevancy test
- Frye standard-general acceptance test
- Coppolino standard - allows new or controversial
science to be used if adequate foundation can be
laid. Expert witnesses used in this case. - Marx standard - court must be able to understand
and evaluate scientific evidence. A university
professor may be brought in to give a lecture of
the concept. - Daubert standard - requires special pretrial
hearings for scientific evidence. Scientific
procedure must be described in a peer-reviewed
journal
37Simpson/Goldman Murder
- Pretrial hearings announced that blood collected
at crime scene matched that of O.J.s - Defense argued that contamination could have
occurred during sample collection and between
collection of different samples - Technician admitted mislabeling samples
- Possibility that evidence might be tainted was
obvious to both the court and the jury - DNA evidence was not allowed as evidence
- When rules of evidence are not followed, DNA
samples lose their value in court.
38Chain of custody
- Requires that collection of evidence must be
systematically recorded and access to evidence
must be controlled - Special challenges for DNA samples
- crime scene may have DNA from people other than
perpetrators of crime. These people could become
suspects based on this DNA - DNA collected from victims in a morgue can become
contaminated by DNA of other bodies previously on
autopsy table - during early days all procedures for processing
DNA was not standardized, people running assays
were not experienced and made mistakes
39Educating the Jury
- Comparison of STR data is a statistically-based
method - Jurors may not understand significance of a 1 in
50 billion chance of a random match - Attorneys must compare chance of random match of
DNA data with chance that people will die by
being hit by lightening over their lifetime to
make them appreciate these numbers - Jurors must understand what DNA evidence offers
in the way of putting suspect at a crime scene
40Paternity testing
- Verifying parents of a child to determine
responsibility for child support - 250,000 cases per year in U.S.
- Using amniocentisis, it is even possible to
verify a childs parents before birth - collect fetal cells from amniotic fluid
- DNA extraction and fingerprinting.
41No longer necessary with PCR technology
PCR amplification, then DNA fingerprinting
42Mothers STRs
Offspring STRs
STRs of suspected Father
-
Is the suspect the father?
43Tracing genealogy through mothers mitochondrial
DNA
- Inside each cell of the body is an organelle
called the mitochondria - The mitochondria has chromosomes that were only
inherited from the mother (MtDNA) - comes from the cytoplasm of the egg.
- The DNA of the mothers mtDNA is the same as her
mothers mtDNA, and so on, back through the
maternal bloodline.
44(No Transcript)
45- mtDNA is used to reunite families separated by
corrupt governments - Junta in Argentina arrested pregnant women and
took their newborn infants and gave them to
supporters of the regime without consent of
mother. - AAAS helped reunite 51 children with their
natural mothers after the Junta regime collapsed. - mtDNA can be used to identify a buried corpse
that has been buried for many years if you have
living relatives whose DNA you can compare it to.
46MtDNA and evolutionary biology
- mtDNA mutates at a relatively constant rate of
2-4 every million years. - Allows scientists to trace gene frequency changes
over time. - Eve hypothesis allowed scientists to trace a
majority of people now living on Earth to a
common female ancestor from ancient Africa - Followed human migration and dispersal from that
location to other parts of the world. - Jared Diamonds Guns Germs and Steel
47Careers in DNA testing
- Laboratory technicians
- must be able to work very meticulously
- forensic science technicians must pass a test to
demonstrate these skills before being let loose
at a crime scene - sometimes have to perform their sample
manipulation in a clean room - Requirements
- B.S. degree in biology, biochemistry or molecular
biology or a specialized Associates degree in
biotechnology and laboratory experience. - Good writing skills (lab notebook entries)
- Good math and communication skills
- Salaries 31K to 45K entry level depending on
location.