DNA
1
DNA
Actually, a human cell contains two types of DNAnuclear and mitochondrial. We will focus first on
nuclear DNA.
 Nuclear DNA strand is a very large molecule made by linking a
series of repeating units called nucleotides.
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HISTORY OF BIOLOGICAL
EVIDENCE IN FORENSICS
DNA fingerprinting


Also known as DNA profiling
Used with a high degree of accuracy
Biological evidence is examined for the presence of inherited traits
Some forensics laboratory techniques were originally developed for
other purposes. Can you name any?
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HEREDITY AND PATERNITY
Paternity testing has historically involved the A-B-O
blood typing system, along with blood factors other
than A-B-O.
Currently, paternity testing has implemented DNA test
procedures that can raise the odds of establishing
paternity beyond 99 percent.
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THE FUNCTION
AND STRUCTURE OF DNA
DNA contains the genetic material of a cell
Chromosomes are located in the cell nucleus
Chromosomes contain long DNA strands wrapped around proteins
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THE FUNCTION
AND STRUCTURE OF DNA
Nitrogenous Bases—pairs of molecules that form the rungs of the DNA
“ladder”
Four types of Bases




A (adenine)
C (cytosine)
G (guanine)
T (thymine)
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NUCLEOTIDES
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THE FUNCTION
AND STRUCTURE OF DNA
Base-Pairing Rules
 adenine (A) binds only with thymine(T)
 Cytosine (C) binds only with guanine (G)
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BASE PAIRING
LEADS TO DOUBLE
HELIX
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DNA AT WORK
DNA directs the production of proteins, which are
made by combining amino acids.
Each group of three nucleotides in a DNA sequence
codes for a particular amino acid.
 Example: G-A-G codes for the amino acid glutamine, while C-G-T
codes for alanine.
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IMPORTANCE OF
BASE PAIRS
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DNA REPLICATION
DNA duplicates itself prior to cell division.
DNA replication begins with the unwinding of the DNA
strands of the double helix.
Each strand is now exposed to a collection of free
nucleotides that will be used to recreate the double
helix, letter by letter, using base pairing.
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REPLICATION
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DNA IDENTIFICATION
Polymorphisms: Non-coded DNA that contain unique
patterns of repeated base sequences that that are
unique to individuals
DNA Fingerprinting (Profiling) isolates and analyzes
polymorphisms
DNA fingerprints appear as a pattern of bands on Xray film.
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DNA PROFILE
Two types of repeating DNA sequences
Variable Numbers of Tandem Repeats (VNTR)

The number of repeats varies from person to person

9 to 80 bases in length
Short Tandem Repeats (STR)

2 to 5 bases in length

Shorter lengths make STRs easier to use than VNTRs
VNTR and STR data are analyzed for


tissue matching
inheritance matching
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POLYMERASE CHAIN REACTION (PCR)
TESTING
Polymerase chain reaction (PCR) is a technique for
replicating small quantities of DNA or broken pieces of
DNA found at a crime scene, outside a living cell.
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PCR
1.
First, the DNA is heated to separate it.
2.
Second, primers (short strands of DNA used to
target specific regions of DNA for replication) are
added, which hybridize with the strands.
3.
Third, DNA polymerase and free nucleotides (A, T,
G, and C) are added to rebuild each of the
separated strands.
4.
Now, this cycle is repeated 25 to 30 times.
 Within a few hours a short strand of DNA can be multiplied a
billionfold.
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Appropriate primers are positioned close to the repeat
units of a DNA segment in order to initiate the PCR
process that will create short or mini STRs.
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STR
STRs are locations on the chromosome that contain
short tandem repeats that repeat themselves within
the DNA molecule.
STRs normally consist of repeating sequences of 3 to 7
bases in length, and the entire strand of an STR is also
very short, less than 450 bases in length.
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POWER OF STR
What makes STRs so attractive to forensic scientists is
that hundreds of different types of STRs are found in
human genes.
The more STRs one can characterize, the smaller will
be the percentage of the population from which a
particular combination of STRs can emanate. Can be
achieved via multiplexing
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RFLP
Restriction Fragment Length Polymorphisms (RFLP)
RFLPs are length differences associated with relatively
long repeating DNA strands and form the basis for one
of the first DNA typing procedures.
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DNA PROFILE MATCHING
Tissue Matching
 Two samples that have the same band pattern are from the same person
Inheritance Matching
 Each band in a child’s NDA fingerprint must be present in at least one parent
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DNA POPULATION DATABASES
Population genetics:


the study of gene variations among groups of individuals
Analyze the probability of a random person with the same alternative form of a
gene (an allele)
Examples


Identifying the suspect in a crime
Identifying an alleged father in a paternity case
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PRODUCT RULE
A high degree of discrimination and even
individualization can be attained by analyzing a
combination of STRs (multiplexing).
Because STRs occur independently of each other, the
probability of biological evidence having a particular
combination of STR types is determined by the product
of their frequency of occurrence in a population.
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SEX DETERMINATION BY STR
The amelogenin gene, which is actually the gene for
tooth pulp, is shorter by six bases in the X
chromosome than in the Y chromosome.
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MITACHONDRIAL DNA
Mitochondrial DNA (mtDNA) is located outside the cell’s
nucleus and is inherited from the mother.
Mitochondria are structures found in all our cells used
to provide energy that our bodies need to function.
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CODIS
CODIS (Combined DNA Index System) is a computer
software program developed by the FBI that maintains
local, state, and national databases of DNA profiles
from convicted offenders, unsolved crime scene
evidence, and profiles of missing persons. It currently
contains about 470,000 profiles from unsolved cases.
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SOURCES OF DNA
Biological evidence: examples include saliva, blood,
skin, hair, seminal fluid
Individual evidence is capable of identifying a
specific person.
Small amounts may be only trace evidence
Polymerase chain reaction (PCR) technique
generates multiple copies of DNA evidence
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COLLECTION AND
PRESERVATION OF DNA
1.
Use disposable gloves and collection instruments
2.
Avoid physical contact, talking, sneezing, and
coughing in the evidence area
3.
Air-dry evidence and put it into new paper bags or
envelopes
4.
Dry or freeze the evidence
5.
Keep evidence cool and dry during transportation
and storage
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PREPARING DNA SAMPLES
FOR FINGERPRINTING
1.
2.
3.
4.
DNA is mixed with special enzymes
Enzymes cut apart the DNA in specific places forming different sized
fragments
DNA is separated within an agarose gel
An electric current is passed through the gel separating the fragments
by size
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PREPARING DNA SAMPLES
FOR FINGERPRINTING
Extraction
1.
Cells are isolated from biological evidence such as
blood, saliva, urine, semen, and hair
2.
The cells are disrupted to release the DNA from
proteins and other cell components
3.
The DNA can be extracted from the cell nucleus
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PREPARING DNA SAMPLES
FOR FINGERPRINTING
Amplification
VNTR analyses—polymerase chain reaction (PCR) can be
used to amplify the DNA that contains the VNTRs
STR profiles—restriction enzymes are unnecessary; PCR
allows the amplification of the strands with STR sequences
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PREPARING DNA SAMPLES
FOR FINGERPRINTING
Electrophoresis
DNA samples are placed in gels through which electronic currents
are passed
DNA fragments line up in bands along the length of each gel
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ELECTROPHORESIS
An electrophoresis apparatus
running five sample of DNA
Arrows show the movement of the
negatively charged DNA
fragments through the gel matrix
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PROBES
DNA probes



identify the unique sequences in a person’s DNA
are made up of different synthetic sequences of DNA bases complimentary to the DNA
strand
bind to complimentary bases in the strand (see the fragmentary DNA bands above)
In most criminal cases, 6 to 8 probes are used
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ANALYSIS OF DNA
FINGERPRINTS AND APPLICATIONS
Bands and widths are significant in
matching samples of DNA
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ANALYSIS OF DNA
FINGERPRINTS AND APPLICATIONS
DNA fingerprinting can
 match crime scene DNA with a suspect
 determine maternity, paternity, or match to another relative
 eliminate a suspect
 free a falsely imprisoned individual
 identify human remains
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. . . . . . . . . . . . . . SUMMARY . . . .
DNA contains the information needed for replication in a
sequence of nitrogenous bases.
DNA analysis allows even a small sample of tissue to be
identified with a single individual.
DNA contains, in non-coding regions called junk DNA, many
repeated sequences that vary in number between individuals.
These differences between individuals can be used to produce a
DNA fingerprint for an individual.
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. . . . . . . . . . . . . . . . . SUMMARY
Polymerase chain reaction (PCR) for DNA amplification has
largely eliminated the problem resulting from the tiny samples
usually available.
DNA evidence must be collected carefully to avoid
contamination with other DNA.
DNA analysis involves extraction, electrophoresis, and
visualization.
DNA profiles are kept by police agencies in electronic
databases.
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