Chapter 10
Advanced
Concepts
in DNA
© 2012 Cengage Learning. All Rights Reserved
Introduction—Objectives
1. Discuss the structure and function of DNA.
2. Explain what causes variation in DNA.
3. Differentiate between the various types of
DNA analysis.
4. Compare and contrast the methods of
extracting DNA.
5. Explain the proper methods of collecting
DNA evidence at a crime scene.
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Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Introduction—Vocabulary
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DNA extraction—process of removing DNA from a
cell; procedures include Chelex extraction, organic
extraction, and differential extraction
homologous chromosomes—pair of chromosomes
in which one chromosome was inherited from the
male parent and the other was inherited from the
female parent
Nucleotide—subunit of DNA and other nucleic
acids; made up of a 5-carbon sugar, a phosphate
group, and a nitrogenous base
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Introduction—Vocabulary
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polymerase chain reaction (PCR)—technique used
to make billions of copies of a specific segment of
DNA; allows very minute samples of DNA to be
copied billions of times
primer—small piece of DNA used to begin and end
replication during PCR
short tandem repeat (STR)—short segment of DNA
in which the same sequence of two to six base pairs
is repeated many times; varying numbers of repeats
found among individuals
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
A 200-Year Old Mystery
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August 13, 1792, the French royal family is
arrested and imprisoned.
The king and queen were executed.
The heir lived two more years.
Did he die in prison or escape?
DNA samples from the remains of
the king, queen and suspected
dauphin were compared.
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Introduction
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Deoxyribonucleic acid (DNA)—the genetic
material that makes each individual unique.
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Only identical twins have the same DNA
DNA copies itself when cells divide
DNA contains the blueprint for making proteins
essential for life processes
DNA is found in biological evidence
DNA is individual evidence
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Structure and Function (Obj 10.2)
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Nucleotides—subunits
of DNA
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Deoxyribose
Phosphate group
Nitrogenous base
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Adenine
Thymine
Cytosine
Guanine
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Structure and Function (Obj 10.2)
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Each nitrogenous base
can form hydrogen bonds
with a specific base on a
complementary strand of
DNA
Adenine always pairs with
thymine
Guanine always pairs
with cytosine
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Structure and Function (Obj 10.2)
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Nitrogenous base pairs form the
rungs of a twisted ladder and
carry a lot of information
Sugar and phosphate groups
form the edges of the ladder
Genes—sections of DNA that
code for a protein
Proteins catalyze, regulate, and
control chemical reactions in the
cell.
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Structure and Function (Obj 10.2)
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Alleles—an alternative form of a gene
for a certain trait.
An individual inherits two alleles for
each gene, one from each parent.
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Structure and Function (Obj 10.2)
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Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
What Causes Variation in DNA? (Obj 10.2)
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99.5% of DNA is the same for everyone
0.5% of DNA is what makes us unique
Homozygous—when two alleles are identical for a
particular trait
Heterozygous—when two alleles are different
Marker—a sequence of DNA base pairs associated
with a specific trait
Matching markers from two samples determines the
likelihood of an individual being the same as the
reference sample
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
DNA Analysis Procedures (Obj 10.3, 10.4)
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Method of extraction varies depending upon
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Chelex Extraction
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The type of biological evidence analyzed
The amount of evidence available
The type of cells present
Used when a small amount of
biological evidence is available
Sample placed in boiling water
with Chelex beads
The cells in the sample burst open
to expose and preserve DNA
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
DNA Analysis Procedures (Obj 10.3, 10.4)
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Organic Extraction—removing biological
evidence from materials (e.g., blood from
bed sheets)
Differential Extraction—separates vaginal
epithelial cells from sperm cells with a mild
solution of phenol and chloroform
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Measuring Quality
and Quantity of DNA (Obj 10.3)
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High molecular weight (HMW)—sample
contains DNA in large chunks
Restriction fragment length polymorphism
(RFLP)—DNA analysis requiring HMW
Polymerase chain reaction (PCR)—analysis
used when only small amounts of DNA are
available, amplification process
Variable Number of Tandem Repeats
(VNTR) —repeating sections of DNA
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
DNA Fingerprinting
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Fingerprinting has several steps
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Isolate and purify DNA
Restriction enzymes indicate where DNA can be
cut into fragments (the RFLPs)
Gel electrophoresis sorts DNA bands by size
Southern blotting process transfers DNA bands
from the gel to a nylon sheet
Population genetics determines probability of
another individual having the same profile
Forensic Science II: Advanced Concepts in DNA, Chapter 10
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DNA Fingerprinting
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Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
PCR
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Polymerase Chain Reaction
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An enzyme separates a region of DNA and then
copies it billions of times
Samples are heated and cooled in a three-step
cycle
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Denaturing—(heating) separating the two DNA strands
Primer annealing—(cooling) adding primers (markers) to
the beginning and ends of targeted sequences
Polymerization—(heating) an enzyme copies the DNA
between the two primers
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
PCR
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Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
PCR
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Forensic Science II: Advanced Concepts in DNA, Chapter 10
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STR
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Short Tendem Repeats—a short segment
of DNA in which the same sequence of 2 to 6
base pairs is repeated many times
Easier and quicker to replicate
Repeating STR patterns are highly variable
between individuals; therefore
Matching multiple STRs between individuals
has a very low probability
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
CODIS
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Combined DNA Index System
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An FBI nationwide database
Stores DNA fingerprints of individuals who have
been convicted of certain crimes
Open to forensic scientists to compare DNA
fingerprints of suspects based on 13 core STR
locations
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
CODIS
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Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Collecting DNA Evidence (Obj 10.5)
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Biological samples
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Stop degradation
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Avoid contamination
Retain original surface when possible
Reference samples for comparative analysis
Protect sample from environmental exposure
Remove moisture by drying out and freezing the
sample
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Collecting Biological Evidence
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Examples
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Skin cells
Blood
Saliva
Semen
Hair
Other possibilites?
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Collecting Biological Evidence
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Guidelines
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Never use bare hands
Avoid sneezing and coughing
Use clean protective gloves for each item
collected
Package each sample separately
Air dry evidence located on objects; store in an
envelope or paper bag, not plastic bags
Establish a chain of custody
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Collecting Reference Samples
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Known samples taken from suspects and
victims
Reference samples are compared to
evidence collected from the crime scene
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Testing for Blood, Semen, & Saliva
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Presumptive tests used at the crime scene
Luminol is the most common method used to
detect blood
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Sprayed on objects to find bloodstains
Detects blood even when diluted
Chemicals and fluorescent dyes are used to
visualize seminal fluid
For saliva, the Phadebas test or starch iodine
test detects the presence of amylase
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Mitochondrial DNA (mtDNA) Analysis
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mtDNA is inherited from the mother
Easier to extract than nuclear DNA
Less likely to degrade
Each cell has hundreds of copies of mtDNA
Typically extracted from teeth, hair, bones
Take care to avoid altering this evidence for
other analyses
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Y-Chromosome Analysis
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Passed down through paternal lineage
Beneficial for these situations
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Sexual assault
Missing person
Genealogy
Does not provide individual evidence
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Chapter Summary
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The discovery of DNA’s existence, structure,
and function dates back to the 1800s.
DNA, shaped like a double helix, is made up
of a sugar and phosphate backbone with
nucleotides on the inside.
The four nitrogenous bases are adenine,
guanine, cytosine, and thymine.
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Chapter Summary
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The genetic code is nearly universal. The code is
always read three bases at a time and codes for
amino acids, which combine to form proteins.
DNA isolation and extraction from biological
evidence such as blood, semen, and saliva must be
done prior to completing a DNA profile.
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Chapter Summary
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Polymerase chain reaction (PCR) is used to collect
and analyze very minute and degraded samples.
Short tandem repeats (STR) are very short
segments of DNA in which the same sequence of
two to six base pairs is repeated several times.
The STR pattern, being highly variable from person
to person, provides matches with higher degrees of
certainty.
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Chapter Summary
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Once DNA is exposed to an environment outside the
body, the DNA begins to degrade, which affects the
ability to retrieve a useful result.
When collecting DNA evidence, extra precautions
must be taken to ensure it is kept in the best
possible condition.
Presumptive tests exist for the three most common
types of body fluids found at a crime scene: blood,
semen, and saliva.
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved
Chapter Summary
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Mitochondrial DNA is inherited from the mother.
A Y chromosome is inherited by males from their
father. Y-chromosome analysis is beneficial in cases
involving sexual assault, missing persons, and
genealogy.
Most sexual assaults involve a male as the
perpetrator. DNA tests that specifically test the
genetic content on the Y chromosome can be
forensically valuable.
Forensic Science II: Advanced Concepts in DNA, Chapter 10
© 2012 Cengage Learning. All Rights Reserved