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Chapter 10
Advanced Concepts in DNA
Introduction—Objectives
1.
2.
3.
4.
5.
Discuss the structure and function of DNA.
Explain what causes variation in DNA.
Differentiate between the various types of DNA analysis.
Compare and contrast the methods of extracting DNA.
Explain the proper methods of collecting DNA evidence at a crime
scene.
Introduction—Vocabulary

DNA extraction—process of removing DNA from a cell;
procedures include Chelex extraction, organic extraction, and
differential extraction
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homologous chromosomes—pair of chromosomes in which one
chromosome was inherited from the male parent and the other
was inherited from the female parent
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Nucleotide—subunit of DNA and other nucleic acids; made up of
a 5-carbon sugar, a phosphate group, and a nitrogenous base

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
A 200-Year Old Mystery
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August 13, 1792, the French royal family is arrested and
imprisoned.
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The king and queen were executed.
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The heir lived two more years.
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publicly accessible website, in whole or in part.
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Did he die in prison or escape?
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DNA samples from the remains of the king, queen and suspected
dauphin were compared.
Introduction
o
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
Structure and Function (Obj 10.2)

Nucleotides—subunits of DNA
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Deoxyribose
Phosphate group
Nitrogenous base
• Adenine
• Thymine
• Cytosine
• Guanine

Each nitrogenous base can form hydrogen bonds with a specific
base on a complementary strand of DNA
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Adenine always pairs with thymine
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Guanine always pairs with cytosine
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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
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Genes—sections of DNA that code for a protein
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Proteins catalyze, regulate, and control chemical reactions in the
cell.
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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.
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publicly accessible website, in whole or in part.
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What Causes Variation in DNA? (Obj 10.2)

99.5% of DNA is the same for everyone
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0.5% of DNA is what makes us unique
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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
DNA Analysis Procedures (Obj 10.3, 10.4)
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Method of extraction varies depending upon
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The type of biological evidence analyzed
The amount of evidence available
The type of cells present
Chelex Extraction
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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
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
o
Measuring Quality and Quantity of DNA (Obj 10.3)
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High molecular weight (HMW)—sample contains DNA in large
chunks
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Restriction fragment length polymorphism (RFLP)—DNA analysis
requiring HMW
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Polymerase chain reaction (PCR)—analysis used when only small
amounts of DNA are available, amplification process
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Variable Number of Tandem Repeats (VNTR) —repeating
sections of DNA
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publicly accessible website, in whole or in part.
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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
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
• 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
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
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Easier and quicker to replicate
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Repeating STR patterns are highly variable between individuals;
therefore
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Matching multiple STRs between individuals has a very low
probability
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
Collecting DNA Evidence (Obj 10.5)
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publicly accessible website, in whole or in part.
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Biological samples
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Avoid contamination
Retain original surface when possible
Reference samples for comparative analysis
Stop degradation
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Protect sample from environmental exposure
Remove moisture by drying out and freezing the sample
Collecting Biological Evidence
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Examples
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Skin cells
Blood
Saliva
Semen
Hair
Other possibilites?
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
Collecting Reference Samples
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Known samples taken from suspects and victims
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Reference samples are compared to evidence collected from the
crime scene
Testing for Blood, Semen, & Saliva
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Presumptive tests used at the crime scene
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Luminol is the most common method used to detect blood
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Sprayed on objects to find bloodstains
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Detects blood even when diluted
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Chemicals and fluorescent dyes are used to visualize seminal fluid
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For saliva, the Phadebas test or starch iodine test detects the
presence of amylase
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publicly accessible website, in whole or in part.
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Mitochondrial DNA (mtDNA) Analysis
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mtDNA is inherited from the mother
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Easier to extract than nuclear DNA
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Less likely to degrade
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Each cell has hundreds of copies of mtDNA
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Typically extracted from teeth, hair, bones
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Take care to avoid altering this evidence for other analyses
Y-Chromosome Analysis
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Passed down through paternal lineage
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Beneficial for these situations
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Sexual assault
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Missing person
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Genealogy
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Does not provide individual evidence
Chapter Summary
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The discovery of DNA’s existence, structure, and function dates
back to the 1800s.
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DNA, shaped like a double helix, is made up of a sugar and
phosphate backbone with nucleotides on the inside.
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The four nitrogenous bases are adenine, guanine, cytosine, and
thymine.
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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.
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DNA isolation and extraction from biological evidence such as
blood, semen, and saliva must be done prior to completing a DNA
profile.
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Polymerase chain reaction (PCR) is used to collect and analyze
very minute and degraded samples.
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publicly accessible website, in whole or in part.
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
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.
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The STR pattern, being highly variable from person to person,
provides matches with higher degrees of certainty.
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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.
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When collecting DNA evidence, extra precautions must be taken
to ensure it is kept in the best possible condition.
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Presumptive tests exist for the three most common types of body
fluids found at a crime scene: blood, semen, and saliva.
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Mitochondrial DNA is inherited from the mother.
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A Y chromosome is inherited by males from their father. Ychromosome analysis is beneficial in cases involving sexual
assault, missing persons, and genealogy.
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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.
© 2012 Cengage Learning. All Rights Reserved. May not be scanned, copied or duplicated, or posted to a
publicly accessible website, in whole or in part.
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