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Chapter 13
DNA - Deoxyribonucleic Acid
 Discovered in 1868 – at that time is was not
understood what it was or why it was there
 Major breakthrough was made in the 1950’s - (won a
Nobel Peace Prize) by James Watson and Francis Crick
 In 1985, Alec Jeffreys discovered that portions are like a
fingerprint (VNTR – variable number of tandem
repeats) he used RFLP
Gene
 Fundamental unit of heredity
 Instructs body cells from hair color to susceptibility to
diseases
DNA continued…
 Is a polymer linked together by nucleotides
 A – Adenine
 C – Cytosine
 G – Guanine
 T – Thymine
 Base pairing A pairs with T, G pairs with C
 100 million base pairs
DNA at work
 Proteins – made up of amino acids (aa’s) polymers that
play a basic role in the structure and function of living
things.
 Amino acids (aa’s) - are building blocks
 There are 20 known aa’s
Forensic DNA Testing
 Scientists first used:
 RFLP – Restriction Fragment Length Polymorphism
 Quarter sized sample needed for testing
 RFLP DNA typing has the distinction of being the first
scientifically accepted protocol in the US used for the
Forensic characterization of DNA.
Replication of DNA
 PCR – Polymerase Chain Reaction (Rxn)
 Technique for replicating/copying a portion of a DNA
strand outside a living cell.


Sample size no longer a limitation – pin size could be used
Is a more viable DNA typing technique, offers increased
sensitivity.
 Restriction Enzymes (NZM or NZYM)
 Chemicals that act like scissors to cut DNA molecules at
specific locations
DNA replication continued…
 STR – Short Tandem Repeats
 Are locations (loci) on the chromosomes that contain
short sequence elements that repeat themselves within
the DNA molecule throughout the human genome.
 IMPORTANT – repeating sequence is short in length –
3-7 bases – entire strand is fewer than 400 bases in
length.


Much shorter than the strands used for RFLP
Much less susceptible to degradation or may often be
recovered from bodies/stains that have been subjective to
extreme decomposition.
Types of DNA
 Gel Electrophoresis – gel plate, buffer and electric
charge is used to “move” proteins – by hand/see
 Capillary electrophoresis – preferred, easier – by
computer (use 13 core loci for CODIS)
Types of DNA
 Nuclear DNA – tells mother – father
 Commonly used, less expensive
 Mitochondrial DNA – tells mother – child
 Analysis is more sensitive than nuclear DNA profiling
 More rigorous and time consuming and more expensive
vs nuclear DNA
DNA and Trials
 General Acceptance came from paternity cases (techniques
and terminology became more known to judges and juries
alike)
 1987 – Tommie Lee Andrews - First person in US convicted
of a crime base on DNA evidence
 OJ Simpson Trial made it nationally known
 DNA evidence has exonerated people wrongly convicted
and imprisoned for a crime
DNA at a Crime Scene
 “It’s like leaving your name, address, and social
security number at the scene of the crime.”
 1998 – FBI lauches CODIS – Combined DNA Index
System
 Offender - individuals
 Forensic – cases with DNA evidence