DNA Forensics

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DNA Forensics

• MUPGRET Workshop

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• DNA evidenceoffers prosecutors important new
  tools for the identification and apprehension of
  some of the most violent perpetrators,
  particularly in cases of sexual assault. At the
  same time, DNA aids the search for truth by
  exonerating the innocent. The criminal justice
  system is not infallible. Janet Reno

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Biological Basis

• The same DNA is found in virtually all cells in
  our bodies.
• The four letters of DNA ATCG spell out
  instructions that give each individual their
  unique appearance.
• The arrangement of the letters is different in
  each individual.
• 1 letter per 1000 is different.

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How Does DNA Carry Information?

• To answer this question we must take a closer
  look at DNA.
• DNA is a biopolymer
• Polymers are molecules made of repeating units or
  building blocks
• DNA has four chemical building blocks symbolized
  by the letters A,G,C, T
• The letters of your DNA are in a specific order
  that carries information about you!!
• So, a DNA polymer can be represented as a string
  of letters

A G C T T A G G G T A A A C C C A T A T A
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DNA Carries Information in the Sequence of DNA
Letters
. . .A G C T T A G G G T A A A C C C A T A G . . .
A gene

• A gene is a length of DNA letters that contain
• an instruction for a cell to follow.
• The cell uses specially designed protein
  machines
• to read the information in genes.

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The Order of DNA Letters Encodes the Genetic
Information
The order or sequence of the A, G, C and T
letters in the DNA polymer encodes the actual
genetic information

• Example of the DNA letters in a gene
• AGCTTAGGGTAAACCATATAGGGCCATACCCTATCGGTAAGCTT
• AGCTTAGGGAAAACCCATATAGGGCCATACCCTATCGGTAAG

• The specific order of the DNA letters carries
• the information.
• Changing the order of the DNA letters will
  change the information carried by the gene.
• We will talk about how this happens later!

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Secret of DNA Fingerprinting Lies in the Ability
to Detect Small Differences in DNA Letters Among
Individual Samples

• Look around the room and see how different we all
  look. Then compare any two human genomes
• The DNA letters are almost the identical order
  (sequence) between any two human genomes!
• A very small number (0.1) of the DNA letters
  differ between any two human genomes.
• Two plants that look very
• similar may be close or
• distantly related because
• humans select for desirable
• traits in new varieties.

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Genes Can Have Hundreds to Millions of DNA
Letters
. . .A G C T T A G G G T A A A C C C A T A G . . .
A gene
It can take hundreds, thousands or even a million
or more letters (bases) to spell out the
instructions in a single gene.
and what for?
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Genes Contain Instructions for Building Proteins

• Genes contain instructions for making proteins,
  one of the major types of the molecules of life,
  or biomolecules
• Proteins, like DNA, are polymers
• Protein building blocks are called amino acids
• Amino acids are strung together into long, linear
  polymers by following the instructions in genes
• In general, a gene encodes the instructions for
  one protein
• When a gene is misspelled, the protein made
  from it
• may be made with an incorrect amino acid
• may not work properly

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Genetic Code is Written in 3-Letter DNA Words
(Codons)
-TACCTCATGATTATACA- DNA(DNA strands separated)
-AUGGAGUACUAAUAUGU mRNA (copied from
DNA) 5-AUGGAGUACUAAUAUGU mRNA 5-AUG GAG UAC
UAA UAU mRNA
mRNA code read by ribosome in TANDEM triplets
called codons. Codon adaptors convert RNA
letters into the correct amino acid building
blocks in the protein chain.

• CODON MEANINGS
• A START PROTEIN SIGNAL AUG
• A STOP PROTEIN SIGNAL UAA, UGA, UAG
• An amino acid building block of a protein
• Codons identified in the Genetic Code Table

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The Universal Genetic Code Table
STOP Codons UAA UAG UGA
Name of Building Block Amino Acid
PhePhenylalanine LeuLeucine IleIsoleucine
AUG CODON Signal to start making the protein.
http//anx12.bio.uci.edu/hudel/bs99a/lecture20/le
cture1_6.html
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A DNA Spelling Mistake Can Alter the Protein Chain
START
ADD
ADD
ADD
ADD
ADD
ADD
ADD
STOP
ATG TTC AGG CCA AAT TTT GTC GCG UAA GGA ATT
ATG TTT AGG CCA AAT TTT GTC GCG
TTC to TTT spelling change causes a different
protein building block to be inserted in the
second position. That is all it takes.
ADD Codon specifies the amino acid specified by
3-letter word ATG/AUG Codon specifies start
and methionine (met) UAA STOP adding amino
acids to protein chain
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Every Cell Has a Complete Copy of Genome DNA

• Virtually every cell in your body contains its
  own complete copy of all your DNA

• A single, complete copy of an organisms DNA is
  called its genome

• The genome is a set of instructions, like a
  master plan, written in a molecular language,
  using DNA instead of paper and ink

• Therefore, each cell in your body has a copy of
  your genome, which is, in essence, a master plan
  for making you.

But Most of My Cells Dont Make Melanin-- Right?
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How BIG is 3.2 Billion DNA Letters?
Human Genome

• Human Genome Has 3.2 Billion
• DNA Letters 3,200,000,000 bp
• 3.2 billion (3.2 x 109) is the same as
• 200 (1000 pages each) New York
• City telephone books
• 3 Gigabyte computer hard drive
• a person typing 60 words/minute
• for 8 hours/day, would take more than 50 years to
  type the entire human genome sequence
• placed end-to-end the DNA in one
• human cell extends almost 6 feet

One DNA base-pair
Genome Facts NOVA Online Access Excellence Cell
to Chromosome to DNA
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Our DNA Story
Variations on the Human Theme! People look very
different from each other. Yet we all have
features in common, 2 arms, 2 legs, one head, one
nose, etc.
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Traits are Inherited

• Traits are characteristics that vary among
  individuals.
• Simple trait
• Eye color Blue, brown, green
• Seed coat color
• Complex traits
• Blood types A, B, AB, O
• Plant height.
• Plant disease resistance.
• Connection between traits and genes TRAITS are
  inherited from parents through GENES!

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Genes are Responsible for the Traits You Inherit

• Genes determine
• physical traits and influence personality
• biological characteristics such as blood type
• level of health risk (heart disease, stroke,
  alcoholism, Alzheimers)
• specific genetic diseases (sickle cell,
  hemophilia, cancer, etc.)
• inherited traits that are passed on to your
  biological children
• However the environment always affects the
  result of genetic inheritance.
• Example genes for growth are influenced by
  nutrition
• available during child development

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DNA forensics

• Makes use of the similarities and differences in
  our DNA sequences to determine whether two
  biological samples come from the same individual
  or not.

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VNTR

• Variable number of tandem repeats.
• Type of DNA marker that is unique to each
  individual.
• DNA of about 80 pairs that is repeated many times
  in a row.
• Number of repeats differs between individuals
  different size DNA fragments on a gel.

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VNTR

• By comparing across multiple VNTR genes we can
  get a more accurate test of whether two samples
  are the same or not.

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Probability

• One gene has 20 alleles. The probability that two
  individuals have the same allele is 1/20.
• If two genes each have 20 alleles and two
  individuals have the same alleles at both the
  probability is 1/20 x 1/20 1/400.

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Probability

• People have two alleles for each gene.
• If there are 20 alleles for that gene.
• There are 210 possibile genotypes for that gene.
• The number of possible combinations at 2 genes
  with 20 alleles is 2102, for 4 genes with 20
  alleles is 2104, etc.

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Related 1 4 2 3
1 2 3 4
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Similar DNA Profiles

• Twins from a single egg have identical profiles.
• Siblings, parents, and other blood relatives
  share some alleles in common. The expected
  proportion depends on how closely related the
  pair of individuals are.

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Uses of DNA Profiling

• Victim identification
• September 11
• Srebrenica Massacre
• Missing persons
• Military personnel
• Paternity Testing

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Use of DNA Profiling

• Criminal Testing
• Conviction
• Exoneration
• Excluding suspects
• Identifying protected species

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Do matches establish guilt?

• Not necessarily
• Must have additional evidence
• Chain of evidence must be preserved
• Person may have had reasonable access to the
  crime scene

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Advantages of DNA Evidence

• More sensitive than blood typing
• More informative
• More resilient

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How reliable is DNA profiling?

• Generally, highly reliable.
• Reliability is affected by methods used to
  collect, store and analyze samples.
• Most common problem is contamination or mixture
  of a sample.
• Standards have been established for forensic
  analysis.

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CODIS

• Convicted Offender database
• Contains for 44,000 DNA profiles in the forensic
  evidence section.
• Over 5 million profiles will be entered in the
  next 4 years.
• Contributed to matches in more 6000 cases.

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CODIS

• Who should be included?
• Currently convicted offenders are required to be
  registered in the UK.
• CODIS rules say only convicted criminals after
  conviction can be entered but not all states
  comply with this.
• Some states submit only certain types of
  criminals.

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CODIS

• Some states allow use for all types of crime
  investigations.
• Others only for sex-related or violent crimes.

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Problems with DNA evidence

• Evidence can be degraded if not properly handled.
• Backlogs in processing (16,000 rape cases
  backlogged in NYC).
• Contamination
• Human error

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Other questions

• Should tissue samples be saved or destroyed after
  profiling?
• Can personal or medical information be obtained
  from DNA profiles?
• Why are innocent people convicted of crimes and
  then exonerated by DNA evidence?

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More questions

• Is it possible for an innocent person to be
  convicted based on DNA evidence?
• How has DNA evidence changed the criminal justice
  system?