4. What is DNA?
What does DNA stand for?
Deoxyribose Nucleic Acid or Deoxyribonucleic Acid
What does DNA do?
• DNA contains genetic information.
• DNA codes for the proteins our
bodies make that are necessary for
survival.
5. What is DNA?
DNA is a code for making proteins
AGC TAG CTT ATA CTC TAT CTC TTT
Amino Amino Amino Amino Amino Amino
Acid Acid Acid Acid Acid Acid
The order of amino acids determines
what type of protein is made.
6. What is DNA?
Some common proteins are:
Hemoglobin - carries oxygen from lungs to cells
Insulin - regulates metabolism
Many types of enzymes - catalyze reactions in the
body, such as the breakdown of sugar for energy
DNA also determines how much of these
proteins each cell makes.
7. What is DNA?
What does DNA look like?
Double Helix
Like a Twisted Ladder
8. What is DNA?
What does DNA look like?
Sugar Phosphate
Backbone
(Sides of Ladder)
Nitrogenous
Base
(Rungs of Ladder)
9. What is DNA?
The DNA ladder is made up of
building blocks called nucleotides.
What is a nucleotide?
Adenine
Phosphate Group Cytosine
Base Guanine
Thymine
Deoxyribose sugar
10. The 4 Bases
A C
Adenine Cytosine
G T
Guanine Thymine
12. The 4 Bases
The bases pair up
to form the rungs
of the ladder.
A pairs with T
G pairs with C
13. What is DNA?
DNA is written as the sequence of these
bases:
AAGTCGATCGATCATCGATCATACGT
• Only one side of the ladder is written.
• In humans, there are three billion (3,000,000,000)
base pairs (letters) in the DNA within each cell.
14. What is DNA?
Among humans, most of the 3 billion bases
in the DNA sequence are exactly the same.
• Our Human DNA is 99.8% similar to each
other, but the 0.2% difference is more than
enough to distinguish us from one another.
• Human DNA is even 98% similar to
chimpanzees.
• NO TWO PEOPLE HAVE IDENTICAL DNA*
*except identical twins
15. What is DNA?
Stupid Facts:
• If two different people started reciting their
individual genetic code at a rate of one letter per
second, it would take almost eight and a half
minutes before they reached a difference.
• If unwound and tied together, the strands of DNA
in one cell would stretch almost six feet but would
be only 50 trillionths of an inch wide.
• If all the DNA in your body was put end to end, it
would reach to the sun and back over 600 times
(100 trillion times six feet divided by 92 million
miles).
17. Where is DNA?
DNA is found in the cells in our body.
Nucleus
(Brain of the cell)
Mitochondria
(more later)
18. Where is DNA?
All types of cells in our body contain a
copy of the same DNA.
Some cells important to forensic science are:
White Blood Cell Sperm Cell Cheek Cell
20. Where is DNA?
Chromosomes
come in pairs
(one from Mother)
(one from Father)
There are 46
chromosomes in
each cell.
(23 pairs)
21. Where is DNA?
What are sources of DNA at a crime scene?
DNA can be recovered from any
substance that contains cells.
• Blood • Bone
• Semen • Teeth
• Saliva • Hair
• Tissue • Maggot Crops
24. How does DNA differ among humans?
DNA is a sequence of 4 possible letters
A G C T
Of the 3 billion letters, 99.8% of the
sequence in all humans is identical.
There are several ways the sequence can be different.
25. How does DNA differ among individuals?
1. One of the bases (letters) can be different.
Person 1 AGCTAGATCGTTATTCCGAG
Person 2 AGCTAGATCGTCATTCCGAG
26. How does DNA differ among individuals?
2. Bases (letters) can be added or
removed.
Person 1 AGCTAGATCGTTATTCCGAG
Person 2 AGCTAGATCGTATTCCGAG
Person 3 AGCTAGATCGTTTATTCCGAG
Person 4 AGCTCCGAG
27. How does DNA differ among individuals?
2. Bases (letters) can be added or removed.
Person 1 AGCTAGATCGTTATTCCGAG
Person 2 AGCTAGATCGTATTCCGAG
Person 3 AGCTAGATCGTTTATTCCGAG
Person 4 AGCTCCGAG
28. How does DNA differ among individuals?
3. Regions of DNA can be repeated a different # of times
Person 1 ..GCCAGCTAGCTAGCTAGCTAGCTAGCTTTCAT..
29. How does DNA differ among individuals?
3. Regions of DNA can be repeated a different # of times
Person 1 ..GCCAGCTAGCTAGCTAGCTAGCTAGCTTTCAT..
1 2 3 4 5 6
Person 2 ..GCCAGCTAGCTAGCTAGCTAGCTTTCAT..
1 2 3 4 5
Person 3 ..GCCAGCTAGCTAGCTAGCTAGCTAGCTAGCTT..
1 2 3 4 5 6
31. Forensic DNA Analysis
Collection of Evidence
Types of Unknown Samples:
• Blood, Semen, Stains, Saliva
• Hair, Tissue, Bones, Teeth
Types of Known Samples:
• Blood or buccal swabs from suspect
or victim or other known person
32. Forensic DNA Analysis
Beware of Contamination
Contamination occurs when DNA from another source gets
mixed in with the sample being collected.
• An investigator touches, sneezes, bleeds on a sample.
• Wear gloves and use disposable instruments
• Package items separately.
• Especially, do not mix known samples (from victim or
suspect) with unknown samples.
33. Forensic DNA Analysis
Packaging Evidence
• Package each item individually.
• Put evidence into paper bags, not plastic.
• Moisture degrades DNA; air dry samples.
• Keep samples at room temperature and out
of sun.
34. Forensic DNA Analysis
Brief History of DNA- (1985)
Multilocus
RFLP
Detects VNTRs:
Variable Number of
Tandem Repeats
35. Forensic DNA Analysis
Brief History of DNA(Late 80s, Early 90s)
Single locus RFLP
D2S44 probe
Lanes 6 and 10 match
Lanes 8 and 11 match
36. Forensic DNA Analysis
Brief History of DNA(Early 90s)
PCR Strips (DQ alpha)
6 Alleles:
1.1, 1.2, 1.3
2, 3 or 4
A person can
have one or
two of these
numbers.
37. Forensic DNA Analysis
Two main types of analyses (90s - Present) :
Short Tandem Repeats (STRs)
• Individual identification possible
• Samples: Blood stains, semen
Mitochondrial DNA
• Used in cases of severely degraded DNA
• Individual identification not possible
• Samples: Bones, hair shafts
38. Forensic DNA Analysis
Short Tandem Repeats (STRs)
• Currently the most used of all forensic markers
• Individual identification possible
• Type of data used in the FBI CODIS database
• People differ in length at these loci
• Are located in the nuclear DNA (chromosomes)
39. Forensic DNA Analysis
Short Tandem Repeats (STRs)
Regions of DNA can be repeated a different # of times
Person 1 ..GCCAGCTAGCTAGCTAGCTAGCTAGCTTTCAT..
1 2 3 4 5 6
Person 2 ..GCCAGCTAGCTAGCTAGCTAGCTTTCAT..
1 2 3 4 5
Person 3 ..GCCAGCTAGCTAGCTAGCTAGCTAGCTAGCTT..
1 2 3 4 5 6
40. Forensic DNA Analysis
Short Tandem Repeats (STRs)
Locus or Loci:
Refers to the location on the chromosome.
Allele:
Refers to the type of DNA.
For STRs, the allele will be the number of repeats.
CCAGATAGATAGATAGATAGATAGATAGATAGATAGATCC
41. Forensic DNA Analysis
Example
Locus: D5S818
Alleles: 7,9
Paternal chromosome 5
CCAGATAGATAGATAGATAGATAGATAGATCC
Maternal chromosome 5
CCAGATAGATAGATAGATAGATAGATAGATAGATAGATCC
43. Forensic DNA Analysis
Basic Steps in Analysis
Extraction:
• Separates DNA from sample
Amplification or PCR:
• Amplifies small portions of DNA (STR regions)
Separation:
• Separates amplified fragments according to size.
45. Forensic DNA Analysis
Basic Steps in Analysis
Extraction:
• Separates DNA from sample
Amplification or PCR:
• Amplifies small portions of DNA (STR regions)
Separation:
• Separates amplified fragments according to size.
47. Forensic DNA Analysis
Basic Steps in Analysis
Extraction:
• Separates DNA from sample
Amplification or PCR:
• Amplifies small portions of DNA (STR regions)
Separation:
• Separates amplified fragments according to size.
59. Forensic DNA (mitochondria)
Mitochondrial DNA
Mitochondrial DNA is
only 16,569 letters long.
(compared to 3 billion
in nuclear DNA)
There is a 900 base pair
region with a 1.7%
difference (D loop).
60. Forensic DNA (mitochondria)
Nuclear DNA vs. Mitochondrial DNA
Double Helix Double Helix
46 Chromosomes One Ring
Multiple copies in
One copy per cell each mitochondria
Multiple mitochondria in
each cell
MtDNA used for old or degraded samples
61. Forensic DNA (Mitochondria)
For nuclear DNA: Length is measured
For mtDNA: Sequence is examined
Different colored peaks correspond to a different base
62. Forensic DNA Analysis
Basic Steps in Analysis
Extraction:
• Separates DNA from sample
Amplification or PCR:
• Amplifies small portion of mtDNA (D loop)
Sequencing:
• Sequence is determined by another reaction
and separation of sequenced fragments
63. Forensic DNA (Mitochondria)
DNA Sequences are compared to each other.
Hair found on
Suspect
AGCTAGATCGTTATTCCGAG
Victim AGCTAGATCGTTATTCCGAG
Conclusion: Hair may have come from the victim.
64. Forensic DNA (Mitochondria)
DNA Sequences are compared to each other.
Hair found on
Suspect
AGCTAGATTGTTATTCCGAG
Victim AGCTAGATCGTTATTCCGAG
Conclusion: Hair did not come from the victim
65. Forensic DNA (Mitochondria)
Cigarette butt
at crime scene
AGCTAGATTGTTATTCCGAG
Suspect #1 AGCTAGATCGTTATTCCGAG
Suspect #2 AGCTAGATTGTTATTCCGAG
Suspect #3 AGCTTGATTGTTATTCCGAG
Suspect #4 AGCTAGATTGTTATTCCGAG
Conclusion: Cigarette could be from Suspects #2, #4
or other person with the same sequence.
66. DNA and Statistics
The final result is presented as a statistic.
Do not say:
“The DNA in the bloodstain is John Doe’s DNA.”
Do Say:
“The chance that another person has this DNA
in the bloodstain is 1 in 300 billion.”
67. DNA and Statistics
Where do the statistics come from?
First, the frequency of each allele is estimated
using data from a population data base.
Allele frequency
Locus: D5S818 from database
7 26%
Alleles: 7,9 9 11%
68. DNA and Statistics
Where do the statistics come from?
Next, the frequency of the genotype at
each locus is calculated.
Locus: D5S818 Genotype frequency
7,9 6%
Alleles: 7,9
69. DNA and Statistics
For total frequency, multiply all of the
frequencies together.
D5 = 6%
D8 = 12%
D18 = 0.5%
Total = 0.004%
