Biochemical Forensic Analysis: DNA
Transcript of Biochemical Forensic Analysis: DNA
Biochemical Forensic
Analysis: DNA
The Cell
• The smallest unit of life
• The nucleus is the
“brain” of the cell
– contains all the genetic
info the cell needs to
exist & to reproduce
• In most types of cells,
genetic information is
organized into structures
called chromosomes
Chromosomes
• In most types of cells, genetic information is organized into structures called chromosomes– usually X shaped
• Y chromosome in males
– 23 pairs in humans• one from mother & one
from father
Genes
• Each chromosome contains
hundreds to thousands
information blocks called
genes
• Each gene is the
blueprint for a specific type
of protein in the body
– only identical twins will have all
the genes identical
Chromosomes
• Each chromosome is a single polymeric molecule called DNA– if fully extended the
molecule would be about 1.7 meters long
– unwrapping all the DNA in all your cells
• cover the distance from earth to moon 6,000 times
Structure of DNA
Nucleotides
• DNA is a polymer
built from monomers
called nucleotides
• Each nucleotide is
consists of
– deoxyribose
• pentose sugar
– phosphoric acid
– a nitrogenous base
The DNA Double Helix
• DNA is normally a
double stranded
macromolecule
• Two polynucleotide
chains are held
together by H-
bonding
– A always pairs with T
– C always pairs with G
5’ T-T-G-A-C-T-A-T-C-C-A-G-A-T-C 3’
3’ A-A-C-T-G-A-T-A-G-G-T-C-T-A-G 5’
In a double helix the strands go in opposite directions
Functions of DNA
• Two Functions
– To transmit information from one generation
of cells to the next
– To provide the information for the synthesis of
components (proteins) necessary for cellular
function
Cell Types
Where can DNA be found?
CellBlood
Sweat
Hair RootsSaliva
Various Tissue
Semen
SAME
Nuclear DNA
Where are the types of DNA
found in a cell?
Mitochondrial DNACell
Sources of Biological Evidence
• Blood
• Semen
• Saliva
• Urine
• Hair
• Teeth
• Bone
• Tissue
Types of objects where DNA may
be found
Blood Stains
Semen Stains
Chewing Gum
Stamps & Envelopes
Penile Swabs
Plant Material
Sweaty Clothing
Bone
Hair
Fingernail Scraping
Saliva
Animal Material
Where DNA Evidence is Found
Blood
Hair Roots
Saliva
Sweat
Tissue
Chemical
DNA
Isolation of DNA
DNA
Solution
Amplification(making copies)
G
T
A
G
A
A T
C
A
T
C
T
Heat
Step one of a single cycle
DENATURE
T
Step two of a single cycle
ANNEAL
Step three of a single cycle
T
EXTEND
1 Cycle2 Cycles
3 Cycles
4 Cycles
5 Cycles
28 Cycles
Amplification
DNA
PCR (Polymerase Chain Reaction)
Analysis of amplified DNA
Amplified
DNA
DNA
Profile
Brief History of Forensic DNA Typing
• 1980 - Ray White describes first polymorphic RFLP marker
• 1985 - Alec Jeffreys discovers multilocus VNTR probes
• 1985 - first paper on PCR
• 1988 - FBI starts DNA casework
• 1991 - first STR paper
• 1995 - FSS starts UK DNA database
• 1996 – First mtDNA case
• 1998 - FBI launches CoDIS database
DNA Use in Forensic Cases
• Most are rape cases or murders
• Looking for match between
evidence and suspect
• Must compare victim’s DNA profile
•Mixtures must be resolved
•DNA is often degraded
•Inhibitors to PCR are often present
Challenges
Human Identity Testing
• Forensic cases -- matching suspect with
evidence
• Paternity testing -- identifying father
• Historical investigations-Czar Nicholas,
Jesse James
• Missing persons investigations
• Mass disasters -- putting pieces back together
• Military DNA “dog tag”
• Convicted felon DNA databases
Blood
Hair Roots
Saliva
Sweat
Tissue
Chemical
DNA
Extraction of DNA
RFLP Analysis
• Enzymes break DNA into restriction fragments
• Measurements taken of fragments that vary in length across people (length polymorphism) because they contain VNTRs
• can produce extremely low random match probabilities
• requires relatively large fresh samples (>50 ng DNA)
• slow and expensive
Which Suspect, A
or B, cannot be
excluded from the
class of potential
perpetrators of
this assault?
DNA Fingerprinting
• The basic structure of everyone’s DNA is the
same
– the difference between people is the ordering of the
base pairs
• Every person can be distinguished by the
sequence of their base pairs
– millions of base pairs make this impractical
– a shorter method uses repeating patterns that are
present in DNA
VNTR’s
• DNA strands contain information which directs
an organism’s development
– exons
• Also contain stretches which appear to provide
no relevant genetic information
– introns
– repeated sequences of base pairs
• Variable Number Tandem Repeats (VNTRs)
• can contain anywhere from 20 to 200 base pairs
VNTRs
• All humans have some VNTRs
• VNTRs come from the genetic information
donated by parents
– can have VNTRs from mother, father or a
combination
D1 = biological daughter of both parents
D2 = child of mother & former husband
S1 = couple’s biological son
S2 = adopted son
VNTR Analysis
• Usually an individual will inherit a different variant of the repeated sequence from each parent
VNTR Analysis
• PCR primers bracket the locus
• PCR reaction forms a nucleotide chain from the
template
VNTR Analysis
• The length of the amplified DNA & its position after electrophoresis will depend on the number or repeated bases in the sequence
Analysis used 3 different VNTR loci for each suspect giving
6 bands
Although some individuals have several bands in common,
the overall pattern is distinctive for each
Suspects A & C can be eliminated
B remains a suspect
• NOVA Exercise
• https://ny.pbslearningmedia.org/resource/t
dc02.sci.life.gen.creatednafingerprint/creat
e-a-dna-fingerprint/#.WOJAUG_yvcs
• NY TIMES piece on DNA Use
• https://www.nytimes.com/video/us/100000
002886783/how-dna-changed-the-world-
of-forensics.html
Mitochondrial DNA
What is mtDNA
Typing?
Database and
statistical issues
A Mitochondrial Exclusion
A Mitochondrial Inclusion
Innocence Project
• https://www.youtube.com/watch?v=Czl2rf2
lZfA
The Future of Forensic
DNA
CODIS
SNP’s & Chips
FBI’s CODIS DNA Database
Combined DNA Index System
• Used for linking serial crimes and
unsolved cases with repeat offenders
• Launched October 1998
• Links all 50 states
• Requires >4 RFLP markers
and/or 13 core STR markers
• Current backlog of >600,000 samples
13 CODIS Core STR Loci
with Chromosomal Positions
CSF1PO
D5S818
D21S11
TH01
TPOX
D13S317
D7S820
D16S539 D18S51
D8S1179
D3S1358
FGA
VWA
AMEL
AMEL
STR Analysis by Hybridization
on Microchips