chapter 13 rflps, vntrs, strs and pcr
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Chapter 13 RFLPs, VNTRs, STRs and PCR. RFLP restriction fragment length polymorphism has been the workhorse of forensic DNA for many years. - PowerPoint PPT PresentationTRANSCRIPT
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Chapter 13 RFLPs, VNTRs, STRs and PCR
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RFLP restriction fragment length polymorphismhas been the workhorse of forensic DNA for many years
•Alec Jeffries an English geneticist first described RFLP in 1985. RFLP makes use of the fact that certain regions in our genomes have DNA sequences that are repeated over and over again. These regions are known as
• VNTR (Variable Number Tandem Repeats).
Jeffries found that the number of repeats at any particular location differs from person to person. Using restriction enzymes Jeffries was able to cut out the VNTRs and compare their sizes directly. (See Human Heredity text page 291- A Boy of Ghanaian descent)
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Minisatellites are sequences of DNA base pairs, ranging from 14 to 100 nucleotides long that are organized into clusters of tandem repeats, for example the sequence:
CCTTCCCTTCCCTTCCCTTC
Is made up of four tandemly repeated copies of the 5 nucleotide sequenceCCTTC. Such clusters are found on all chromosomes.
There are many different minisatellite sequences, and the number of repeats at each site is variable ranging from 2 to more than 100. Each variant is an allele, and they are inherited codominantly.
These minisatellites are called variable number tandem repeats (VNTRs)
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VNTR’s can be cut with restriction enzymes and visualized with Gel Electro-phoresis
http://ihome.cuhk.edu.hk/~z045513/virtuallab/animation/vntr.html
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VNTR
vs
STR ShortTandemRepeats
STRs are similar to VNTRs, since they both describe regions of DNA where one finds DNA sequences repeated over and over again. STRs are shorter versions of VNTRs. Sometimes only 2 nucleotides repeat in STRs. Both types of repeats are inherited from ones parents.For each locus one comes from the mom and one comes from the dad.
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VNTR
vs
STR ShortTandemRepeats
STRs (are much easier to analyze than VNTRs because they are small and easy to amplify with PCR (Polymerase Chain Reaction).
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What is DNA profiling?
DNA profiling or fingerprinting is the use of molecular genetic methods to determine the exact genotype of a DNA sample in a way that can basically distinguish one human being from another
The unique genotype of each sample is called a DNA profile.
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How do crime scene investigators create a DNA profile?
1. Evidence is collected at the crime scene:
Blood Tissue Semen
Urine HairTeeth Saliva Bone
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2. DNA is extracted from sources at the crime scene and from victim and suspects
How do crime scene investigators create a DNA profile?
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How do crime scene investigators create a DNA profile?
3. DNA samples are processed
Sample Obtained from Crime Scene or
Paternity Investigation Biology
DNAExtraction
DNAExtraction
DNAQuantitation
DNAQuantitation
PCR Amplificationof multiple (STR) markers
PCR Amplificationof multiple (STR) markers
Technology
Separation and Detection of PCR Products
(STR Alleles)
Separation and Detection of PCR Products
(STR Alleles)
Sample Genotype Determination
Sample Genotype Determination
GeneticsComparison of Sample
Genotype to Other Sample Results
If match occurs, comparison of DNA profile to population
databases
Generation of Case Report with Probability of Random
Match
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Since humans are 99.5% identical where do crime scene investigators look for differences in DNA profiles? The remaining 15 million nucleotides determines the difference between people
4. Crime Scene Investigators search in areas of the genome that are unique from individual to individual and are “anonymous” (control no known trait or function)
The areas examined are Short Tandem Repeats or STR’s
STR region
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Example of an STR: TH01
The TH01 locus, found on chromosome #11, contains repeats of TCAT.
CCC TCAT TCAT TCAT TCAT TCAT TCAT AAA
1 2 3 4 5 6
This example has 6 TCAT repeats.
There are more than 20 known TH01 alleles.
Each individual inherits 1 allele from each parent.
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Determining genotypes for individuals using STRs
Mr. Zynda’s TH01 locus for his two chromosomes is given below.
CHROMOSOME from MOM
CCC TCAT TCAT TCAT TCAT TCAT TCAT AAA
CHROMOSOME from DAD
CCC TCAT TCAT TCAT TCAT TCAT TCAT TCAT TCAT TCAT TCAT TCAT TCAT TCAT TCAT AAA
His genotype would be 6*14*
What is his TH01 genotype?
Instead of “A a”we could express it as:
6* 14*
Where *= the number of TCAT repeats
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To determine the genotype (DNA profile) Crime Scene Investigators make billions of copies of the target sequence using PCR (Polymerase Chain Reaction).This done instead of cutting the tandem repeat out with restriction enzymes.
Starting DNA Template
5’
5’
3’
3’
Target DNA
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What’s the
point of
PCR?
• PCR, or the polymerase chain reaction, makes copies of a specific piece of DNA
• PCR allows you to look at one specific piece of DNA by making copies of *only* that piece of DNA
Literary Analogy:• PCR is like photocopying just one page
rather then checking out all the books in a library
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The PCR Reaction
What do you need?
What is needed for PCR?
• Template (containing the STR you want to amplify for the study)
• Sequence-specific primers flanking the target sequence
• Nucleotides (dATP, dCTP, dGTP, dTTP)
• Magnesium chloride (enzyme cofactor)
• Buffer, containing salt• Taq polymerase
5’
3’5’
3’3’ 5’
3’5’
Forward primer
Reverse primer
Target sequence
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The PCR Reaction
How does it work?
Heat (94oC) to denature DNA strands
Cool (52oC) to anneal primers to template
Warm (72oC) to activate Taq polymerase, which extends primers and replicates DNA
Repeat 35 cycles
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Denaturing Template DNA
Heat causes DNA strands to separate
3’
5’
5’
3’
Denaturation of DNA at 94oC
5’
3’
3’
5’
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Annealing Primers
Primers bind to the template sequence
Taq polymerase recognizes 3’ end of primer + template strand
Primers anneal at 52oC
5’
3’
3’
5’
5’
3’
3’
5’
3’5’3’ 5’
3’ 5’3’5’
Taq extends at 72oC
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STR DNA is replicated
Repeat denaturing, annealing, and extending 35 cycles
Cycle 1
Cycle 2
Cycle 3The exact-length target product is made in the third cycle
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http://highered.mcgraw-hill.com/olc/dl/120078/micro15.swf
PCRAnimation
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(14)
(12)
(11)
(9)
(8)
(7)
(6)
(5)
(4)
(3)
(13)
(10)
TH01 alleles
Alleleladder
Mother Father Child C Child D Child E2 points to this slide:
1. A graphic visualization of what the PCR products can look like.
2. In addition to crime scene investigations, STR DNA profiling can also be used to determine familial relationships. The children of any two parents will have a combination of alleles provided by the parents.
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Milestones in Forensic DNA analysis
1985 Alec Jeffries develops RFLP
1990 PCR analysis using single locus STR begins
1992 FBI initiates STR work
1994 DNA Identification Act: provides funding for national DNA database
1995 OJ Simpson trial focuses public attention on DNA evidence
1998 FBI starts CODIS database; Swissair disaster – all remains identified using STR DNA profiling
2001 World Trade Center disaster in NYC – many remains identified using a combination of DNA profiling approaches
2004 Indian Ocean tsunami; Interpol and other world agencies use DNA profiling to identify
victims
Today Trace your Genetic Genealogy; commercially available packages can trace paternal/maternal ancestry
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DNA Testing Today
GeneTree.com & Ancestry.com provide DNA tests from $99-$200 to trace genealogy
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Crime scene investigators use techniques that are fast, cost effective, and have a high Power of Discrimination
The Power of Discrimination is the ability of a test to distinguish between different samples (genotypes)
slow fast
low
high
mtDNA Blood group typing
RFLP analysis
STR analysis
Speed of Analysis
Pow
er
of
Dis
crim
inati
on
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Statistics of Chance: M&M Locus
6 Possible Alleles:• Green• Red• Yellow• Blue• Brown• Orange
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Probabilities• One allele from each parent means 2
copies of gene/locus
1
6
1
6X =
1
36
Frequency of any M&M genotype
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Probabilities
Mike & Ikes: 5 alleles
Jolly Rancher: 5 alleles
1
6
1
6
1
5
1
5
1
5
1
5
1
36
1
25
1
25
Locus
M&M6 alleles
Jolly Rancher5 alleles
Mike & Ikes5 alleles
x =
x =
x =
1
22,500=Chance an individual has a given genotype
1
506,250,000=Chance 2 people have
the same genotype
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Who can’t we exclude from the pool of suspects?
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The Power of Discrimination increases with the number of loci profiled
TPOX genotype 8-12 D3S1358 genotype 16-17
FGA genotype 21-23
0.099
0.050
0.044
0.044 x 0.099 x 0.050 = 2.18 x 10-4
1 / 2.18 x 10-4
or 1 in 4591
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The Power of Discrimination increases with the number of loci profiled
TPOX 8-12 0.044
D3S1358 16-17 0.099
FGA 21-23 0.050
VWA 14-14 0.0088
Random Match Probability
= 1 in 5.3 x 105
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Real-WorldProbabilities • Forensics labs use 13 different loci
with multiple alleles
• Allele frequencies DO NOT follow mathematical principles - allele frequencies vary by population.
• These 13 loci allow for discrimination of any two people in the world (with the exception of identical twins), living or dead.
• Probability of a random match when all 13 loci typed: ~1 in 3 trillion.
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Butler et al 2003 J Forensic Sci. www.cstl.nist.gov/biotech/strbase/pub_pres/Butler2003a.pdf
Allele Caucasians
n=302
African American
n=258
Latinos
n=1405 .002 .004
6 .232 .124 .214
7 .190 .421 .279
8 .084 .194 .096
9 .114 .151 .150
10 .008 .002 .014
11 .002
TH01 Published Allele Frequencies by Population
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CODIS COmbined DNA Index System
A federally maintained database used by law enforcement officials
13 loci guarantees high power of discrimination
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