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

CODIS

• https://www.youtube.com/watch?v=ga1Mx

mwniJ0