DNA Fingerprinting

DNA Fingerprinting

Applications• To link a suspect to a crime scene and for criminal exonerations.

O.J. Simpson and Colin Pitchfork

• I.D. Missing persons or Dead Person.

• Link a suspect to an event.

Clinton – Lewinsky Scandal

• Paternity Cases.

DNA Fingerprinting Applications

DNA fingerprinting is routinely used to test samples of blood, hair root, saliva and other bodily fluids.

DNA Fingerprinting

Why is DNA an Indispensable Tool in Forensics Science?

Study into the structure of the human genome has ledto the discovery that portions of DNA structure ofcertain genes are as unique to each individual as fingerprints.

What is DNA?

In 1953 James Watson & Francis Crick discovered the structureDeoxyribonucleic Acid – DNA.

It is found in the nucleus of all living cells, with the exception of Red Blood Cells.

Double Helix Structure

What is DNA?

• A person has 46 chromosomes, over 100,000 genes, 3 billion bases of DNA.

• Each leg of the ladder in the double helix consists of alternating sugar and phosphate units.

• The nucleotide bases consist of the following:

Adenine (A) which pairs with

Cytosine (C) which pairs with

Thymine (T)

Guanine (G)

What is DNA?

Replication of a single strand of DNA

A T A G C C A A C C A A

T A T C G G T T G G T T

What is DNA?

The order of the bases in a molecule of DNA provide instructions used to build proteins.

A piece of DNA that contains all the information needed to build one protein is called a gene.

What is DNA Fingerprinting? In 1985 an English geneticist, Alec Jeffreys, was

studying inherited variation within genes and among individuals.

He studied a genetic peculiarity known as the intron. Introns are sequences of “junk” DNA that do not code for a specific protein.

He noticed that some introns are made up of the same DNA base pair sequences and that the number of repetitions varies from person to person.

DNA Fingerprinting Techniques

• Restriction Fragment Length Polymorphisms – RFLP are the repetitive intron fragments

RFLP

• How RFLP works:

• DNA from several individuals was “cut” with a restriction enzyme.

• The “cut” DNA is sorted by size.

RFPL animation

DNA Fingerprinting Techniques

Disadvantages of RFLP• Expensive.

• Requires a large sample size.

RFLP (continued)

• Time consuming.

• Labor intensive

Advantages of RFLP

• Very confident in results

example:

Using RFLP on a sample ofblood from the O.J. Simpson murder trial said the chancessomeone other than Simpsonleft their DNA at the crimescene was 1 in 6 Billion.

DNA Fingerprinting Techniques

PCR

• Polymerase Chain Reaction – PCR.

• A technique used to “copy” DNA.

• Can be used with minute amounts of DNA.

• Uses DNA’s natural replication process to multiple sample size.

PCR animation

DNA Fingerprinting Techniques

PCR (continued)

Disadvantages of PCR

• Results are not as impressiveas RFLP.

Advantages of PCR

• Small sample size needed.

DNA Fingerprinting

The Power of DNA Fingerprinting

Several drops of O.J. Simpson’s blood were foundat the Nicole Brown-Simpson and Ronald Goldmancrime scene. Both RFLP and PCR tests were performed. Here are theofficial results:

PCR (3 Drops of Blood were tested)

1/240,000 chance blood belonged to someone other than O.J. Simpson.

Drop #1: 1/170 Million chance blood belonged to someone other than O.J. Simpson.

RFLP (2 Drops of Blood were tested)

Drop #2: 1/57 Billion chance blood belonged to someone other than O.J. Simpson.

Source: Criminalistics, An Intro to Forensics Science, Richard Saferstrein

DNA Fingerprinting

A picture of DNA fingerprinting

DNA Fingerprinting

Mitochondrial DNA

• Another structure in the cell that contains DNA is the mitochondria.

• Each cell contains thousands ofmitochondria, each containing severalLoops of DNA.• Unlike nuclear DNA, this DNA is onlyinherited from your mother.

• This makes any individual with the samematernal lineage indistinguishable.

DNA Fingerprinting

How DNA is used to determine Paternity

• DNA fingerprinting can be used to determine a child’s parents.

• We know you receive 50% of you nuclear DNA from mom, andthe other 50% from dad.

DNA Fingerprinting

Interpret the results of the following DNA fingerprints

•Both daughter 1 and son 1 sharegenes with mom and dad, whiledaughter 2 has genes of mom butnot the dad, and son 2 does notshare genes with either parent.

What is CODIS?

• CODIS is an acronym for:

Combined DNA Index System

• Administered by the FBI, system which stores DNA ofcertain convicted felons.