DNA Fingerprinting

DNA Fingerprinting Also known as DNA profiling Used in criminal and legal cases since

the 1980’s to determine identity or parentage

Also used to identify victims or war and large scale disasters

No two people (except identical twins) have the exact same DNA

History of Biological Evidence in Forensics

Most technologies for analyzing biological evidence were originally developed for medical diagnostic purposes

Ex.; blood typing, chromosome analysis

Structure and Function of DNA DNA is the “blueprint” of life DNA contains the genetic material of

an organism DNA is made up of sugars,

phosphates and nitrogenous bases in a double-stranded molecule (double helix)

DNA Bases DNA bases are adenine (A), cytosine

(C), guanine (G), and thymine (T) Adenine (A) bonds with thymine (T) Guanine (G) bonds with cytosine (C) The order of bases in one strand is

complementary to the order of bases in the other strand Ex.; CGTCTA in one strand matches up to

GCAGAT in the other strand

Chromosomes Structures that contain the genetic

information stored in molecules of DNA

There are 23 pairs of chromosomes in the nucleus of most body cells

One chromosome in each pair is inherited from the mother and the other is inherited from the father

Nuclear DNA vs Mitochondrial DNA

Nuclear DNA – the DNA in chromosomes

It is virtually identical in all cells of the human body

Mitochondrial DNA – found in the mitochondria of cells

Inherited only from the mother

Genes and Alleles Genes – DNA sequences that have

instructions that determine our inherited characteristics

Allele – one of two or more alternative forms of a gene

One allele comes from the mother, one from the father

Human genome – the total amount of DNA in a cell, which is contained in chromosomes and mitochondria

DNA Identification Polymorphisms – differences in DNA

sequences Individuals have unique patterns of repeated base

sequences

DNA Fingerprinting – a technique to isolate and analyze these variable areas

Appears as a pattern of bands on Xray film The unique pattern of repeated base pairs is

analyzed and can be used to identify an individual because each individual’s DNA has a unique band pattern

Examination of DNA Profiles Helps forensic scientists decide if two

or more DNA samples are from the same individual, related individuals or unrelated individuals.

Focus is on two types of repeating DNA sequences

VNTR’s –variable number of tandem repeats STR’s – short tandem repeats

VNTR’s Variable number of tandem repeats –

noncoding sections of DNA that repeat multiple times

Different people can have the same repeated base sequence, but it occurs different amounts of times for each person

Ex.; one person can have it 3 times, another could have it 7 times

Can be from 9 to 80 bases in length

STR’s Short tandem repeats – usually only

two to five pairs in length This is the preferred method of analysis

because of its accuracy and because small and partially degraded DNA samples can be used

DNA Profile Also called a DNA fingerprint Can be developed when several different

VNTR’s and STR’s are examined Two main purposes for DNA fingerprints

1. crime scene matching (same band pattern for suspect and evidence)

2. inheritance matching (each band in a child’s DNA fingerprint must be present in at least one parent)

Population genetics The study of variation in genes among a

group of individuals The proportion of people in a population who have

a particualr characteristic Ex.; very few Asians have blue eyes

Calculations can be made to determine the probability that a random person in the population would have the same allele as the suspect in a crime or an alleged father in a paternity case

Sources of DNA Individual evidence – biological evidence

that contains unique DNA Ex.: saliva, blood, semen, skin, hair

Trace evidence – when the amount of evidence is very small Use of polymerase chain reaction (PCR) technology

allows the small amount of trace DNA to be copied, enabling forensic investigators to perform testing without worrying about using up all the trace evidence

Collection and Preservation of DNA Evidence

1. Wear disposable gloves and change them often. 2. Use disposable instruments for handling each

sample. 3. Avoid touching the area where DNA exists. 4. Avoid talking, sneezing & coughing over evidence. 5. Avoid touching your face, nose & mouth when

collecting and packaging evidences. 6. Air dry evidence thoroughly before packaging. 7. Put evidence into new paper bags or envelopes. 8. If wet evidence cannot be dried, it may be frozen. 9. Keep evidence cool and dry (avoid moisture or

storing it in hot places).

Preparing DNA Samples For Fingerprinting

DNA is mixed with special enzymes that cut the DNA in specific places, forming different sized fragments of DNA.

Using the process of gel electrophoresis, these different sized DNA fragments are separated with a gel and form different band patterns, or fingerprints, within the gel

Steps of DNA Fingerprinting 1. Extraction 2. Restriciton Fragments 3. Amplification 4. Electrophoresis

Steps of DNA Fingerprinting 1. Extraction – DNA is extracted from

the cells. 2. Restriction fragments – restriction

enzymes recognize a unique pattern of DNA bases and will cut the DNA at that specific location.

This forms restriction fragments of varying lengths

Steps of DNA Fingerprinting (continued)

3. Amplification – polymerase chain reaction (PCR) can be used to amplify certain pieces of DNA that contain the VNTR’s

4. Electrophoresis – the method of separating the molecules within an electric field based on the size of the DNA fragments

Electrophoresis (continued) samples are placed at the top of a tray filled

with gel (the gel is the medium through which the DNA travels)

An electric current is passed through the gel which caused the DNA fragments to move

Smaller DNA fragments move farther All the DNA fragments line up in bands along

the length of the gel, giving a “fingerprint”

DNA Probes Used to identify unique sequences in

each person’s DNA Made up of complimentary sequences The probe binds to its complimentary

sequence The probe can be tagged with a

radioactive isotope to reveals the band patterns

Applications of DNA Profiling

Crime-scene DNA matching with suspect’s DNA

Paternity and maternity determination Identify family members and relatives Suspect elimination Freeing those who have been falsely

convicted Identification of human remains

CODIS Combined DNA Index System Electronic database of DNA profiles Consists of 2 separate indexes

Forensic Index – contains DNA profiles from crime scene evidence

Offender Index – contains DNA profiles from sex offenders and violent crime

** as of 2007, FI profiles = 194,785 and OI profiles = 5,070,473!! UNREAL!!! **