DNA Profiling in Forensic Science

Introduction

DNA Profiling is the analysis of DNA samples to determine if they came from the same individual.

Since its introduction in 1986, DNA profiling has become a standard tool of forensics.

Introduction

DNA Profiling can be used to… Identify murder victims

Prove/disprove an presence at a crime scene

Solve paternity disputes

Protect endangered species by proving the origin of contraband animal products

Study DNA from fossilized plants and animals, even mummified humans

DNA Profiling Techniques

In forensic applications DNA from two or more samples needs to be compared (i.e. Does the DNA collected at the crime scene match the suspect’s DNA?)

To compare DNA samples two different techniques may be used…

Short Tandem Repeat (STR) Analysis

Restriction Fragment Length Polymorphism (RFLP) Analysis

STR Analysis

STR compares a small number (usually 13) of short segments of noncoding DNA (sections of DNA that do not code for protein)

The segments used for comparison are called short tandem repeats (STRs)

STR Analysis

Short tandem repeats consists of short sequences of nucleotides that are repeated many times tandemly.

STR Analysis

Short tandem repeats consists of short sequences of nucleotides that are repeated many times tandemly.

STR Analysis

Short tandem repeats consists of short sequences of nucleotides that are repeated many times tandemly.

STR Analysis

Short tandem repeats consists of short sequences of nucleotides that are repeated many times tandemly.

STR Analysis

We typically have the same sequences (i.e. AGAT or GATA) at the same locations but unrelated individuals vary widely in the number of repeats.  

The likelihood that any two individuals (except identical twins) will have the same 13-loci DNA profile can be as high as 1 in 1 billion

or greater.

STR Analysis

To create a DNA profile, forensic scientists use polymerase chain reaction (PCR) to amplify the regions of DNA that include the STR sites.

STR Analysis

After amplification via PCR, the DNA profiles can be compared using gel electrophoresis

STR Analysis

Recall that electrophoresis separates DNA fragments based on size, thus any differences in the number of repeats at the STR sites can be observed in the gel.

RFLP Analysis

RFLP analysis works by comparing DNA samples that have been exposed to a restriction enzyme.

The restriction enzyme cuts the DNA into fragments

RFLP Analysis

The fragments of DNA produced by the restriction enzyme can then be separated and compared using gel electrophoresis