What is DNA Fingerprinting and how does it work?

Andrea Taktak, Elizabeth Friedl
  • Author
    Andrea Taktak

    Mrs. Taktak is in her 21st year of teaching high school science courses. She has designed curriculum and lessons for Forensic Science and Sports Medicine, and has taught Honors Biology, Anatomy and Physiology, Physical Science, and Environmental Science. Mrs. Taktak is a Master Teacher with a Teacher Leader Endorsement and has a Masters Degree in Education from Graceland University as well as a Bachelors of Science degree from Northern Kentucky University. She also most recently earned her Ohio Certified Volunteer Naturalist certificate while studying and volunteering at the Cincinnati Nature Center.

  • Instructor
    Elizabeth Friedl

    Elizabeth, a Licensed Massage Therapist, has a Master's in Zoology from North Carolina State, one in GIS from Florida State University, and a Bachelor's in Biology from Eastern Michigan University. She has taught college level Physical Science and Biology.

Learn about DNA fingerprinting, including the steps involved. Understand why DNA fingerprinting is more conclusive when proving innocence and examples of its use. Updated: 12/30/2021

Table of Contents


DNA Fingerprinting Definition

Every human being has a unique set of fingerprints which can be used for identification. Historically, this was especially helpful when attempting to solve a crime. Fingerprints detected at the crime scene would be compared and hopefully matched to criminals in a fingerprint database. After the discovery of DNA and along with advances in technology, another method of identification besides fingerprinting was developed called DNA fingerprinting. DNA Fingerprinting can be defined as a method of using DNA to create a unique genetic pattern for every person. For example, if any cells are left behind at a crime scene, DNA can be obtained from those cells and put through a series of steps that will ultimately create a unique pattern for that individual. This unique pattern is what is referred to as the DNA fingerprint. Like traditional fingerprinting, this DNA fingerprint is then compared to a known sample to see if there is a match.

This image demonstrates what a DNA Fingerprint looks like. Observe the bands which create unique patterns.

DNA Fingerprint

Another term that is used synonymously with DNA fingerprinting is DNA profiling. Like DNA fingerprinting, DNA profiling is also a technique that identifies patterns of DNA sequences that are unique to an individual. The unique patterns created from DNA profiling can also be used to identify human remains, determine paternity, or compare two species for genetic similarities.

DNA can be taken from any cell in the body, including liquid tissue such as blood. DNA is often left behind in small amounts at a crime scene, unbeknownst to the criminal. DNA can also be collected from hair follicles if hair has been forcibly removed, from skin cells or saliva left on a cigarette butt, or from fingernail scrapings of a victim.

Although no two people have the same DNA fingerprint (except for identical twins), DNA fingerprinting often works best when eliminating a suspect to prove innocence. If the DNA sample from a suspect does not match the crime scene sample, then it can be reasonably concluded that person was not at the crime scene and thus did not commit the crime. A matching sample, in contrast, simply means that the suspect was present at the crime scene at some point in time and does not prove that the suspect committed the crime.

A great early example of the use of DNA fingerprinting to prove innocence is the case of Kirk Bloodsworth, charged with child rape and murder in 1985 and put on death row. Bloodsworth adamantly maintained that he was innocent, claiming he was charged based on faulty eyewitness testimony. In 1993, Bloodsworth was exonerated by DNA fingerprinting because the drops of semen that were left behind at the scene did not match his DNA. Many other wrongfully convicted persons have been freed based on DNA evidence, which is largely attributed to the Innocence Project that was founded in 1992.

Kirk Bloodsworth was exonerated of the rape and murder of a child because of DNA Fingerprinting.

Kirk Bloodsworth

How are DNA Profiles Stored?

DNA profiles, also called DNA types, are created through the process of gel electrophoresis. Once created, these profiles are photographed and then stored in national databases. In 1994, congress passed the DNA Identification Act which gave the FBI permission to begin storing profiles of criminals (starting with sex offenders) in a database. This database is called the National DNA Index System (NDIS) and has expanded to include all types of criminals and suspects. This is particularly helpful to law enforcement agencies if a suspect crosses state lines.

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  • 0:03 What Is DNA Fingerprinting?
  • 1:07 How it Works
  • 3:09 Uses of DNA Fingerprinting
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DNA Fingerprinting Steps

A very small amount of DNA is needed to create a DNA fingerprint. The following steps explain this process:

Step 1 (DNA Extraction): DNA is isolated from the cells containing it through a process called DNA extraction. During this process, the cell membrane and nuclear membrane will be chemically broken down to allow the DNA to flow out. Additional chemical reactions are performed to isolate and purify the DNA to get it ready for the next step.

Step 2 (PCR): Once the DNA is extracted, it is put through a process known as a Polymerase Chain Reaction (PCR). In this process, the DNA sample is copied many times over until there is an amount that is sufficient to create the DNA fingerprint.

Step 3 (Restriction Enzyme Treatment): The copies of DNA will be cut up or digested using special molecular scissors known as restriction enzymes. These enzymes will cut DNA at specific genetic sequences. Because each person has unique DNA, the places that the restriction enzymes cut will be unique to each person. This will create fragments of DNA that are different sizes. Each person will have fragments of different lengths. These fragments are given the name of Short Tandem Repeats (STRs), referred to as such because most of our DNA is made up of non-coding areas that have repeating sequences. These regions are typically where the restriction enzymes will make their cuts.

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Frequently Asked Questions

What are the four steps of DNA fingerprinting?

DNA fingerprinting has four main steps:

Step 1: DNA extraction from other cellular components.

Step 2: Polymerase chain reaction to amplify the extracted DNA.

Step 3: Restriction enzyme treatment to cut the DNA at specific sequences, resulting in fragments that are unique in size to each person.

Step 4: Gel electrophoresis to separate these DNA fragments by size for comparison.

What is DNA fingerprinting and what is it used for?

DNA fingerprinting is a technique that involves creating a unique pattern of DNA from a crime scene sample and comparing it to potential suspects. DNA fingerprinting began with forensic uses but has more recently been used to determine paternity and ancestry, identify human remains, and compare two species to detect genetic similarities.

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