DNA Fingerprinting: Modern Day Sherlock Holmes

Do you remember watching crime TV shows when you were a kid? Remember how in CID whenever a crime would take place the detectives would search for fingerprints. This approach has been employed by several detectives for many years. Even Sherlock Holmes and other famous detectives would gather fingerprints from the crime site and match them with fingerprints of suspects. However, this technique is not very useful in the modern world. This is simply because individuals who are committing crimes can wear gloves that prevent them from leaving fingerprints on any surfaces if they touch them by chance. Some criminals can even burn their thumbprints to avoid leaving fingerprints at the crime scenes and hence it becomes difficult for detectives to catch them. Let’s have a look at the Principle of DNA Fingerprinting, a new technique adopted in crime issues. 

DNA Fingerprinting

To begin with, the advent of DNA fingerprinting by Alec Jeffreys in 1984 made things easier. DNA fingerprinting is a technique in which just a small amount of DNA of any individual can be taken from the crime scene in the form of hair follicles, blood, sperm etc. This DNA sample can then be analysed to generate a unique fingerprint of any individual. Whenever a crime takes place, even small micrograms of DNA that are left behind at the crime can be used. In this scenario, the detective takes the samples to the DNA fingerprinting lab and with the help of this technique, a fingerprint of that individual is generated.  

 This technique was developed by Alec Jeffreys in 1984. In India, this technique was started by Lalji Singh.

Another key point is DNA samples of all the suspects of the crime can be taken and their DNA fingerprint is matched with the DNA which was found at the crime scene. Hence this technique allows the modern-day Sherlock to find the killer and prove him/her guilty.

Image source: newsclick. in

Principle of DNA Fingerprinting

Let’s talk about the principle of DNA fingerprinting.

How exactly are we able to create fingerprints of every individual that is unique to him/ her?

The principle of DNA fingerprinting is based on DNA polymorphism. 99.9% of the DNA of every individual is the same. Only 0.1% of the DNA of every individual is different i.e. unique to him/her. In DNA fingerprinting we deal with this 0.1% of DNA. When we obtain the DNA sample from the crime scene or any other situation, we analyse it by density centrifugation. Two peaks are seen after centrifugation; the majority of the DNA is termed bulk DNA and maintains the higher peak. The 0.1 % of DNA that is unique to every individual maintains the lower peak. The majority of this 0.1% DNA is repetitive DNA and is termed Satellite DNA.

The Satellite DNA can be divided into minisatellite and microsatellite. This differentiation is based on three key factors:-

• The number of tandem repeats
• Length of DNA fragment
• A: G and C: G ratio

Image Source: youtube.com

Minisatellite DNA shows a high degree of polymorphism. Polymorphism (also called variation at the genetic level) arises due to mutations. We can count the number of repeats and observe that every chromosome has a unique number of repeats. 

Moreover, in DNA fingerprinting we consider VNTRs (variable number of tandem repeats); they belong to minisatellite DNA. As the name indicates VNTR have a variable number of repetitive DNA on every chromosome that shows a high degree of polymorphism.

How is DNA Fingerprinting done?

Let us find out how exactly DNA fingerprinting is done after we have received the DNA sample.

• In the first step we take the DNA sample in the form of the hair follicle, blood, sperm, bone etc. we first use enzymes to digest all the other components of the cell: plasma, organelles, cell wall etc. we only have DNA left in the test tube now.
• With the help of chilled ethanol, we take out the DNA. This is called spooling.
• After taking the DNA out we put it through the PCR machine (polymerase chain reaction). This machine helps us to create several copies of the DNA so that we can easily put them through the rest of the procedure without losing the precious DNA that we have found at the crime scene.
• After the PCR is done and several DNA fragments have been obtained, those fragments are cut into smaller pieces with the help of restriction endonuclease enzymes.

After having several fragments of DNA these fragments are passed on to the gel electrophoresis machine. In the gel electrophoresis, machine agarose gel is present and electrodes are connected with a positive and a negative terminal. Since the DNA is negatively charged due to the presence of the phosphate group it moves toward the positive charge. However, due to the presence of agarose gel movement of this DNA, it is dependent upon the size of the DNA fragments. Hence we separate the DNA based on their size.

Image Source: youtube.com

Steps to visualize

• Then these DNA fragments are stained in ethidium bromide and passed through UV rays to obtain orange bands.
• The DNA fragments are now mounted on the nitrocellulose membrane. This is called southern blotting. 
• DNA that we have obtained now on the nitrocellulose membrane needs to be used to make up a DNA fingerprint. We use probes for this method. Probes are radiolabelled single-stranded DNA molecules that have the required coding sequence that we need to see in the DNA. For example, we already know the entire sequence of the VNTR that is present on the chromosome. We will use radiolabeled probes of such VNTR sequences.
• Furthermore, these VNTRs are put into the same solution as the single-stranded  DNA fragments on the nitrocellulose membrane. Radiolabelled VNTR Probes bind to them to form double-stranded vntr sequences. We put the nitrocellulose membrane through the radiographic film. This is called Auto-radiography.
• After this, we can observe the VNTRs on the radiographic film. A distinct unique pattern of every individual can be obtained based on the number of VNTR that he or she has on his or her chromosome. See the below image to see patterns of different individuals.

We can easily see Suspect 3 is the Criminal. Image Source: youtube.com

Thus, we can easily find out who the criminal is by comparing the DNA fingerprint that we have found at the crime scene versus the DNA of every suspect in the crime. 

Limitations of DNA Fingerprinting

There are several limitations to DNA fingerprinting as well. For example, if an individual has a twin brother or sister then DNA fingerprinting will not work for it because monozygotic (identical) twins have the same number of VNTR and hence both will have the same DNA fingerprint. 

Applications

To summarize, the most important application of DNA fingerprinting is in crime scenarios. Thus, it is used in forensic science and is extremely useful for modern-day detectives. DNA fingerprinting can help individuals track their lineage and their ancestors as well. It plays a key role in determining population and genetic diversity.

DNA Fingerprinting also plays a key role in parental disputes. We can confirm with this technique who the real parents of any individual are. Hence it is very helpful in will quarrels and other civil disputes.

Thank you for reading Biology Detectives!

Team MBD

Want to pursue a career by using the principle of DNA fingerprinting? M.Sc. Forensic Science (du.ac.in)  Check out!

What is DNA? Find out- Did you know our DNA is 99.9% same? – My Biology Dictionary