DNA fingerprinting | How is DNA fingerprinting used to identify a criminal?

What is DNA Fingerprinting?

DNA fingerprinting is also known as DNA profiling is a process to determine an individual identity from a sample of DNA by looking at unique patterns in their DNA.

DNA fingerprinting is a forensic technique used for criminal investigation. It is also used in paternity testing in case of disputes. DNA fingerprinting is mostly used in medical research for the diagnosis of inherited diseases.

Some important facts about DNA fingerprinting

  • The Human genome contains approx 3 billion base pairs.
  • DNA fingerprinting was by invented by Sir, Alec Jeffreys in 1984.
  • Pretty much every cell in our body contains our DNA
  • On average, about 99.9% of the base sequence among human beings are the same.
  • The remaining 0.1% of the base sequence is non-coding and unique among human beings. That’s makes possible to distinguish one individual from another, unless they are an identical twin.
  • DNA fingerprinting uses a repetitive sequence that is variably known as Variable number tandem repeats (VNTRs).
  • In specific short tandem repeats (STRs) is known as microsatellites. Microsatellites are short sequence contains 10-60 base pairs long of repetitive DNA.
  • In India DNA fingerprinting was first started by Dr VK Kashyap and Dr Lalji Singh. Lalji Singh is popularly known as the ” Father of Indian DNA fingerprinting”.

How is DNA fingerprinting used to identify a criminal?

A DNA profile can produce by Traditional techniques as well as modern-day techniques.

Let’s discuss it one by one.

DNA profile produced by Traditional Techniques

Step-1. Extract DNA from Sample

DNA is extracted from a sample such as blood, Saliva.

Step-2. Break DNA into Small fragments

Extracted DNA is further cut into small fragments by using the Restriction endonuclease enzyme. Restriction endonuclease is also known as molecular scissors. Restriction endonuclease enzymes cut DNA at their specific site (Pallandrom sequence).
An example of a palindrome sequence is
5/–G A A T T C–3/
3/–C T T A A G–5/

Step-3. Separating the DNA fragments.

DNA fragments run on agarose gel electrophoresis to separate DNA from small fragments. DNA has a negative charge due to the negative charge of its phosphate components because of negative charge moves towards a positive charge on a gel plate. Low molecular weight DNA moves faster. After that, the gel is observed under the ultraviolet chamber. In-gel there is a chemical dye used called “Ethidium bromide” which sticks with DNA fragments that make the DNA bands is visible under ultraviolet light.

Step-4. Transferring DNA onto Nitrocellulose paper.

The gel separated DNA fragments were further transferred to white nitrocellulose paper. The nitrocellulose paper now carries an exact replica of the DNA on the gel known as Southern blotting.

Step -5. Adding Radioactive probe.

The probes are small fragments of DNA labelled with radioactive phosphorus. The probes are only attached to the DNA that is complementary.

Step-6. Setting up X-ray film

In a dark room, nitrocellulose paper is placed against a piece of x-ray film. The x-ray film recorded the pattern of radioactivity on the paper. when x-ray film is exposed to radioactive a pattern of more than 30 dark bands appear on the film where the labelled DNA. This pattern is DNA fingerprinting. Therefore, to compare sample DNA fingerprint with suspect sample all the suspect’s samples run side by side on the same gel electrophoresis.

DNA-fingerprinting steps
Illustration Showing the steps in DNA fingerprinting [Image credit: Genome Research Limited]

DNA profile produced by Modern-day techniques:

We have to take DNA samples from a crime scene that are usually degraded because of a number of reasons. such as environmental exposures are a usually common cause. sometimes an only a tiny amount of samples can be found in a crime scene such as a hair sample. A very small amount of DNA is present in one hair sample. It is almost impossible to DNA profiling accurately with a small amount of DNA. With the invention of Modern-day Polymerase chain reaction (PCR), all the cases can solve accurately in modern-day DNA Profile techniques. Let’s discuss the modern-day techniques of DNA profile step by step-

Step-1. Extraction of DNA

DNA is extracted from a biological sample that is taken from a crime scene. In modern-day techniques, it only requires a tiny amount of someone’s DNA to produce accurate results. Samples including blood, saliva, hair.

Step-2. Polymerase Chain Reaction (PCR)

The modern-day techniques of DNA fingerprinting method, do not use restriction enzyme cut DNA. Instead, it uses PCR. PCR is an automated process that generates lots of the same copies of the DNA sequence. It only requires a tiny amount of DNA sample, even it can make copies of DNA that are partially degraded.
In PCR small sequence of DNA is called primer. Primer bind to the complementary sequence of the DNA of interest.
The primer for each short tandem repeats (STR) is labelled with a specific coloured fluorescent tag. This makes it easier to identify and record the STR sequence after PCR.

Step-3. Electrophoresis

Once enough copies of the sequence are produced by PCR, electrophoresis is used to separate the DNA according to its size.

Step-4. Visualisation

Each fragment of DNA sequence passes by a laser which causes the fragments with a fluorescent tag to glow with a specific colour. The output is displayed as a series of coloured peaks ( As shown in the figure below).

DNA profiling Steps
Illustration Showing in the steps of DNA profiling [Image credit: Genome Research Limited]

Therefore, a match made between the crime scene DNA profile and an individual profile identifies a possible suspect.

DNA fingerprinting matching
Illustration Showing DNA fingerprint matching [Image credit: Genome Research Limited]

To make more clear about DNA fingerprinting Steps watch this animation link below

https://www.youtube.com/watch?v=eWMlB9xchTQ