How is DNA fingerprinting done in forensics?

In one line

The rule

DNA fingerprinting is a technique that compares specific regions of DNA, called variable number tandem repeats (VNTRs), to find unique patterns for each individual.

There is no single formula, but the process uses the fact that certain DNA sequences repeat a different number of times in each person.

Let's understand it simply

Imagine you are at a cricket match and everyone is wearing a jersey with their own number. Even if two people look similar, their jersey numbers are different. DNA fingerprinting works like reading these jersey numbers, but inside your cells. Your DNA has special sections that repeat, like a song chorus. The number of repeats is different for each person, just like everyone’s ringtone can be different. Scientists "read" these repeats to make a pattern, which is almost always unique to you.

How it works

1. Collect a sample: Take a tiny amount of blood, hair, saliva, or skin from the crime scene.
2. Extract DNA: Use chemicals to break open the cells and pull out the DNA.
3. Cut DNA into pieces: Add special enzymes (restriction enzymes) that act like scissors and cut the DNA at certain spots.
4. Separate pieces by size: Place the cut DNA on a gel and pass electricity through it (gel electrophoresis). Smaller pieces move faster, so they spread out.
5. Transfer to a membrane: Move the DNA from the gel to a nylon sheet to make it easier to study (Southern blotting).
6. Add DNA probes: Add small pieces of DNA with a label (like a glow-in-the-dark sticker) that stick only to the repeating sections.
7. Visualize the pattern: The labeled sections show up as bands (lines). The pattern of bands is your DNA fingerprint.
8. Compare patterns: Match the pattern from the crime scene with the suspect’s DNA. If they match, the DNA is from the same person.

The story behind it

DNA fingerprinting was discovered by Sir Alec Jeffreys in 1984 in England. He was studying how DNA varies between people and found that some parts of DNA are highly variable. One day, he saw that these variations could make a unique pattern for each person. The first real case solved using DNA fingerprinting was in 1986, helping to catch a criminal and free an innocent man. Jeffreys’ discovery changed forensic science forever.

Where you see it in real life

• Crime scenes: Police collect hair or blood to find out who was present.
• Paternity tests: To check if a man is the biological father of a child.
• Identifying disaster victims: After a train or plane accident, DNA helps identify bodies.
• Wildlife protection: To catch poachers by matching animal DNA from seized products.
• Missing person cases: Comparing DNA from relatives to find missing people.
• Sports doping: To check if athletes are using someone else’s urine or blood.

Why it matters

• Solves crimes: Helps catch real criminals and free innocent people.
• Proves family relationships: Used in courts for inheritance and custody cases.
• Disaster management: Identifies bodies when other methods fail.
• Protects endangered animals: Tracks illegal hunting and trading.
• Medical research: Finds genetic diseases in families.

Worked example

Let’s say a blood stain is found at a crime scene. Police have a suspect.

Given: Blood stain at crime scene, blood sample from suspect
Step 1: Extract DNA from both samples
Step 2: Cut DNA with restriction enzymes
Step 3: Run both samples on gel electrophoresis
Step 4: Transfer DNA to nylon membrane
Step 5: Add labeled DNA probes to both samples
Step 6: Visualize band patterns for both samples
Step 7: Compare the band patterns
Step 8: If the patterns match, the blood is from the suspect

Common mistakes

• Thinking all DNA is compared: Only specific repeating regions are used, not the whole DNA.
• Believing DNA fingerprints are 100% unique: Identical twins have the same DNA fingerprint, but everyone else’s is unique.
• Assuming it is instant: The process takes hours or days, not minutes.
• Mixing up DNA fingerprinting and regular fingerprints: DNA fingerprinting uses genetic material, not the patterns on your fingers.

Remember it

DNA fingerprinting is like your Aadhaar card inside your cells—unique and hard to fake.