What DNA Fingerprinting Actually Is
Your fingerprints are unique — no two people (except identical twins) have the same ridges. DNA fingerprinting exploits a similar idea at the molecular level: certain regions of your DNA have sequences that are repeated, and the number of repeats varies from person to person.
This technique, developed by Alec Jeffreys in 1984 at the University of Leicester, allows us to create a unique “molecular identity card” for every individual. It has revolutionised forensic science, paternity testing, and evolutionary biology.
CBSE Class 12 Biology (Chapter 6 — Molecular Basis of Inheritance) gives 2-3 marks to this topic. NEET frequently asks about the principle, steps, and applications.
The Molecular Basis — VNTRs
The key to DNA fingerprinting is a class of sequences called Variable Number Tandem Repeats (VNTRs), also called satellite DNA or minisatellites.
What are VNTRs? Short DNA sequences (10-60 base pairs) that are repeated in tandem (one after another) at specific locations (loci) in the genome. The core sequence is the same, but the number of repeats at each locus varies between individuals.
For example, at a particular locus:
- Person A might have the sequence AATG repeated 8 times
- Person B might have it repeated 14 times
- Person C might have it repeated 3 times
Since we have two copies of each chromosome (one from each parent), each person has two “alleles” at each VNTR locus — a maternal and a paternal copy with different repeat numbers.
Why are they unique? The probability that two unrelated individuals have the same number of repeats at multiple loci simultaneously is astronomically small — roughly 1 in 30 billion with 10 or more probes.
The Process — Step by Step
Step 1: DNA Extraction
DNA is extracted from any biological sample — blood, semen, saliva, hair follicles, fingernails. The extraction method depends on the sample type (phenol-chloroform, CTAB for plants, etc.).
Step 2: Restriction Digestion
The extracted DNA is cut with restriction endonucleases (restriction enzymes). These are molecular scissors that cut DNA at specific sequences called restriction sites. The VNTR regions, bounded by these sites, are released as fragments of different lengths depending on how many repeats they contain.
Longer repeats → longer fragments. Shorter repeats → shorter fragments.
Step 3: Gel Electrophoresis
The DNA fragments are separated by size using agarose gel electrophoresis. An electric field drives the fragments through the gel — smaller fragments move farther, larger ones move less. This creates a pattern of bands, like a barcode.
Step 4: Southern Blotting
The gel is treated with alkali (NaOH) to denature (separate) the double-stranded DNA into single strands. These single-stranded fragments are then transferred from the fragile gel onto a more durable nitrocellulose membrane or nylon membrane. This transfer is called Southern blotting (named after Edwin Southern, its inventor).
Step 5: Hybridisation with Probes
Radioactively labelled DNA probes — synthetic single-stranded DNA sequences complementary to specific VNTR core sequences — are added to the membrane. These probes hybridise (bind by base pairing) only to their complementary sequences on the membrane.
Modern methods use chemiluminescent or fluorescent probes instead of radioactive ones.
Step 6: Autoradiography
X-ray film is placed over the membrane. Wherever a probe has bound, radioactivity (or chemiluminescence) exposes the film, producing dark bands. The pattern of bands is the DNA fingerprint.
NEET 2022 asked to identify the correct sequence of steps in DNA fingerprinting. The order is: DNA extraction → restriction digestion → gel electrophoresis → Southern blotting → hybridisation → autoradiography. Memorise this sequence.
Key Terms
Restriction endonuclease: Enzyme that cuts DNA at specific palindromic sequences. Example: EcoRI cuts at 5’-GAATTC-3’.
Gel electrophoresis: Separation of DNA fragments by size using an electric field in an agarose matrix.
Southern blotting: Transfer of DNA from gel to nitrocellulose membrane while preserving the spatial arrangement of bands.
Hybridisation: Pairing of complementary single-stranded nucleic acid sequences (probe with target).
Autoradiography: Detection of radioactively labelled molecules by exposure to X-ray film.
VNTR (Variable Number Tandem Repeat): Repeated short DNA sequences whose copy number varies between individuals. The basis of DNA fingerprinting.
Probe: A short, labelled single-stranded DNA sequence used to detect its complementary sequence by hybridisation.
Applications
Forensic Science
DNA from a crime scene (blood stain, hair, semen) is fingerprinted and compared with suspects’ DNA. The pattern of bands either matches or doesn’t — there are no “partial matches.” This evidence is accepted in courts worldwide.
Famous cases: The first criminal conviction using DNA fingerprinting was the Colin Pitchfork case in 1986 in the UK. In India, DNA fingerprinting was used in the Priyadarshini Mattoo case.
Paternity/Maternity Testing
Each person gets half their DNA from their mother and half from their father. In a DNA fingerprint, every band in a child’s pattern must match a band either in the mother’s or the father’s pattern. If the alleged father’s bands cannot account for all the child’s paternal bands, paternity is excluded.
Identification of Missing Persons
DNA from a body can be matched against samples from family members. The National Missing Persons DNA Database uses this for identification.
Wildlife Conservation and Ecology
DNA fingerprinting is used to determine relatedness in wild populations, detect illegal wildlife trade (matching seized animal products to specific poached animals), and study gene flow between populations.
Disease Diagnosis
Certain genetic diseases show characteristic restriction fragment patterns. DNA fingerprinting can diagnose diseases like sickle cell anaemia, cystic fibrosis, and some cancers.
Solved Examples
Example 1 — CBSE Level
Explain why DNA fingerprints of identical twins are the same.
Solution:
Identical twins arise from a single fertilised egg that splits into two embryos. They carry exactly the same DNA sequence, including the same VNTR patterns at all loci. Since DNA fingerprinting identifies differences in VNTR repeat numbers, and twins have none, their fingerprints are indistinguishable by this technique.
This has legal implications — traditional DNA fingerprinting cannot distinguish identical twins. Advanced methods examining somatic mutations or methylation patterns may differentiate them.
Example 2 — NEET Level
A child has a VNTR band pattern: 5, 8, 12, 15. The mother’s pattern is: 5, 12, 17, 20. Which of the bands in the child’s pattern came from the father?
Solution:
Bands 5 and 12 in the child match bands in the mother. Therefore, bands 8 and 15 in the child came from the father. The alleged father must show bands 8 and 15 in his own pattern to be confirmed as the biological father.
Example 3 — Conceptual
Why is it necessary to use Southern blotting before hybridisation?
Solution:
Gel electrophoresis separates DNA fragments, but the gel is fragile and the DNA within it cannot be easily processed. Southern blotting transfers the DNA onto a nitrocellulose/nylon membrane that is chemically robust. The membrane can be handled, dried, stored, and then incubated with probes. Additionally, the denaturation step (NaOH treatment) before blotting converts double-stranded DNA to single strands — only single-stranded DNA can hybridise with probes.
DNA Fingerprinting in India
The Centre for DNA Fingerprinting and Diagnostics (CDFD) in Hyderabad is India’s premier institute for this technology. Lalji Singh (1947-2017), known as the “Father of DNA Fingerprinting in India,” developed the technique’s first Indian application and conducted landmark cases including the Rajiv Gandhi assassination investigation.
CBSE Class 12 sometimes asks about Lalji Singh or CDFD in 1-mark questions. The CDFD is located in Hyderabad. Alec Jeffreys developed the technique in 1984 at Leicester, UK. Keep these facts straight.
Common Mistakes to Avoid
Mistake 1: Saying “DNA fingerprinting sequences DNA.” It does not sequence DNA. It only detects the presence and number of specific repeat sequences using probes. Sequencing would determine the exact base-pair order of the entire genome.
Mistake 2: Confusing Southern blotting with Northern and Western blotting. Southern = DNA, Northern = RNA, Western = Proteins. The “North, South, West” directions are a mnemonic — note that “Eastern” blotting is a later addition for lipids.
Mistake 3: Saying the technique is 100% foolproof. DNA fingerprinting is extremely reliable but can be affected by sample degradation, contamination, and laboratory error. Courts require careful handling protocols and often independent verification.
Mistake 4: Confusing VNTRs with SNPs. Single Nucleotide Polymorphisms (SNPs) are single base differences in the genome, used in genome-wide association studies. VNTRs are tandem repeats — these are used in traditional DNA fingerprinting. Modern forensics also uses STRs (Short Tandem Repeats, like VNTRs but shorter: 2-7 bp per repeat).
Mistake 5: Forgetting that restriction enzymes cut at both flanking sites, not within the VNTR. The enzyme cuts the DNA on either side of the VNTR region, releasing the repeat region as a fragment whose length depends on the number of repeats.
Practice Questions
Q1. What are the components of the VNTR region that make it useful for DNA fingerprinting?
VNTRs consist of a core sequence (10-60 bp) repeated in tandem multiple times. The key features are: (1) the number of repeats varies between individuals (variation), (2) the same loci are present in all individuals of a species (consistency of location), (3) they are inherited in a Mendelian fashion, and (4) they are non-coding so their variation is neutral and accumulates without selection pressure.
Q2. How does a DNA fingerprint help establish paternity?
Every band in a child’s DNA fingerprint must match a band in either the mother’s or father’s fingerprint. Half the child’s VNTR alleles come from the mother and half from the father. If the alleged father’s fingerprint contains bands corresponding to all the paternal bands in the child’s fingerprint, paternity is confirmed with high probability. If paternal bands cannot be accounted for, paternity is excluded.
Q3. Why is it necessary to have DNA probes for specific VNTR sequences?
After gel electrophoresis and Southern blotting, the nitrocellulose membrane contains thousands of DNA fragments of varying sizes. Without probes, you cannot identify which specific bands correspond to VNTR loci. Probes that are complementary to the VNTR core sequence hybridise specifically to only those fragments containing that sequence, allowing selective visualisation of VNTR bands while ignoring the rest.
Q4. DNA from a crime scene is degraded. How does this affect the reliability of DNA fingerprinting?
Degraded DNA may be partially broken or modified. This can: (1) produce faint or missing bands if VNTR-containing fragments are degraded, (2) generate additional bands from non-specific hybridisation, (3) affect the accuracy of band size estimation. Modern techniques like PCR-based STR typing can amplify small amounts of DNA before analysis, improving reliability with degraded samples. Forensic labs use strict quality controls and often require a minimum number of matched loci for identification.
FAQs
Can DNA fingerprinting distinguish between siblings? Siblings share approximately 50% of their DNA, so their fingerprints share many bands but are not identical. The patterns overlap significantly but differ in enough bands to tell them apart. Twins’ fingerprints are identical (for identical twins).
Is DNA fingerprinting admissible in Indian courts? Yes. Indian courts accept DNA fingerprinting as evidence under the Evidence Act, though it is treated as strong scientific evidence rather than absolute proof. Lab accreditation, proper chain of custody, and expert testimony are required.
How many loci are tested in modern forensics? Modern forensic labs use Combined DNA Index System (CODIS) in the USA, which tests 20 STR loci. Indian forensic labs typically test 15-20 loci. The probability of two unrelated individuals matching all loci is less than 1 in a quadrillion.
Does DNA fingerprinting reveal medical information about a person? Traditional VNTR-based fingerprinting tests non-coding regions and reveals little medical information. However, newer genomic techniques do use coding regions. There are ongoing ethical debates about privacy, data retention, and misuse of DNA databases.
What is the difference between DNA fingerprinting and genome sequencing? DNA fingerprinting detects specific VNTR patterns at selected loci — it does NOT read the full genetic code. Genome sequencing determines the exact base-pair sequence of the entire (or a portion of the) genome. Fingerprinting is fast and cheap; sequencing is more informative but expensive.