Question
What is DNA fingerprinting? Explain the role of VNTRs and STRs in this technique. Describe the steps involved and list its forensic and medical applications.
(NEET 2022, similar pattern)
Solution — Step by Step
DNA fingerprinting is a technique to identify individuals based on their unique repetitive DNA sequences. Just as no two people have the same fingerprints, no two individuals (except identical twins) have the same pattern of DNA repeats.
Developed by Alec Jeffreys in 1984.
VNTRs (Variable Number Tandem Repeats): Sequences of 10-100 base pairs repeated in tandem. The number of repeats varies between individuals. Used in the original DNA fingerprinting (Southern blot-based).
STRs (Short Tandem Repeats): Sequences of 2-6 base pairs repeated in tandem. Preferred in modern forensics because they can be amplified by PCR from tiny samples.
Both VNTRs and STRs are inherited — you get one set from each parent. The combination of repeat numbers across multiple loci creates a profile unique to each individual.
- DNA extraction from the sample (blood, saliva, hair root, semen)
- Restriction digestion — cut DNA with restriction enzymes that flank the VNTR/STR regions
- Gel electrophoresis — separate fragments by size
- Southern blotting — transfer DNA bands from gel to a nylon membrane
- Hybridisation with a radioactive or fluorescent probe (complementary to the VNTR sequence)
- Autoradiography — expose to X-ray film to visualise the banding pattern
Modern method (STR-based): Steps 2-6 are replaced by PCR amplification of specific STR loci followed by capillary electrophoresis. Much faster and works with nanogram quantities of DNA.
Forensic: Matching suspect DNA to crime scene evidence (blood, hair, skin cells)
Paternity/maternity disputes: Child’s DNA bands must match one band from each parent
Immigration cases: Proving biological relationships
Identifying disaster victims: Matching remains with family DNA
Evolutionary studies: Comparing DNA profiles across species
Why This Works
The power of DNA fingerprinting lies in the extreme variability of repeat numbers. At a single VNTR locus, there might be 30 different alleles in a population. When you examine 10-15 loci simultaneously, the probability of two unrelated individuals having the same profile is less than 1 in a billion.
Each person inherits one allele (repeat number) from their mother and one from their father at each locus. That’s why a child’s DNA fingerprint shows bands that match one parent or the other — never a band that neither parent has.
Alternative Method — PCR-Based STR Profiling
For NEET, the NCERT focuses on the VNTR/Southern blot method. But know that modern forensics uses PCR-based STR profiling — it’s faster, works with degraded samples, and needs far less DNA. If NEET asks about “modern DNA fingerprinting,” PCR + STR is the answer. The CODIS database (used by FBI) profiles 20 STR loci.
Common Mistake
Students confuse VNTRs with satellite DNA. While VNTRs are a type of satellite DNA (specifically minisatellites), not all satellite DNA is used in fingerprinting. Satellite DNA is a broader category that includes highly repetitive sequences found near centromeres. VNTRs and STRs are specific subtypes chosen because of their high variability between individuals. In NEET, be precise — say “VNTR” or “STR,” not just “satellite DNA.”