DNA Fingerprinting
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Direct answer
DNA fingerprinting is a technique that identifies individuals by analysing polymorphic regions of their DNA, chiefly the variable number tandem repeats (VNTRs) of satellite DNA whose repeat numbers differ from person to person. DNA is isolated, cut with restriction enzymes, separated by gel electrophoresis, denatured, transferred to a membrane and hybridised with a labelled probe, and the resulting band pattern is visualised by autoradiography. Since every cell of an individual carries the same DNA and no two unrelated individuals share the same pattern, the method settles forensic and paternity questions definitively.
What you must remember
- The basis: DNA polymorphism — variations in DNA sequence that are inheritable; satellite DNA in non-coding regions shows high variation in repeat number and length between individuals.
- VNTR: variable number tandem repeats; the number of repeats at a locus differs between individuals and between the two homologous chromosomes of one individual, giving bands of different sizes.
- Identity rule: DNA is identical in every cell of a person, so leucocytes, sperm or hair-root cells all yield the same fingerprint; monozygotic twins are the sole exception, sharing the same pattern.
- Steps in order: isolation of DNA; digestion with restriction endonucleases; separation of fragments by agarose gel electrophoresis; denaturation of the separated fragments into single strands; blotting onto a synthetic membrane; hybridisation with a labelled VNTR probe; and autoradiography to reveal the band pattern.
- Sensitivity: the technique works with minute samples, which can be amplified beforehand, making it suitable for old or degraded crime-scene material.
- Applications: identification of criminals in forensic cases such as rape and murder, settling paternity and maternity disputes, and determining population and evolutionary relationships.
- Interpretation: bands shared by child and alleged father, compared against the mother's bands, establish or exclude biological parentage.
Common confusion
DNA fingerprinting is not whole-genome sequencing — it samples only polymorphic marker loci, and the comparison is of band positions, not complete sequences. Students also confuse the probe (a known labelled VNTR sequence used for hybridisation) with the sample DNA, and forget that electrophoresis separates restriction fragments by size, which is precisely why different repeat numbers produce bands at different positions.
Exam-focused takeaway
NEET-UG loves the ordered steps of the technique, especially that restriction enzymes cut the DNA, that electrophoresis separates fragments by size, and that a labelled VNTR probe detects them after blotting. Questions also ask the basis of polymorphism (satellite DNA and VNTR) and the classic applications — crime identification and paternity testing. Assertion-reason stems test the fact that all cells of an individual carry identical DNA and that identical twins share the same fingerprint.
Frequently asked questions
What is the chemical basis of DNA fingerprinting?
DNA polymorphism, particularly the variable number tandem repeats of satellite DNA, whose repeat numbers differ greatly among individuals.
Which enzyme is used to cut DNA in fingerprinting?
Restriction endonucleases, which cleave DNA at specific recognition sequences to produce the fragments later separated by electrophoresis.
What is a VNTR probe?
A labelled, known sequence of tandem repeats used to hybridise with complementary VNTR sequences on the membrane, making the relevant bands detectable.
Why does the same individual give the same fingerprint from any tissue?
Because every somatic cell of an individual contains the same genomic DNA, so the VNTR pattern is identical whether the sample is blood, semen or a hair root.
What are the main applications of DNA fingerprinting?
Forensic identification of suspects in crimes, determining paternity and other biological relationships, and studying genetic diversity and evolutionary links among organisms.