Q.(a) Expand VNTR and describe its role in DNA fingerprinting.

Step 1 — The starting material must be obtained first

Before anything else can be done, DNA has to be extracted from the biological sample. This is the isolation of DNA (d), which must come first.

Step 2 — The isolated DNA is cut into fragments

Once DNA is available, it is treated with specific restriction endonucleases, which cut the DNA at defined sequences into fragments of varying lengths — this is step (a), and it can only happen after the DNA is isolated.

Step 3 — The fragments are separated by size

The resulting mixture of differently sized fragments is then resolved using gel electrophoresis (e), which separates fragments according to their size — this requires the fragments from step (a) to already exist.

Step 4 — The separated fragments are transferred to a membrane …

Step 1 — What DNA fingerprinting actually compares.

$99.9\%$ of human DNA is identical between any two people. Fingerprinting therefore targets the small fraction that varies a lot — repetitive DNA.

Step 2 — Satellite DNA and VNTRs.

In a density-gradient centrifugation, bulk DNA gives a major peak plus small satellite peaks — these are the highly repetitive sequences. A particular class, minisatellites / VNTRs (Variable Number of Tandem Repeats), consists of a short sequence repeated in tandem, and the number of repeats differs from person to person (it is inherited from the parents). That copy-number polymorphism is what makes the pattern individual-specific (except in identical twins).

Step 3 — The workflow (Alec Jeffreys' method). …

Step 1 — Why PCR needs a special polymerase

One PCR cycle has three steps:

1. Denaturation — heat to about $94^\circ$C to separate the two DNA strands.
2. Annealing — cool to about $50\text{–}60^\circ$C so the two primers bind.
3. Extension — hold at about $72^\circ$C while DNA polymerase extends the primers.

An ordinary (e.g. E. coli) DNA polymerase would be denatured and destroyed at $94^\circ$C, so fresh enzyme would have to be added every cycle. The whole technique only becomes practical with a thermostable polymerase.

Step 2 — Identify the source organism

Thermus aquaticus is a thermophilic bacterium living in hot springs. Its DNA polymerase — Taq polymerase — remains active at the high temperatures of PCR, so the same enzyme survives all 30–40 cycles. …

Step 1 — What "early diagnosis" demands. A method that can flag the infection while the pathogen is still present in very low concentration, i.e. before the immune response or clinical symptoms have developed.

Step 2 — How PCR delivers that. The Polymerase Chain Reaction repeatedly copies a target stretch of nucleic acid (denaturation → primer annealing → extension by Taq polymerase), doubling it each cycle:

So even a handful of bacterial/viral genome copies is amplified to a detectable amount. If the pathogen's nucleic acid is detected, the person is infected. (For RNA viruses, reverse transcription first makes cDNA — RT-PCR.) NCERT's Biotechnology and its Applications explicitly gives PCR as the routine method for early detection of bacterial and viral infections, including HIV in suspected AIDS patients.

Step 3 — Why the others don't fit. …