Q.Causative agents of HIV-AIDS and COVID-19 belong to the same group of viruses. To diagnose and amplify the genetic material for further study of COVID-19 virus, ‘RT-PCR’ test is carried out.

Concept: PCR (Polymerase Chain Reaction) — a technique to amplify specific DNA sequences.

(a) RT-PCR stands for Reverse Transcription Polymerase Chain Reaction.

(b) The PCR technique involves three repeated steps:

1. Denaturation: The reaction mixture is heated to ~94–98°C to separate the double-stranded DNA into single strands.
2. Annealing: The temperature is lowered to ~50–65°C, allowing short primers to bind (anneal) to complementary sequences on each single-stranded template.
3. Extension: The temperature is raised to ~72°C, and Taq DNA polymerase extends the primers by adding nucleotides, synthesizing new complementary strands. …

The Core Concept

The question connects two major viral diseases — HIV-AIDS and COVID-19 — by pointing out that their causative agents (HIV and SARS-CoV-2) are both RNA viruses. This is the key: because their genetic material is RNA, not DNA, you cannot directly use standard PCR (which works only on DNA). You first need to reverse transcribe the RNA into complementary DNA (cDNA). That is exactly what the "RT" in RT-PCR does.

Once you have cDNA, the "PCR" part takes over — a beautifully elegant cycling process that makes millions of copies of a specific DNA segment in just a couple of hours. Understanding PCR is not about memorising steps; it is about seeing how temperature control mimics what happens inside a cell during DNA replication, but in a tube.

(a) What RT-PCR Stands For

RT-PCR stands for Reverse Transcription Polymerase Chain Reaction.

(b) The Steps of PCR Technique

PCR is a cyclic process. Each cycle roughly doubles the amount of target DNA. After 25–35 cycles, you have billions of copies. Here is how one cycle works, broken into its three temperature-controlled steps.

1. Denaturation (at ~94–98°C)

The double-stranded DNA (your template) is heated to near-boiling. This breaks the hydrogen bonds between complementary base pairs, separating the two strands into single-stranded DNA. Think of it as unzipping a zipper.

2. Annealing (at ~50–65°C)

The reaction is cooled down. This allows short, synthetic DNA fragments called primers to bind (anneal) to their complementary sequences on the single-stranded template DNA. Primers are designed to flank the target region you want to amplify — they define the boundaries of the copied segment.

The annealing temperature is critical: too high, and primers won't bind; too low, and they bind to the wrong places (non-specific binding), giving you junk.

3. Extension (at ~72°C)

This is the copying step. A special heat-stable DNA polymerase — most famously Taq polymerase (from the thermophilic bacterium Thermus aquaticus) — attaches to each primer and starts adding nucleotides (dNTPs) to build a new complementary strand. It works best at around 72°C. The enzyme moves along the template, extending the primer until it either falls off or reaches the end of the template. …