Polymerase Chain Reaction
பாலிமரேஸ் சங்கிலி வினை
A single DNA molecule is invisible to a microscope, yet it holds the blueprint of a pathogen. PCR is not just a technique; it is the molecular amplifier that makes the invisible detectable. If you cannot master the three steps of PCR, you are not ready for Biotechnology.
The idea
PCR (Polymerase Chain Reaction) involves three critical steps: Denaturation, Annealing, and Extension. The process uses thermostable DNA polymerase, specifically Taq polymerase, isolated from the bacterium Thermus aquaticus. This enzyme remains active at high temperatures, facilitating the repeated replication of the target DNA segment. Millions of copies are generated in a cyclic process.
Why it matters
It exists to overcome the limitation of low sample quantity, enabling the detection of minute amounts of viral or bacterial DNA.
NEET lens
NEET focuses on the temperature requirements and the source of Taq polymerase. Memorize that denaturation occurs at 94-98°C and extension at 72°C. Do not confuse the steps; the sequence is fixed. Expect questions on why Taq polymerase is essential for the automation of this process.
Clinical significance
PCR is the gold standard for detecting HIV and early-stage cancer mutations. Without PCR, clinicians would be blind to viral loads in the blood during the window period of an infection. It is the difference between early intervention and terminal diagnosis.
Introduction
PCR (polymerase chain reaction) makes millions of copies of a specific DNA segment in a test tube (in vitro), using a heat-stable DNA polymerase.
Core concept
Requirements: template DNA, two sets of primers (small chemically synthesised oligonucleotides complementary to the regions of DNA to be copied), deoxynucleotides and a thermostable DNA polymerase. Steps (one cycle): (1) Denaturation — high temperature separates the two DNA strands; (2) Annealing — primers bind to complementary regions of the single strands; (3) Extension — DNA polymerase extends the primers using the nucleotides. Repeated many times (around 30 cycles), the segment is amplified to approximately a billion times (2^n copies after n cycles). The thermostable DNA polymerase, Taq polymerase, is isolated from the bacterium Thermus aquaticus and remains active during the high temperature used for denaturation. The amplified fragment can be ligated with a vector for further cloning.
Go deeper
PCR is used in molecular diagnosis: it can detect very low amounts of a pathogen's DNA (e.g. HIV in suspected AIDS patients) or mutations in genes of suspected cancer patients, before symptoms appear.
From Class 10
PCR is like a photocopier for DNA — it can make billions of copies of one small piece of DNA in a few hours.