Polymerase Chain Reaction
பாலிமரேஸ் சங்கிலி வினை
Stop assuming DNA replication happens only inside a cell. PCR proves we can force billions of copies in a test tube. If you confuse 'in vitro' with 'in vivo', you lose marks on the simplest questions.
The idea
PCR is an in vitro technique to amplify a specific gene segment. It requires genomic DNA as a template, two sets of primers, and a thermostable DNA polymerase, specifically Taq polymerase. The process involves three steps: denaturation at 94°C, annealing of primers, and extension using dNTPs. Taq polymerase, isolated from Thermus aquaticus, remains active at high temperatures, ensuring the reaction continues without needing fresh enzyme addition.
Why it matters
It allows for the amplification of a specific DNA segment to billions of copies, making it easier to study or clone.
NEET lens
NEET focuses on the three steps and the source of Taq polymerase. Memorize the temperature cycles and the role of primers. Traps involve confusing the direction of synthesis or the enzyme's origin. Expect at least one question on the necessity of thermostable enzymes.
Clinical significance
PCR is the diagnostic gold standard for detecting viral loads like HIV or SARS-CoV-2 in blood samples. Without this amplification, low-titer pathogens would remain undetected by standard assays. Failure of PCR precision leads to false-negative results, directly impacting patient prognosis.
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.