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During the process of gene amplification using PCR, if very high temperature is not maintained in the beginning, then which of the following steps of PCR will be affected first?
Denaturation is the very first step of PCR requiring high thermal energy (~94–96 °C) to break inter-strand hydrogen bonds. Without initial high heat, dsDNA cannot melt into ssDNA, so Denaturation fails immediately!
In vitro enzymatic gene amplification relies on cyclic repetition of three sequential, temperature-dependent stages: 1. Denaturation (approx. 94–96 °C): High thermal energy disrupts the weak hydrogen bonds holding complementary purine-pyrimidine base pairs together, unzipping double-stranded DNA (dsDNA) into two single-stranded DNA (ssDNA) templates. 2. Annealing (approx. 50–60 °C): Temperature is lowered to enable synthetic oligonucleotide primers to hybridize specifically to complementary flanking sequences at the ends of ssDNA. 3. Extension (approx. 72 °C): Thermostable *Taq* DNA polymerase (isolated from thermophilic bacterium *Thermus aquaticus*) adds deoxynucleotide triphosphates (dNTPs) onto the primer ends in the direction.
Because denaturation is the initial gatekeeper step requiring maximum thermal energy (~94 °C), failure to reach or maintain high temperature prevents the separation of double-stranded DNA into single strands. Consequently, the primer target sites remain buried within duplex DNA. Hence, Denaturation is the very first step affected.
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Incorrect. Annealing is Step 2 and requires lower temperatures (~50–60 °C). It cannot proceed without preceding denaturation, but it is not the step requiring initial high heat.
Incorrect. Extension is Step 3 operating at 72 °C by Taq polymerase.
Correct. Denaturation is the initial step and specifically demands high heat (~94 °C) to break inter-strand hydrogen bonds.
Incorrect. Ligation is catalyzed by DNA ligase during recombinant plasmid construction and is not a component step of PCR thermocycling.
While annealing and extension ultimately fail if denaturation fails, the question explicitly asks which step is affected *first* in the cycle. Denaturation is directly and immediately paralyzed by lack of initial high heat.
"Denaturation requires high temperature (~94-96 °C) to separate complementary DNA strands before primer annealing."
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