PCR

PCR

Radioactive label not incorporated in PCR is

In DNA polymerization, α-phosphate of deoxynucleoside triphosphate is incorporated into growing phosphodiester backbone, while β and γ phosphates are released as pyrophosphate driving reaction forward. Therefore α-32P-dNTP results in internally labeled products detectable by autoradiography, and 5' end-labeling uses γ-32P-ATP via polynucleotide kinase to label primer terminus. Gamma-32P-labeled dNTP cannot be internally incorporated because γ-phosphate is not retained in final strand; polymerase requires cleavage between α and β phosphates. Consequently, experiments requiring internal radioactive tracing cannot utilize gamma-labeled precursors, distinguishing end-labeling from incorporation strategies in labeling techniques.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

PCR

Primer-dimer leads to

Primer-dimer formation occurs when primers anneal to each other due to 3' complementarity and are extended by polymerase, generating short spurious products. These dimers consume limited primers, dNTPs, and polymerase activity, reducing availability for genuine target amplification. On agarose gels they appear as diffuse low-molecular-weight bands below 100 base pairs, and in real-time PCR they elevate background fluorescence and produce non-specific melting peaks. High primer concentration, low annealing temperature, and poor design exacerbate phenomenon, leading to failed assays and misleading quantification.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

PCR

Which method can amplify a small DNA amount?

Among routine molecular techniques, PCR uniquely amplifies specific DNA sequences from minute starting material through exponential replication, achieving detectable quantities from single copies. ELISA detects proteins via antibody-antigen interactions without nucleic acid amplification. Northern blot and FISH involve hybridization for detection but lack exponential synthesis, requiring microgram amounts of RNA or DNA. Hence forensic samples, ancient DNA, viral loads, and biopsy specimens are analyzable only by PCR, including conventional, nested, and quantitative real-time variants that drastically enhance sensitivity and enable diagnostics and research.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

PCR

Thermal cycler is used for

Polymerase chain reaction demands rapid, precise transitions between denaturation near 94°C, annealing at 50-65°C, and extension at 72°C for 30-40 cycles. Thermal cycler uses Peltier elements and engineered aluminum blocks to achieve ramp rates of several degrees per second, ensuring uniform heating and cooling of microliter volumes in tubes. This automation eliminates manual water bath transfers, improves reproducibility, and enables hot-lid technology to prevent evaporation. It does not perform electrophoresis or single-step incubation, but orchestrates complete temperature profile required for exponential DNA amplification.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

PCR

Buffer in PCR helps to

PCR buffer typically contains Tris-HCl, potassium chloride, and magnesium chloride adjusted to pH 8.3-8.8 at 25°C, which shifts to approximately 7.2 at extension temperature due to temperature coefficient of Tris. This system stabilizes pH against proton changes from dNTP hydrolysis, maintains ionic strength required for enzyme activity, and supports polymerase conformation. It does not hydrolyze DNA, dissolve template, or cool reaction, as thermal cycling is mediated by instrument block. Stable pH environment maximizes enzyme processivity, fidelity, and reproducibility across cycles.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

PCR

Which PCR component gives specificity?

Specificity in polymerase chain reaction arises primarily from sequence complementarity between primers and template. Each primer of 18-22 bases is designed to anneal uniquely to flanking regions of target locus under stringent annealing temperatures, thereby defining start and end of amplicon. dNTPs are generic building blocks, polymerase catalyzes extension indiscriminately, and template provides coding information but does not select region. Well-designed primers with unique sequences, appropriate GC content, and avoidance of repeats prevent mispriming, non-specific bands, and genomic cross-reactivity, ensuring specific amplification.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

PCR

Taq polymerase lacks

Taq polymerase retains 5' to 3' polymerase activity necessary for nucleotide incorporation and 5' to 3' exonuclease activity useful for nick translation, but lacks 3' to 5' exonuclease function that mediates proofreading. This absence means polymerase cannot reverse and remove incorrectly paired bases at the growing 3' terminus, leading to fixed mutations in amplicons. The structural basis is absence of exonuclease domain. Knowledge of domain architecture explains error spectrum and rationale for blending Taq with proofreading enzymes for long PCR.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

PCR

Which DNA Pol has proofreading activity?

DNA polymerases vary in fidelity due to presence of 3' to 5' exonuclease domain. Tli polymerase, also called Vent polymerase from Thermococcus litoralis, possesses intrinsic proofreading activity, excising mispaired nucleotides and increasing accuracy approximately 5-10 fold over Taq. Taq from Thermus aquaticus and Tth from Thermus thermophilus lack proofreading, exhibiting higher error rates. For high-fidelity PCR, cloning, mutagenesis, and sequencing applications requiring accurate replication, proofreading enzymes like Tli, Pfu, and Q5 are preferred despite slower extension rates and higher cost.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

PCR

For primer Tm = 4(G+C)+2(A+T), Tm of 3 GC and 4 AT is

The Wallace rule approximates melting temperature as Tm = 4(G+C) + 2(A+T), where GC contributes three hydrogen bonds and AT contributes two, providing higher thermal stability for GC pairs. For a primer composition containing three GC and four AT bases, energetic contribution from GC equals 4×3 =12°C and from AT equals 2×4=8°C. Summation yields 20°C, representing temperature at which half of duplex dissociates into single strands. Although modern nearest-neighbor thermodynamic algorithms provide greater accuracy for longer oligos, this formula remains fundamental for NEET-type quick calculations and introductory primer design exercises taught academically.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

PCR

In primer design, self-complementarity results in

Self-complementarity in primers, particularly at 3' termini, enables undesirable interaction between two primer molecules or identical molecules, generating primer-dimer artifacts. During annealing, complementary bases pair transiently, and polymerase extends each primer using the other as template, creating short double-stranded products that accumulate exponentially and compete aggressively with target amplicon for reagents. This reduces overall yield, generates low-molecular-weight bands on gels, and interferes with quantitative assays like real-time PCR and melt curve analysis. Effective design avoids self-complementary stretches and evaluates 3' complementarity scores using specialized software.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

PCR

Radioactive primer labelling is best done with

Radioactive labeling of primers for detection in Sanger sequencing, footprinting, or probe tracking is best achieved by transferring γ-phosphate of [γ-32P]ATP to 5' hydroxyl terminus of oligonucleotide using T4 polynucleotide kinase, yielding 5' 32P-labeled primer. This end-label does not interfere with polymerase extension from 3' end. Alpha-32P labeled deoxynucleotides incorporate internally via polymerase synthesis but not suitable for pre-synthesized primer labeling. Beta and generic notations are not used. Five-prime labeling provides high specific activity enabling autoradiographic detection with superior sensitivity.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details

PCR

Cycle number in PCR is generally

Exponential amplification follows 2^n where n is cycle number. Typically 30 to 40 cycles generate 10⁹-fold amplification, converting trace template into detectable product visible on agarose gel. Fewer than 25 cycles may produce insufficient yield, especially for low-copy targets. Beyond 40 cycles, reaction reaches plateau due to depletion of dNTPs, primers, polymerase inactivation, and accumulation of inhibitors, increasing risk of non-specific products, primer-dimers, and background smearing. Therefore 25-35 cycles for abundant templates and 30-40 cycles for rare templates balances sensitivity with specificity in analytical PCR.

Ref: NCERT Biology Class XII Principles on Klenow fill-in labeling, Lehninger Chapter 9 DNA cloning techniques, and Molecular Cloning by Sambrook Chapter 10 documenting end-labeling of cohesive termini.

View Details