Introduction
Molecular diagnostics is one of the fastest-growing and most technologically advanced segments in the in vitro diagnostics (IVD) field. By detecting and amplifying specific nucleic acid sequences (DNA or RNA), it enables the identification of pathogens, genetic mutations, and gene expression patterns with unparalleled sensitivity and specificity. From COVID-19 testing during the pandemic to oncology companion diagnostics, from infectious disease detection to genetic screening, molecular diagnostics has become an indispensable tool in modern medicine. A high-quality molecular diagnostic reagent system — whether PCR, qPCR, or RT-PCR — heavily depends on the proper selection and combination of enzymes (DNA polymerases, reverse transcriptases), nucleotides (dNTPs), buffers (Tris, KCl, Mg²⁺), stabilizers (BSA, trehalose), chelating agents (EDTA), and preservatives (ProClin 300). This article, as the general outline of the molecular diagnostics raw material series, systematically introduces the core position of molecular diagnostics in IVD, the three major PCR technology platforms, a panorama of core raw materials, and general principles for raw material selection, laying the foundation for subsequent in-depth platform-specific analyses.
I. Core Position of Molecular Diagnostics in IVD
1.1 Market Position of Molecular Diagnostics
Molecular diagnostics is one of the fastest-growing segments in the IVD market. According to industry data, molecular diagnostics accounts for approximately 15-20% of the global IVD market, with a compound annual growth rate (CAGR) significantly higher than clinical chemistry and immunoassay. The COVID-19 pandemic dramatically accelerated the adoption of molecular diagnostic technologies, and the demand continues to grow for infectious disease testing, oncology, genetic screening, and personalized medicine. Key drivers include:
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Pandemic response and infectious disease control: Demand for rapid and accurate detection of emerging pathogens (influenza, RSV, COVID-19, mpox)
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Oncology and personalized medicine: Companion diagnostics for targeted therapies (EGFR, KRAS, BRAF mutations)
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Genetic and rare disease screening: Newborn screening, carrier testing, prenatal diagnosis
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Antimicrobial resistance monitoring: Detection of drug-resistant bacteria and resistance genes
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Technological advancements: Automation, sample-to-answer systems, and point-of-care molecular testing
1.2 Major Technology Platforms in Molecular Diagnostics
| Technology Platform | Principle | Key Applications | Advantages | Limitations |
|---|---|---|---|---|
| PCR (Polymerase Chain Reaction) | Thermal cycling amplification of DNA | Pathogen detection, genetic testing | Gold standard, robust, quantitative | End-point analysis only |
| qPCR (Real-Time PCR) | Fluorescence monitoring during amplification | Viral load quantification, gene expression | Quantitative, no post-PCR processing | Equipment cost higher |
| RT-PCR (Reverse Transcription PCR) | RNA → cDNA → amplification | RNA virus detection (HIV, HCV, SARS-CoV-2) | Enables RNA target detection | Two-step process can be complex |
| Digital PCR (dPCR) | Partition-based absolute quantification | Rare mutation detection, copy number variation | Absolute quantification, high precision | High cost, lower throughput |
| Isothermal Amplification | Constant temperature amplification (LAMP, RPA) | POCT molecular diagnostics | No thermal cycler needed, fast | Complex primer design |
| Next-Generation Sequencing (NGS) | High-throughput parallel sequencing | Comprehensive genomic profiling, liquid biopsy | Massive data, discovery capability | High cost, complex analysis |
1.3 PCR, qPCR, and RT-PCR — The Workhorses of Molecular Diagnostics
This series focuses on the three most widely used PCR-based platforms:
| Platform | Full Name | Target | Real-Time Detection | Reverse Transcription | Primary Use Case |
|---|---|---|---|---|---|
| PCR | Polymerase Chain Reaction | DNA | No | No | Pathogen detection, genotyping |
| qPCR | Quantitative/Real-Time PCR | DNA | Yes | No | Viral load, gene expression |
| RT-PCR | Reverse Transcription PCR | RNA | Optional | Yes | RNA virus detection (HIV, HCV, SARS-CoV-2, RSV, influenza) |
| RT-qPCR | Reverse Transcription qPCR | RNA | Yes | Yes | RNA quantification (gold standard for RNA virus detection) |
1.4 Molecular Diagnostics in IVD vs. Other Technologies
| Comparison Item | Molecular Diagnostics | Immunoassay | Clinical Chemistry |
|---|---|---|---|
| Target | Nucleic acids (DNA/RNA) | Proteins, hormones, antibodies | Metabolites, electrolytes, enzymes |
| Sensitivity | Extremely high (1-10 copies/reaction) | High (fM-pM) | Medium-high (μM-nM) |
| Specificity | Very high (sequence-specific) | High | Medium |
| Detection Time | 1-4 hours | 15 min - 2 hours | 5-30 minutes |
| Unit Cost | High | Medium | Low |
| Automation Level | Medium-High | Medium-High | High |
| Primary Application | Infectious diseases, oncology, genetics | Infectious diseases, hormones, oncology | Routine chemistry, metabolic panels |
Core Conclusion: Molecular diagnostics has an irreplaceable position in nucleic acid detection — for infectious diseases requiring high sensitivity (low viral load samples), genetic mutation analysis, and early cancer detection — complementing rather than replacing immunoassay and clinical chemistry.
II. Three Core PCR Technology Platforms Overview
2.1 PCR (Polymerase Chain Reaction) — The Gold Standard for DNA Amplification
Principle: Thermal cycling amplifies specific DNA sequences through denaturation (94-98°C), annealing (50-65°C), and extension (72°C) steps.
Detection Method: End-point analysis (gel electrophoresis or fluorescent dye after completion)
Primary Applications:
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Pathogen DNA detection (bacteria, DNA viruses)
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Genetic testing (genotyping, mutation detection)
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Food safety testing
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Forensic DNA analysis
Key Raw Materials:
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DNA polymerase (Taq, Hot-start Taq)
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dNTPs (dATP, dCTP, dGTP, dTTP)
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Primers (forward and reverse)
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Buffer (Tris-HCl, KCl, MgCl₂)
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Template DNA
2.2 qPCR (Quantitative/Real-Time PCR) — Quantitative DNA Detection in Real Time
Principle: Same as PCR but with fluorescent probes or dyes that emit signal proportional to the amount of amplified product, monitored in real-time during each cycle.
Detection Method: Real-time fluorescence monitoring (SYBR Green, TaqMan probes, Molecular Beacons)
Primary Applications:
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Viral load quantification (HBV, CMV)
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Gene expression analysis (mRNA quantification)
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Copy number variation (CNV) analysis
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Pathogen quantification
Key Raw Materials:
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DNA polymerase (hot-start, with high processivity)
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dNTPs (high purity)
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Fluorescent probes/dyes (FAM, VIC, SYBR Green I)
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Passive reference dye (ROX)
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Buffer (optimized for real-time detection)
2.3 RT-PCR (Reverse Transcription PCR) — RNA Detection
Principle: RNA is first reverse transcribed into complementary DNA (cDNA) using reverse transcriptase, followed by PCR amplification.
Variants:
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Two-step RT-PCR: Reverse transcription and PCR are performed separately
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One-step RT-PCR: Both reactions occur in a single tube
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RT-qPCR: Reverse transcription followed by real-time qPCR (gold standard for RNA virus detection)
Primary Applications:
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RNA virus detection (HIV, HCV, SARS-CoV-2, RSV, influenza)
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Gene expression analysis
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miRNA detection
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Transcriptome analysis
Key Raw Materials:
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Reverse transcriptase (MMLV, AMV)
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RNase inhibitor
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Random primers or Oligo(dT)
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DNA polymerase
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dNTPs
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RNA template
III. Panorama of Core Raw Materials for Molecular Diagnostics
The performance of molecular diagnostic reagents essentially depends on the proper selection and combination of the following major raw material categories. Each category plays an irreplaceable role: Enzymes (DNA polymerases, reverse transcriptases) are the "engine" of amplification, dNTPs are the "building blocks," primers and probes are the "navigation system," buffers and salts are the "environment stabilizers," and stabilizers and preservatives are the "protective layers" extending reagent shelf life.
3.1 Enzymes — The "Engine" of Amplification
| Enzyme | Function | Recommended Specification | Key Considerations |
|---|---|---|---|
| DNA Polymerase (Taq) | DNA amplification in PCR | 5-10 U/μL, with 5'→3' polymerase activity | Hot-start version reduces non-specific amplification |
| Hot-start Taq Polymerase | Prevents non-specific amplification at low temperatures | Chemically modified or antibody-inactivated | Essential for multiplex PCR and high-sensitivity assays |
| High-Fidelity DNA Polymerase | Reduces error rate for sequencing/cloning | Fidelity ≥ 50x Taq, 2-10 U/μL | For mutation detection and cloning |
| Reverse Transcriptase (MMLV) | RNA → cDNA conversion | 200 U/μL, RNase H+ or RNase H- | RNase H- gives higher cDNA yield |
| Reverse Transcriptase (AMV) | RNA → cDNA conversion (high secondary structure) | 10-20 U/μL | Better for high-temperature RT (e.g., 50-55°C) |
| RNase Inhibitor | Protects RNA from degradation | 20-40 U/μL | Essential for RT-PCR and RNA handling |
| UNG (Uracil-N-Glycosylase) | Prevents carryover contamination | 1 U/μL | Use with dUTP in PCR master mixes |
Enzyme Selection Guide:
| Application | Recommended Enzyme | Rationale |
|---|---|---|
| Routine PCR detection | Hot-start Taq polymerase | Prevents non-specific bands |
| Multiplex PCR (4-20 targets) | Hot-start Taq with enhanced processivity | Balanced amplification |
| High-sensitivity detection | Hot-start Taq + UNG system | Prevents carryover contamination |
| Mutation detection/sequencing | High-fidelity DNA polymerase | Low error rate |
| RNA virus detection (RT-PCR) | MMLV RT (RNase H-) + RNase inhibitor | High cDNA yield |
| RT-qPCR (one-step) | Engineered RT + hot-start DNA polymerase blend | Convenient, reproducible |
3.2 Nucleotides (dNTPs) — The "Building Blocks"
| Raw Material | CAS No. | Recommended Specification | Function |
|---|---|---|---|
| dATP (Deoxyadenosine triphosphate) | 1927-31-7 | ≥99%, HPLC, DNase/RNase free | Adenine nucleotide |
| dCTP (Deoxycytidine triphosphate) | 2056-98-6 | ≥99%, HPLC, DNase/RNase free | Cytosine nucleotide |
| dGTP (Deoxyguanosine triphosphate) | 2564-35-4 | ≥99%, HPLC, DNase/RNase free | Guanine nucleotide |
| dTTP (Deoxythymidine triphosphate) | 365-08-2 | ≥99%, HPLC, DNase/RNase free | Thymine nucleotide |
| dUTP (Deoxyuridine triphosphate) | 1175-53-7 | ≥99%, HPLC, DNase/RNase free | Used with UNG for carryover prevention |
| dNTP Mix | — | 10 mM or 25 mM each, ≥99% | Balanced mixture for PCR |
dNTP Quality Requirements:
| Parameter | Requirement | Impact |
|---|---|---|
| Purity | ≥99% (HPLC) | Low purity reduces amplification efficiency |
| DNase/RNase free | No detectable activity | Prevents template degradation |
| Equal concentration in mix | ±5% for each nucleotide | Imbalance causes misincorporation |
| Storage stability | Stable at -20°C for 24 months | Avoid freeze-thaw cycles (>10 cycles) |
3.3 Primers and Probes — The "Navigation System"
| Raw Material Type | Function | Recommended Specification | Key Considerations |
|---|---|---|---|
| Primers (Forward & Reverse) | Initiate DNA synthesis | ≥90% purity, HPLC or PAGE purified, Tm 55-65°C | 5' end can have modifications |
| Fluorescent Probes (TaqMan) | Sequence-specific detection | Dual-labeled (fluorophore + quencher), HPLC purified | FAM, VIC, HEX, CY5, TAMRA |
| SYBR Green I Dye | Intercalating dye for generic detection | 10,000× concentrate in DMSO | No sequence specificity |
| Molecular Beacons | Hairpin probes with fluorophore + quencher | HPLC purified, Tm of stem and loop optimized | Higher specificity than TaqMan |
| FRET Probes | Two probes for melting curve analysis | HPLC purified | Used in genotyping (e.g., LightCycler) |
Common Fluorophores and Quenchers:
| Fluorophore | Excitation (nm) | Emission (nm) | Common Quencher |
|---|---|---|---|
| FAM | 494 | 518 | BHQ1, TAMRA |
| VIC / HEX | 535-538 | 554-556 | BHQ1 |
| ROX | 575 | 602 | BHQ2 |
| CY5 | 650 | 670 | BHQ2, BHQ3 |
| SYBR Green I | 497 | 520 | None (intercalating dye) |
3.4 Buffers and Salts — The "Environment Stabilizers"
| Raw Material | CAS No. | Recommended Concentration | Function |
|---|---|---|---|
| Tris-HCl | 1185-53-1 | 10-50 mM, pH 8.0-9.0 | Maintains pH for polymerase activity |
| KCl | 7447-40-7 | 20-100 mM | Provides ionic strength, primer annealing |
| (NH₄)₂SO₄ | 7783-20-2 | 15-25 mM | Alternative to KCl, increases specificity |
| MgCl₂ | 7786-30-3 | 1.5-4.5 mM | Cofactor for DNA polymerase activity |
| BSA | 9048-46-8 | 0.1-1 mg/mL | Stabilizes enzyme, reduces adsorption |
| DTT (Dithiothreitol) | 3483-12-3 | 1-10 mM | Reducing agent for reverse transcriptase |
| EDTA | 60-00-4 | 0.1-1 mM | Chelates metal ions, optional in PCR |
Buffer Optimization Guide:
| Parameter | Recommended Range | Impact on PCR |
|---|---|---|
| pH (Tris-HCl at 25°C) | 8.0-9.0 | Lower pH reduces polymerase activity |
| Mg²⁺ concentration | 1.5-4.5 mM | Too low → no product; too high → non-specific bands |
| KCl concentration | 20-100 mM | Higher salt → higher annealing stringency |
| (NH₄)₂SO₄ | 15-25 mM | Increases specificity, good for GC-rich templates |
3.5 Stabilizers and Preservatives
| Raw Material | CAS No. | Recommended Concentration | Main Application | Precautions |
|---|---|---|---|---|
| BSA (Molecular Biology Grade) | 9048-46-8 | 0.1-1 mg/mL | Stabilizes enzymes, reduces polymer adsorption | DNase/RNase free |
| Trehalose | 6138-23-4 | 2-5% | Lyoprotection, thermal stability for master mixes | Extends shelf life |
| Glycerol | 56-81-5 | 5-50% | Cryoprotection for enzymes | Store at -20°C |
| ProClin 300 | — | 0.02-0.05% | Preservative for non-enzyme buffers | ⚠️ Avoid in PCR master mixes (inhibits polymerase) |
| NaN₃ (Sodium azide) | 26628-22-8 | — | ⚠️ Prohibited in PCR reagents | Strongly inhibits DNA polymerase |
Critical Warning:
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⚠️ Polymerase chain reaction (PCR, qPCR, RT-PCR) reagents must NEVER contain sodium azide or ProClin 300 — both strongly inhibit DNA polymerase activity
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⚠️ Use only molecular biology grade reagents certified DNase/RNase free
3.6 Master Mix Components — All-in-One Formulations
| Master Mix Type | Components | Key Applications |
|---|---|---|
| PCR Master Mix (2×) | Taq polymerase, dNTPs, MgCl₂, buffer, loading dye (optional) | Routine PCR amplification |
| qPCR Master Mix (2×) | Hot-start Taq polymerase, dNTPs, MgCl₂, buffer, passive reference dye (ROX) | Real-time quantitative PCR |
| RT-PCR Master Mix (2×) | Reverse transcriptase, RNase inhibitor, DNA polymerase, dNTPs, buffer | One-step RNA detection |
| UNG Master Mix | 2× master mix + UNG enzyme + dUTP | Carryover contamination prevention |
IV. General Principles for Molecular Diagnostic Raw Material Selection
4.1 Purity and Quality Requirements
| Requirement | Explanation | Testing Method |
|---|---|---|
| DNase/RNase free | No nuclease contamination that degrades template | Incubate with nucleic acid, check degradation |
| ≥99% purity (HPLC) | High purity for nucleotides and primers | HPLC analysis |
| Low endotoxin | Endotoxin-free for clinical assays | LAL test |
| Batch-to-batch consistency | Reproducible performance across lots | In-house PCR/qPCR validation |
| Stability data | Real-time and accelerated stability studies | 2-8°C, -20°C stability testing |
4.2 Critical Warnings Summary
| Warning | Applicable Reagents | Reason |
|---|---|---|
| ⚠️ PCR reagents must NOT contain sodium azide | PCR, qPCR, RT-PCR master mixes | Sodium azide inhibits DNA polymerase |
| ⚠️ PCR reagents must NOT contain ProClin 300 | PCR, qPCR, RT-PCR master mixes | ProClin 300 inhibits DNA polymerase |
| ⚠️ Use molecular biology grade water | All PCR reactions | DEPC-treated or nuclease-free water required |
| ⚠️ Avoid repeated freeze-thaw of enzymes | Polymerases, reverse transcriptases | Freeze-thaw reduces enzyme activity |
| ⚠️ Protect fluorescent probes from light | qPCR probes, SYBR Green | Photobleaching reduces fluorescence |
4.3 Quality Control Recommendations for Raw Material Suppliers
| QC Parameter | Specification | Recommended Frequency |
|---|---|---|
| Purity (HPLC) | ≥99% | Each batch |
| DNase activity | None detected | Each batch |
| RNase activity | None detected | Each batch |
| Functional testing | PCR/qPCR performance | Each batch |
| Moisture content (lyophilized) | ≤3% | Each batch |
| Stability (accelerated) | 14 days at 37°C | Periodic |
V.Frequently Asked Questions (FAQ)
Q1: What is the difference between PCR, qPCR, and RT-PCR?
A: Here is a quick comparison:
| Platform | Target | Real-Time? | Output | Primary Use |
|---|---|---|---|---|
| PCR | DNA | No | End-point (gel) | Pathogen detection, genotyping |
| qPCR | DNA | Yes | Quantification (Ct value) | Viral load, gene expression |
| RT-PCR | RNA | No/Yes | cDNA → PCR product | RNA virus detection |
| RT-qPCR | RNA | Yes | RNA quantification | RNA virus quantification |
Q2: What type of DNA polymerase should I choose for my assay?
A: Selection depends on your application:
| Application | Recommended Polymerase |
|---|---|
| Routine PCR detection | Hot-start Taq (prevents non-specific bands) |
| Multiplex PCR (4-10 targets) | Hot-start Taq with enhanced processivity |
| High-sensitivity detection | Hot-start Taq + dUTP/UNG system |
| Mutation detection / sequencing | High-fidelity DNA polymerase (e.g., Pfu, Q5) |
| Long amplicons (>5 kb) | Long-range Taq or blend polymerases |
Q3: What is the optimal Mg²⁺ concentration for PCR?
A: Mg²⁺ is a cofactor for DNA polymerase. The optimal concentration is typically 1.5-4.5 mM:
| Mg²⁺ Concentration | Effect |
|---|---|
| Too low (<1.0 mM) | Little or no PCR product |
| Optimal (1.5-4.5 mM) | Efficient, specific amplification |
| Too high (>5.0 mM) | Non-specific bands, smears |
Note: dNTPs chelate Mg²⁺ — adjust accordingly when changing dNTP concentration.
Q4: Can I use sodium azide or ProClin 300 as a preservative in PCR reagents?
A: Absolutely NOT.
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Sodium azide (NaN₃): Strongly inhibits DNA polymerase activity
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ProClin 300: Also inhibits DNA polymerase
Recommendation: Use molecular biology grade water (DEPC-treated, nuclease-free) and aseptic technique. For master mixes that require stability, use trehalose and BSA as stabilizers instead of antimicrobial preservatives.
Q5: How should I store enzymes and master mixes?
A: Follow these guidelines:
| Reagent | Storage Temperature | Special Handling |
|---|---|---|
| DNA polymerase | -20°C | Avoid freeze-thaw (>10 cycles) |
| Reverse transcriptase | -20°C | Keep on ice during use |
| RNase inhibitor | -20°C | Avoid repeated freeze-thaw |
| dNTPs | -20°C | Stable for 24 months |
| Primers/Probes (liquid) | -20°C | Protect probes from light |
| Primers (lyophilized) | -20°C or room temperature | Reconstitute in TE buffer |
| 2× Master Mix (liquid) | -20°C | Thaw completely, mix well |
| 2× Master Mix (lyophilized) | 2-8°C or room temperature | Reconstitute per instructions |
Q6: What are the most common causes of PCR failure?
A: Common issues and solutions:
| Problem | Possible Cause | Solution |
|---|---|---|
| No product | Missing component | Check master mix, primers, template |
| No product | Inhibitors in sample | Purify template, dilute sample |
| No product | Annealing temperature too high | Lower Tm by 2-5°C |
| Non-specific bands | Annealing temperature too low | Increase Tm by 2-5°C |
| Non-specific bands | Too much Mg²⁺ | Reduce Mg²⁺ concentration |
| Smear on gel | Too much template | Dilute template 10-100 fold |
| Smear on gel | Poor primer design | Redesign primers |
| Low yield | Insufficient cycle number | Increase cycles (35-40) |
| Low yield | Polymerase expired | Use fresh polymerase |
Q7: What is the difference between one-step and two-step RT-PCR?
A:
| Feature | One-Step RT-PCR | Two-Step RT-PCR |
|---|---|---|
| Workflow | RNA → cDNA → PCR in a single tube | Separate RT reaction, then PCR |
| Time | Faster (2-3 hours) | Slower (3-5 hours) |
| Sensitivity | Lower (no cDNA storage) | Higher (can use all cDNA) |
| Convenience | Higher (less handling) | Lower (more steps, more contamination risk) |
| Multiplexing | Limited | Possible (store cDNA for multiple PCRs) |
| Best for | Routine RNA detection (e.g., SARS-CoV-2) | Experiments requiring cDNA storage or multiple targets |
Q8: What is the purpose of ROX in qPCR?
A: ROX (passive reference dye) normalizes well-to-well fluorescence variations caused by:
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Pipetting inaccuracies
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Evaporation during thermal cycling
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Optical differences between wells
Note: Not all qPCR instruments require ROX. Check your instrument compatibility (e.g., Applied Biosystems requires ROX; Bio-Rad does not).
艳 李
