Which feature best distinguishes PCR (Polymerase Chain Reaction) from DNA sequencing when comparing their primary diagnostic applications in a molecular pathology laboratory?

## PCR vs. DNA Sequencing: Primary Distinction ### Core Function Difference **Key Point:** The fundamental distinction lies in what each technique accomplishes: - **PCR**: **Amplifies** (exponentially copies) a specific DNA target sequence - **DNA Sequencing**: **Reads** (determines the nucleotide order) of a DNA template ### Functional Comparison Table | Feature | PCR | DNA Sequencing | | --- | --- | --- | | **Primary function** | Amplification of target DNA | Determination of nucleotide sequence | | **Output** | Millions of copies of target | Nucleotide order (A, T, G, C) | | **Diagnostic use** | Presence/absence of mutation or pathogen | Identification of specific mutations, variants, or sequence changes | | **Thermal cycling** | Required (denature, anneal, extend) | Not required (uses chain terminators or other methods) | | **Polymerase type** | Taq polymerase (thermostable) | DNA polymerase + ddNTPs or next-gen chemistry | | **Workflow** | Standalone or precursor to sequencing | Often follows PCR amplification | ### High-Yield Concept **High-Yield:** PCR is an **amplification tool**; sequencing is a **reading tool**. In modern molecular diagnostics, PCR often precedes sequencing: amplify the region of interest (PCR) → sequence the amplicon (sequencing) → identify mutations. ### Clinical Pearl **Clinical Pearl:** In a molecular pathology lab, PCR is used for rapid detection of pathogens (e.g., SARS-CoV-2, TB) or known mutations (e.g., BRCA1/BRCA2 hotspots), while sequencing is used when the exact mutation is unknown or when comprehensive variant detection is needed (e.g., whole-exome sequencing for cancer genomics). ### Mnemonic **Mnemonic:** **PCR** = **P**roduce (amplify) copies; **SEQ** = **S**ee (read) the sequence [cite:Molecular Cloning: A Laboratory Manual Ch 8]