Polymerase Chain Reaction (PCR) & DNA Fingerprinting – Comprehensive Study Notes

Comic Contrast: Real vs. Movie Scientists

Real laboratory dialog emphasises measured verification (e.g.
“Let’s confirm with a PCR after lunch”), whereas cinematic tropes leap to
spectacular, unsupported claims ("It’s the cure to every disease ever!").
This humorous pre-amble foreshadows the lecture’s focus on sober,
stepwise experimental design.

Lecture Learning Outcomes

By the end of the session you should be able to:

1. Explain the principles of in-vitro DNA synthesis.

2. Describe, in detail, the Polymerase Chain Reaction (PCR) cycle.

3. Differentiate among multiple specialised PCR formats.

4. Outline DNA-fingerprinting methodologies (RFLP, VNTR, STR).

5. Distinguish microsatellites vs minisatellites.

6. List broad and niche applications of PCR technology.

Core Concept – The Polymerase Chain Reaction

• PCR is a rapid, inexpensive, in-vitro technique that amplifies a defined DNA region into millions of copies.
• Invented in $1983$ by Kary Mullis at Cetus Corporation, winning the $1993$ Nobel Prize in Chemistry.
• Relies on thermal cycling: iterative exposure of reagents to precisely controlled temperatures to drive three enzymatic/biophysical events.

Required Reagents

1. DNA Template
   • Must be double-stranded initially; becomes single-stranded during the $95^{\circ}C$ denaturing step (in vivo this role is served by DNA helicase).
   • DNA is thermostable; RNA would degrade at these temperatures.

2. Thermostable DNA Polymerase
   • Classical enzyme: Taq polymerase from Thermus aquaticus.
   • Optimal activity window $70{-}100^{\circ}C$, peak at $72^{\circ}C$.

3. Primers
   • Synthetic, single-stranded oligonucleotides, $20{-}25$ bp.
   • Complementary to sequences flanking the target’s $3'$ ends on sense & antisense strands.
   • Anneal at $50{-}60^{\circ}C$.

4. dNTPs (dATP, dTTP, dCTP, dGTP) in equimolar mix.

5. Ionic Buffer
   • Monovalent $K^+$ and divalent $Mg^{2+}/Mn^{2+}$ ions—essential cofactors for Taq catalytic activity.

The Thermal Cycle (One PCR “Cycle”)

1. Denaturing – $94{-}96^{\circ}C$ for $\approx1{-}2\,\text{min}$ breaks H-bonds between strands.

2. Annealing – temperature lowered (empirically chosen, typically $50{-}60^{\circ}C$).
   • Too high ⇒ weak/no primer binding ⇒ low yield.
   • Too low ⇒ non-specific binding ⇒ off-target amplicons.

3. Extension/Elongation – $72{-}75^{\circ}C$ for $\approx30\,\text{s}$–$1\,\text{min}$.
   • Taq extends from primers’ $3'$ ends, incorporating dNTPs.

A typical assay repeats these three stages $20{-}40$ times, generating a theoretical amplification factor of $2^{n}$ (where $n$ = cycle number).

Spectrum of PCR Variants (>37 recorded, key examples below)

1. Reverse-Transcriptase PCR (RT-PCR)

2. Multiplex PCR

3. Nested PCR

4. Real-Time / Quantitative PCR (qPCR)

5. Inverse, Hot-Start, Long, Touch-Down, Asymmetric, Colony, In-situ, Solid-Phase, Mini-primer, Overlap-Extension, Assembly PCR, etc.

Reverse-Transcriptase PCR (RT-PCR)

Purpose: Detect gene expression by converting cellular RNA → cDNA → PCR.
Steps:

1. RNA Isolation from cells/tissues.

2. cDNA synthesis using reverse transcriptase.

3. PCR amplification with gene-specific primers.
   Approaches:
   • One-Step – RT + PCR in same tube (minimal handling, lower contamination risk).
   • Two-Step – cDNA synthesis first, aliquot into multiple downstream PCRs (greater flexibility for multiple targets).

Multiplex PCR

Single reaction, several primer pairs:
• Single-template multiplex – diverse loci on one genome.
• Multiple-template multiplex – several genomes (e.g.
pathogen panels).
Benefit: time & cost saving; Caveat: primer-dimer formation and specificity trade-off.

Nested PCR

Enhances specificity by sequential amplification:

1. Outer primers amplify a broad segment.

2. The resulting product becomes template; inner (nested) primers bind internally, yielding a shorter, highly specific amplicon.
   Reduces background from mis-primed products.

Real-Time / Quantitative PCR (qPCR)

• Monitors product accumulation during cycling by fluorescence.
• Detection chemistries:
– Non-specific dyes (e.g., SYBR Green) intercalate any dsDNA.
– Sequence-specific probes (e.g., TaqMan) bearing fluorophore & quencher; probe degradation by Taq’s $5'\to3'$ exonuclease releases the fluorophore.
• Generates a C\textsubscript{t} (threshold cycle) value inversely related to starting template quantity; enables absolute or relative quantitation.

DNA Fingerprinting / Profiling

Invented $1984$ by Alec Jeffreys; exploits polymorphic loci representing the $\approx1\%$ inter-individual genomic variation.

Sample Types

Hair, blood, semen, saliva, skin cells, tissues—any source of intact genomic DNA.

Three Principal Laboratory Techniques

1. Restriction Fragment Length Polymorphism (RFLP)
   • Genomic DNA digested with restriction enzymes; fragments ($2{-}60\,\text{kbp}$) separated by gel electrophoresis.
   • Southern-blot visualisation.
   • Pros: highly reproducible; Cons: costly, labour-intensive, large DNA input, slow (up to $\sim1\,\text{month}$).

2. Variable Number Tandem Repeats (VNTR)
   • Minisatellites ($10{-}60$ bp repeat unit).
   • Same wet-lab workflow as RFLP but with probes specific to tandem repeat loci; needs less DNA.

3. Short Tandem Repeats (STR)
   • Microsatellites ($2{-}6$ bp).
   • Current forensic gold standard—fast, inexpensive, requires minute DNA quantities.
   • STR assays rely on PCR rather than restriction digestion.
   • Fluorescently labelled primers → capillary electrophoresis → digital allele calling.

STR Workflow Recap

1. DNA isolation.

2. PCR with fluorescent STR primers.

3. Fragment size separation.

4. Laser detection & software allele designation.

Accuracy & Statistics

Probability of two unrelated people sharing a full STR profile ≈ $1/3\times10^{10}$ (except identical twins).

Applications of DNA Fingerprinting

• Detection of inherited disorders pre- & post-natally.
• Paternity/maternity confirmation via VNTR/STR inheritance.
• Criminal forensics—crime-scene evidence matching.
• Immigration (family reunion proof), conservation biology (genetic diversity), GMO verification, palaeogenomics, pathogen tracing in food safety.

PCR in Sequencing Workflows

Whole Genome Sequencing (WGS) pipeline:

1. DNA Extraction (lyse + purify).

2. Fragmentation via enzymes/mechanical shearing.

3. Library Preparation – adapters ligated, fragments PCR-amplified to create a “DNA library”.

4. High-throughput Sequencing – millions of “reads”.

5. Bioinformatic Assembly – reads stitched into contiguous genome; enables outbreak tracking & comparative genomics.

PCR in Diagnostics (SARS-CoV-2 example)

A. Sample Collection – nasopharyngeal swab in viral transport buffer.
B. RNA Extraction – lysis & silica/chemistry purification.
C. Reverse Transcription + qPCR – convert viral RNA → cDNA, amplify with probe-based real-time detection.
D. Result Interpretation – fluorescence curves vs negative/positive controls determine infection status.

PCR in Personalised Medicine

• Pharmacogenomics leverages PCR genotyping to stratify patients: correct drug, dose, or biologic therapy (e.g., KRAS mutation testing in colorectal cancer guiding anti-EGFR treatment).
• Outcome: maximised therapeutic efficacy, minimised adverse effects.

Microsatellites vs Minisatellites – Key Distinctions

• Microsatellites (STR): repeat unit $2{-}6$ bp, total array length usually (<$100$ bp).
• Minisatellites (VNTR): repeat unit $10{-}60$ bp, arrays can reach $\sim1{-}20\,\text{kbp}$.
• Both display length polymorphism useful for identity tests; detection methodology differs chiefly by PCR reliance (STR) vs restriction digestion (VNTR, RFLP).

Ethical, Philosophical & Practical Considerations

• Privacy: DNA profiles are uniquely identifying; storage in databases demands stringent governance.
• Equity: Personalised medicine hinges on access to sequencing & PCR diagnostics—global disparities persist.
• Contamination risk: Trace DNA can create false forensic leads; rigorous lab practice & chain-of-custody essential.
• Bioterror/Biosecurity: PCR simplifies pathogen detection but also, theoretically, pathogen engineering; dual-use oversight required.

Lecture Wrap-Up (Key Points)

1. PCR relies on cycles of $\text{Denature} \rightarrow \text{Anneal} \rightarrow \text{Extend}$ to achieve exponential DNA amplification.

2. Critical reagents: template, thermostable polymerase, primers, dNTPs, ionic buffer.

3. Numerous specialised PCR formats tailor sensitivity, specificity, throughput, and quantification to experimental goals.

4. Downstream applications span forensics, diagnostics, sequencing, genetic engineering, epidemiology, and precision therapeutics.

5. Understanding reagent chemistry, temperature parameters, and variant selection is fundamental to experimental success and data reliability.

Suggested Further Reading & Resources

(See full reference list provided in lecture slides: ThermoFisher, abm, BitesizeBio, MicrobeNotes, CDC PulseNet, Frontiers in Immunology, EUPATI Toolbox.)