Applications of PCR

After PCR, you cannot see DNA with your naked eye—even 1 billion copies is microscopic. Gel electrophoresis separates DNA fragments by size and visualizes them using a fluorescent dye. DNA is negatively charged at physiological pH, so it migrates through an agarose gel matrix toward the positive electrode. Smaller fragments move faster; larger fragments move slower—this creates separation by size.

Standard Gel Interpretation:

A typical PCR gel contains: (1) A DNA ladder (lane 1)—known size standards (e.g., 100 bp, 500 bp, 1 kb, 3 kb), (2) A negative control—PCR with no template, should show no bands, (3) A positive control—PCR with known template, confirms technique worked, and (4) Sample lanes—your experimental PCR products. What success looks like: Bright bands at the expected size. Band brightness indicates concentration—darker bands mean more DNA.

Common problems:

• No bands in samples but positive control works fi Your template is absent or too dilute
• Bands at unexpected sizes fi Non-specific primer binding or contamination
• Faint bands fi Low template amount, too few cycles, or degraded primers
• Smear instead of sharp bands fi DNA degradation or contamination with nucleases Example: Gene Expression Testing in Three Tissues A researcher wants to check if a gene (e.g., CFTR, cystic fibrosis transmembrane conductance regulator) is expressed in three tissues: lung, liver, and kidney. PCR is performed on cDNA from each tissue with CFTR-specific primers (500 bp product).

Gel result:

Lane 1: Ladder

Lane 2: Lung cDNA -> bright band at 500 bp (CFTR is expressed in lung) Lane 3: Liver cDNA -> faint band at 500 bp (CFTR weakly expressed)

Lane 4: Kidney cDNA -> no band (CFTR not expressed in kidney)

Lane 5: Negative control -> no band (confirms template-dependent amplification) Conclusion: CFTR expression is tissue-specific, highest in lung. This validates that primers work and shows relative expression levels.

Quantifying Band Intensity

Densitometry (image analysis) measures band brightness, converting it to DNA quantity. This allows semi-quantitative PCR where product amount approximates template amount, though less precise than real-time qPCR.

PCR enables rapid, sensitive detection of genetic mutations in patient samples. Instead of waiting weeks for traditional genetic testing, doctors can now identify disease-causing variants in hours, allowing faster diagnosis and earlier treatment. PCR is the workhorse of clinical genetics laboratories.

Genetic screening uses PCR to identify disease-causing alleles in populations without symptoms (yet). It differs from diagnosis—screening happens before symptoms; diagnosis happens after. Screening programs target high-risk groups or specific diseases of public health importance.

PCR revolutionized infectious disease diagnosis. Unlike culture-based methods (which take days to weeks), PCR can detect pathogens in hours, enabling faster treatment initiation. It's especially powerful for fastidious organisms and obligate intracellular pathogens.

Pathogens Commonly Detected by PCR:

• Tuberculosis (TB): Mycobacterium tuberculosis. Culture takes 2-8 weeks; PCR result in 2-4 hours.
• HIV: PCR detects viral RNA (RT-PCR). Useful for acute infection detection and viral load monitoring.
• Chlamydia trachomatis: Obligate intracellular; cannot be cultured in standard media. PCR is the gold standard.
• Neisseria gonorrhoeae: Also difficult to culture; PCR preferred. Often tested with Chlamydia (combo kit).
• SARS-CoV-2: RT-PCR detects viral RNA in respiratory samples. Gold standard for COVID-19 diagnosis. Case Study: Chlamydia trachomatis Diagnosis Problem: Chlamydia is an obligate intracellular bacterium that cannot be grown on standard media.

PCR Solution:

1. Sample: First-void urine (males) or endocervical swab (females)

2. DNA isolation: Extract using magnetic bead kits

3. PCR amplification: Chlamydia-specific primers (omp1 gene) amplify a 260 bp product

4. Detection: 260 bp band on gel = positive; absence = negative

Advantages over culture: Faster (4 hours vs. weeks), more sensitive, non-invasive sampling possible. Impact: PCR-based Chlamydia testing enables rapid diagnosis and treatment, preventing pelvic inflammatory disease and infertility.

Viral Load Monitoring

For ongoing infections like HIV, PCR quantifies viral RNA copies per mL of blood. Patients with <50 copies/mL are undetectable (cannot transmit sexually). PCR provides real-time feedback on whether antiviral therapy is working.

Forensic DNA profiling is perhaps PCR's most dramatic application: connecting individuals to crime scenes through microscopic traces of biological evidence. A single hair follicle, a drop of blood, saliva on a cigarette butt contain enough DNA for PCR amplification and identification.

Historical Context

Sir Alec Jeffreys (1984): British geneticist discovered DNA fingerprinting using RFLP. He published in Nature and recognized its forensic potential immediately.

Colin Pitchfork Case (1988): First criminal conviction using DNA evidence. Two girls were murdered in Leicestershire, England. Police arrested Richard Buckland, who confessed to one murder but denied the other. Jeffreys performed DNA testing on crime scene semen. Result: Buckland's DNA did NOT match. He was exonerated—first exoneration by DNA.

Police then conducted mass DNA screening of 4,000 adult males. Colin Pitchfork, a local bakery worker, matched both crime scenes. He confessed and was convicted. This case established DNA's power to both exonerate and convict.

Why Forensic DNA Works

Every person (except identical twins) has a unique DNA profile. Crime scenes contain biological evidence left by perpetrators. PCR amplifies DNA from these traces, and matching profiles link suspect to crime scene. The combination of multiple markers across the genome makes false matches vanishingly rare.

DNA Databases

Many countries maintain DNA databases of convicted felons. Crime scene profiles are searched against the database. Matches generate investigative leads. Innocent people's DNA is typically removed from databases after exoneration.

All humans share ~99.9% of their DNA sequence. The remaining ~0.1% accounts for all visible differences and genetic variation exploited by forensic profiling. This variation is called polymorphism (many forms)— different alleles at the same locus.

Out of 3 billion nucleotides in the human genome, approximately 3 million positions vary between individuals (0.1% diversity). The most common variants are SNPs (Single Nucleotide Polymorphisms)—single base pair differences occurring roughly once every 1,000 bases.

Example:

Individual A: ...ATGCAGCTA...

Individual B: ...ATGCGGCTA...

This AfiG SNP is a polymorphism. In a population, allele frequencies describe how common each variant is. Polymorphisms are incredibly valuable—you and I differ at ~3 million positions in our genomes. Most differences are silent, but they're useful genetic markers for identification and population studies.

Population Genetics

SNPs and other polymorphisms are exploited in forensic DNA profiling. Different alleles create unique profiles. The combination of multiple polymorphic markers makes false matches extremely rare.

The pioneering forensic DNA technique

RFLP was the first widely used forensic DNA technique (1980s-1990s). Though largely superseded by STR profiling, understanding RFLP is conceptually important.

Mechanism: RFLP exploits variations in restriction enzyme recognition sites. If a mutation destroys a cut site, longer fragments result. If a site is gained, more fragments are produced.

Example:

Allele 1: ...GAATTC... (EcoRI site present) fi cut by EcoRI

Allele 2: ...GACTTC... (AfiC mutation destroys site) fi NOT cut

After digestion: Allele 1 fi shorter fragments, Allele 2 fi longer fragments. Gel electrophoresis separates these by size, creating a banding pattern unique to each genotype.

Limitations: Required large DNA samples (200+ ng), slow (weeks), subjective band interpretation. RFLP fell out of favor with PCR-based methods (STRs).

Historical Significance

RFLP proved DNA could uniquely identify individuals—the conceptual breakthrough that enabled modern forensics. While superseded by faster methods, RFLP remains relevant for paternity testing and population studies.

VNTRs are segments of DNA containing short sequences repeated multiple times in tandem. The number of repeats varies between individuals, creating a polymorphism.

Structure: A VNTR locus might contain a 16 bp repeat unit. Individual A might have 7 copies (112 bp total), while Individual B has 12 copies (192 bp total). This 80 bp size difference is easily detected by gel electrophoresis. Advantages over RFLP: VNTRs are highly polymorphic and don't require restriction enzymes—just PCR amplification. However, VNTRs can be unstable (repeat numbers change during meiosis), and alleles can be difficult to distinguish.

Usage: VNTRs were used in forensics during the 1990s-2000s. Now largely replaced by STRs, which are more stable and smaller (allowing PCR amplification of degraded DNA from old crime scenes).

Mutation Rate

VNTR repeat numbers can change when passing through meiosis, introducing instability. This limits their utility in paternity testing. STRs are more stable and preferred.

STRs (Short Tandem Repeats), also called microsatellites, are the current standard for forensic DNA profiling. An STR locus contains a short repeat unit (2-6 bp) repeated multiple times. The number of repeats varies between individuals.

Structure Example: The STR locus D3S1358 contains a 4 bp repeat (AGAT):

Individual A: 9 copies, 10 copies -> genotype 9,10 (heterozygous)

Individual B: 8 copies, 8 copies -> genotype 8,8 (homozygous)

On a gel, these appear as bands at different sizes.

Why STRs are superior:

1. Small size: ~100-300 bp, allowing PCR from degraded DNA

2. Multiplex PCR: 13-20 loci amplified simultaneously in one reaction

3. Standardization: The FBI established 13 core STR loci (CODIS database) used worldwide

4. Stability: Unlike VNTRs, STR repeat numbers are stable through inheritance

5. Speed: PCR-based results in 4-8 hours vs. weeks for RFLP

Probability of match: Using 13 loci, the probability that two unrelated individuals match at all 13 loci is roughly 1 in 1 billion—essentially unique identification.

Stutter Bands

During PCR amplification of repeats, Taq polymerase sometimes slips, creating products one repeat unit shorter or longer than the true allele ('stutter bands'). Experienced analysts interpret these correctly—stutter is expected.

Comparison: RFLP vs VNTR vs STR

Evolution of forensic DNA profiling

Understanding the progression from RFLP fi VNTR fi STR shows how molecular techniques evolve. Each generation improved upon its predecessor in speed, sensitivity, standardization, and statistical power.

Case 1: Colin Pitchfork (1988) — First DNA Conviction

Crimes: Two 15-year-old girls, Lynda Mann (1983) and Dawn Ashworth (1986), were raped and murdered in Leicestershire, England. Semen was recovered from both crime scenes.

Initial suspect: Richard Buckland, a local teenager, confessed to one attack but denied the other. DNA testing: Sir Alec Jeffreys performed RFLP analysis on semen and blood from Buckland. Result: Buckland's DNA did NOT match either crime scene sample. This was the first exoneration by DNA. Investigation continuation: Police requested voluntary DNA testing from all adult males in the area—over 4,000 men. Colin Pitchfork, a local bakery worker, was identified. His DNA matched both crime scenes. He confessed and was convicted in 1988.

Significance: This case established DNA's dual role—exonerating the innocent and convicting the guilty. It demonstrated that mass DNA screening could solve crimes.

Case 2: OJ Simpson (1995) — 'DNA Wars'

Crimes: Nicole Brown Simpson (ex-wife) and Ronald Goldman (waiter) were murdered in Los Angeles on June 12, 1994.

DNA evidence: Blood from crime scenes matched Simpson's DNA at multiple loci. Probability of random match: 1 in 170 million.

The controversy: Despite overwhelming DNA evidence, Simpson was acquitted in 1995. The defense argued evidence collection was sloppy (potential contamination) and a detective with a racism history may have planted evidence.

Lesson: Even perfect DNA evidence can be undermined by chain-of-custody concerns. Proper documentation and secure handling are essential. Forensic scientists must meticulously track sample handling from collection to analysis.

Case 3: Death Row Exonerations

Impact: Since 1989, DNA testing has exonerated over 375 death row inmates in the U.S. Many were convicted on eyewitness testimony (which is surprisingly unreliable—witness misidentification is the leading cause of wrongful convictions) or faulty forensic evidence.

Example—Roy Criner: Convicted of rape in 1990 in Texas based on eyewitness identification. DNA testing in 1997 excluded him. He was exonerated and released after 9 years in prison.

Impact: DNA exonerations have exposed systemic problems (eyewitness reliability, tunnel vision) and led to reforms (DNA testing becoming a right, improved eyewitness protocols).

Case 4: Lydia Fairchild — The Chimerism Case

Background: Lydia Fairchild applied for welfare benefits in Washington state in 2002. Routine DNA testing to verify paternity showed that she could not be the biological mother of her own children—her DNA didn't match theirs.

The mystery: Lydia was a chimera—a person with two genetically distinct cell lines. She was a fraternal twin; in utero, blood vessels fused between the twins. Her twin brother died, but his blood cells mixed with hers. Today, Lydia's body contains cells from two different individuals.

Resolution: DNA testing from different tissues (blood vs. skin) showed the two different genomes. Her case established that chimerism exists and can complicate DNA testing. She was exonerated. Lesson: DNA is not always unique within a single person. Chimerism (from blood transfusion, pregnancy, or organ transplant) can introduce multiple DNA profiles—important when interpreting unexpected results.

DNA Evidence Strengths and Limitations

DNA profiling is powerful but not infallible. Contamination, degradation, sample mix-ups, and biological phenomena like chimerism can complicate interpretation. Proper protocols and statistical rigor are essential.

PCR-based STR profiling is the gold standard for determining paternity and establishing biological relationships. It's used in legal disputes, inheritance cases, and personal genealogy.

Principle: Each person inherits one allele at each STR locus from each parent. At locus D3S1358:

Mother: alleles 9, 10 (genotype 9,10)

Alleged father: alleles 8, 12 (genotype 8,12)

Child: allele 10 (from mother) and allele 8 (from father)

-> Child's genotype: 8,10

The child's 8 allele must come from the father. If the alleged father doesn't have an 8 allele, he is excluded as the biological father.

Testing protocol:

1. DNA is extracted from blood or buccal swabs

2. STR typing of 13+ loci is performed

3. Alleles are compared

4. Exclusion: One mismatched locus = excluded (probability of exclusion: 99.9%+)

5. Inclusion: All loci consistent = paternity established (probability: 99.9%+) Statistical reporting: Likelihood Ratio (LR) compares probabilities:

LR = 1,000:1 means 1,000 times more likely he IS the biological father than if he isn't. LR > 100:1 = inclusion; LR < 0.01:1 = exclusion.

Non-Invasive Testing

Paternity testing no longer requires blood draws. Cheek swabs (buccal cells) or saliva provide DNA. This makes testing convenient and accessible, though rare contamination from environmental DNA is possible.

Beyond forensics and medical diagnostics, PCR is indispensable in research. It's a cornerstone of molecular biology, genomics, and evolutionary studies.

Gene Expression Studies: RT-PCR (reverse transcription-PCR) amplifies mRNA to measure gene expression in specific tissues. Quantitative RT-PCR (qRT-PCR) measures expression levels, allowing comparison between healthy and diseased cells.

Cloning: PCR amplifies a gene of interest, which is inserted into a plasmid and introduced into bacteria to produce recombinant protein. This is how insulin, growth hormone, and many therapeutic proteins are manufactured.

DNA Sequencing: Before next-generation sequencing (NGS), samples were prepared by PCR amplification. PCR is still used for enrichment of specific regions (target-capture PCR, digital PCR). Evolutionary Studies: PCR amplifies DNA from fossils, museum specimens, or archaeological samples to infer evolutionary relationships. Ancient DNA (aDNA) from Neanderthals, woolly mammoths, and other extinct species reveals interbreeding and evolution.

Pathogen Detection: PCR screens for pathogens in food (E. coli, Salmonella) and water, enabling rapid food safety testing and outbreak investigation.

Metagenomics: PCR amplifies 16S rRNA genes to identify bacterial species in environmental samples (soil, water, gut microbiota). This revolutionized understanding of microbial communities. PCR vs. Whole Genome Sequencing (WGS)

Whole Genome Sequencing (WGS) has become faster and cheaper, but PCR remains superior for many applications:

PCR advantages: Targeted, fast (hours), cheap, works with degraded DNA, minimal template required. Best for: rapid diagnostics, forensics, single-locus testing.

WGS advantages: Unbiased, comprehensive, detects novel mutations. Best for: rare disease diagnosis, population genetics, detailed evolutionary studies.

Future: PCR and sequencing are complementary. PCR will remain the standard for rapid, targeted diagnostics. Sequencing will expand for rare disease diagnosis and personalized medicine.

Digital PCR and Droplet Technology

Emerging digital PCR partitions reactions into thousands of tiny water-in-oil droplets, each undergoing PCR independently. This allows absolute quantification and detects rare mutations with exquisite sensitivity. Expected to expand clinical diagnostics.

Use these mnemonics to consolidate key concepts for exam preparation. Memory devices are powerful tools for active recall.

Use these 40 questions to assess your understanding. Cover the answer section and attempt each question. If you struggle, review the relevant section. Active recall is one of the most effective study techniques.

Q: 1. What are the four essential components of PCR?

Answer: Template DNA, primers (forward and reverse), dNTPs, and Taq polymerase.

Q: 2. What is the function of primers in PCR?

Answer: Primers are short oligonucleotides (18-25 bp) complementary to 3' ends of target sequences. They define amplification boundaries and provide the 3'-OH for Taq to begin synthesis. Q: 3. Why is Taq polymerase used instead of other DNA polymerases?

Answer: Taq is heat-stable (survives 95°C). Most polymerases denature at high temperatures. Taq comes from thermophilic bacteria and remains active through PCR cycles.

Q: 4. What happens during denaturation? At what temperature?

Answer: Double-stranded DNA is heated to 94-95°C, breaking hydrogen bonds and producing single-stranded DNA, needed for primer annealing in the next step.

Q: 5. Explain the annealing step and why annealing temperature matters.

Answer: Primers bind to complementary sequences on denatured template at 55-65°C. Too high = no binding (no amplification); too low = non-specific binding (unwanted products).

Q: 6. What is the extension step, and why is timing important?

Answer: At 72°C, Taq polymerase synthesizes DNA complementary to template, adding dNTPs at ~1000 bases/sec. Time must be sufficient for target length (~1 minute per 1 kb). Q: 7. What is the formula for exponential PCR growth? Copies after 25, 30, 35 cycles? Answer: 2^n copies after n cycles. 25 cycles = 33 million, 30 cycles = 1 billion, 35 cycles = 34 billion.

Q: 8. Why do we run PCR products on an agarose gel?

Answer: Gel electrophoresis separates DNA by size and visualizes it using fluorescent dyes. DNA is negatively charged and migrates toward positive electrode; smaller fragments move faster.

Q: 9. What should every PCR gel include?

Answer: DNA ladder (size standards), negative control (should show no band), positive control (should show band), and sample lanes.

Q: 10. What does a smear instead of sharp bands indicate?

Answer: DNA degradation or contamination with nucleases (enzymes that cut DNA). Q: 11. Name three medical applications of PCR.

Answer: BRCA1/BRCA2 testing (cancer predisposition), hemophilia gene mutation detection, cystic fibrosis screening.

Q: 12. What is HaloPlex BRCA Panel Kit, and why is it needed?

Answer: BRCA1/BRCA2 genes are very large (5-10 kb). HaloPlex uses >1000 overlapping PCR primers to simultaneously amplify the entire genes, then sequences all products.

Q: 13. Describe prenatal genetic screening: timing, sample, methods.

Answer: Amniocentesis (14-20 weeks, amniotic fluid, 0.1% MC risk) or CVS (10-13 weeks, chorionic villi, 0.2% MC risk). PCR/karyotyping detects trisomy 21, aneuploidies, monogenic diseases. Q: 14. What is newborn screening? Name three diseases screened for.

Answer: Blood spot testing within 24-48 hours of birth. Detects PKU, congenital hypothyroidism, sickle cell disease, CF. Early detection prevents intellectual disability.

Q: 15. Define carrier screening and explain its purpose.

Answer: Identifies heterozygous individuals carrying one recessive disease allele but phenotypically normal. If both parents are carriers, 25% of children are affected. Used for reproductive planning. Q: 16. What is PGD (Preimplantation Genetic Diagnosis)? When is it used?

Answer: PCR on a day-3 embryo cell during IVF detects disease-causing mutations. Allows selection of unaffected embryos for implantation.

Q: 17. Why is PCR better than culture for diagnosing Chlamydia?

Answer: Chlamydia is obligate intracellular—cannot grow on standard media. PCR detects chlamydial DNA in urine (non-invasive) in 4 hours vs. culture, which is slow, expensive, impossible. Q: 18. Name four pathogens commonly diagnosed by PCR.

Answer: TB (Mycobacterium tuberculosis), HIV, Chlamydia trachomatis, Neisseria gonorrhoeae. Q: 19. Describe the Colin Pitchfork case. What was its significance?

Answer: 1988: First DNA conviction. Two girls (Lynda Mann, Dawn Ashworth) were murdered. Richard Buckland was initially suspected but DNA excluded him (exoneration). Mass screening identified Colin Pitchfork (matched both scenes, convicted).

Q: 20. What is RFLP, and why is it largely obsolete?

Answer: Restriction Fragment Length Polymorphism exploits variations in restriction enzyme cut sites. Disadvantages: requires large DNA (200+ ng), slow (weeks), subjective interpretation. Superseded by STRs.

Q: 21. Explain VNTR: what does it stand for and how does it work?

Answer: Variable Number Tandem Repeats contain sequences repeated multiple times in tandem. Number varies between individuals, creating size differences detected by gel electrophoresis. Q: 22. Define STR and explain why it's the modern gold standard.

Answer: Short Tandem Repeats are 2-6 bp sequences repeated multiple times. Small size (100-300 bp) allows PCR from degraded DNA. Multiplex (13+ loci). Standardized (CODIS). Probability of false match: ~1 in 1 billion. Fast (4-8 hours). Stable inheritance.

Q: 23. What is the CODIS database, and how many core STR loci does it include? Answer: Combined DNA Index System is the FBI's DNA database of convicted felons and crime scene profiles. Contains 13 core STR loci used worldwide.

Q: 24. Compare RFLP, VNTR, and STR across at least four dimensions.

Answer: Repeat size: RFLP (restriction sites), VNTR (10-100+ bp), STR (2-6 bp). Time: RFLP (4-6 weeks), VNTR (2-4 weeks), STR (4-8 hours). Multiplex: RFLP (no), VNTR (limited), STR (yes, 13+ loci). Stability: RFLP (stable), VNTR (unstable), STR (stable).

Q: 25. How does DNA profile matching provide evidence of guilt or exoneration? Answer: Match = suspect profile matches crime scene profile (probability <1 in billion). No match = suspect is excluded. Absence of match is powerful exoneration.

Q: 26. What was the 'DNA Wars' controversy in OJ Simpson trial?

Answer: Despite overwhelming DNA evidence (1 in 170 million probability), jury acquitted Simpson. Defense argued evidence collection was sloppy (contamination) and a racist detective planted evidence. Taught that even perfect DNA can be undermined by chain-of-custody concerns. Q: 27. What is the Lydia Fairchild chimerism case, and what does it teach?

Answer: Lydia's body contained cells from two genetically different individuals (herself and deceased twin). Her saliva DNA didn't match her children's. Further testing revealed dual genomes. Lesson: chimerism can introduce multiple DNA profiles, complicating interpretation.

Q: 28. Describe the principle of paternity testing using STRs.

Answer: Each child inherits one allele at each locus from each parent. If putative father lacks a required allele, he's excluded. All 13 loci consistent = paternity established (>99.9%).

Q: 29. At how many STR loci does one mismatch exclude paternity?

Answer: One. Absence of a required allele at even one locus excludes paternity. However, mutations and lab error must be considered before definitive exclusion.

Q: 30. What is a Likelihood Ratio (LR) in paternity testing?

Answer: LR compares probability of DNA results if 'he IS the father' vs. 'he is NOT the father.' LR = 1,000:1 means 1,000 times more likely if he is father. LR > 100:1 = inclusion; LR < 0.01:1 = exclusion. Q: 31. Name three research applications of PCR beyond forensics.

Answer: Gene expression (RT-PCR), cloning (amplify genes for recombinant protein), evolutionary studies (ancient DNA), metagenomics (bacterial identification), food safety (pathogen detection).

Q: 32. Compare PCR and Whole Genome Sequencing. Which is better?

Answer: Neither is better—complementary. PCR: targeted, fast, cheap, degraded DNA OK, minimal template. WGS: unbiased, comprehensive, detects novel variants. PCR for rapid diagnostics; WGS for rare disease diagnosis.

Q: 33. What is digital PCR, and how might it change diagnostics?

Answer: Digital PCR partitions reactions into thousands of tiny droplets, each undergoing PCR independently. Allows absolute quantification and detects rare mutations with high sensitivity. Q: 34. Define polymorphism. What fraction of the human genome is polymorphic? Answer: Polymorphism: multiple alleles at a locus (different variants in population). Humans share ~99.9% DNA; ~0.1% is polymorphic (~3 million differences in 3 billion nucleotides). Q: 35. What is 'stutter' in STR PCR products, and how should it be interpreted? Answer: During PCR, Taq sometimes slips on repeats, creating products one repeat unit shorter/longer than true allele. E.g., true is 8 repeats; stutter creates 7 or 9 repeat bands. Analysts recognize stutter as PCR artifact.

Q: 36. Explain what happens if annealing temperature in PCR is too high.

Answer: Primers don't anneal efficiently—few bind. Result: weak or absent PCR product. Q: 37. Explain what happens if annealing temperature is too low.

Answer: Primers can bind to sequences with partial complementarity (mismatches tolerated). Result: non-specific amplification of unwanted fragments. Gel shows multiple bands at unexpected sizes. Q: 38. Why is 'chain of custody' critical in forensic DNA evidence?

Answer: Even if DNA matches perfectly, a broken link in chain of custody (improper handling, labeling, storage, transfer) introduces reasonable doubt. Proper documentation and secure handling are essential.

Q: 39. Describe Sir Alec Jeffreys' contribution to forensics.

Answer: Jeffreys developed DNA fingerprinting using RFLP (1984). Demonstrated DNA could uniquely identify individuals. Enabled first exoneration (Richard Buckland) and conviction (Colin Pitchfork). Founded forensic DNA profiling field.

Q: 40. What is the difference between 'negative control' and 'positive control' in PCR? Answer: Negative control: PCR with no template. Should show NO band. Proves reaction is template-dependent. Positive control: PCR with known template. Should show band at expected size. Confirms entire reaction works.