Biotechnology Techniques - Chapter 10: PCR & Forensic Analysis Study Notes

Polymerase Chain Reaction (PCR): A technique used to amplify DNA segments.
Forward and Reverse PCR Primers: Short synthetic pieces of DNA designed to anneal to specific sequences for DNA amplification.
Forensic Analysis: The application of scientific methods to analyze physical evidence and identify individuals.
Locus: The specific physical location of a gene or DNA sequence on a chromosome.
Allele: Variants of a gene at a specific locus; humans are diploid, having two alleles per locus (except at sex chromosomes).
VNTR (Variable Number Tandem Repeats): Repeated sequences of DNA that vary in number among individuals.
STR (Short Tandem Repeats): A type of VNTR characterized by shorter repeat sequences.
Multiloci VNTRs: VNTRs found in multiple loci across the genome.
Single-locus VNTRs: VNTRs located at only one locus.
Dideoxy Terminator Sequencing: A sequencing method that uses dideoxynucleotides to terminate DNA strand synthesis, resulting in fragments of different lengths for analysis.

PCR Overview:
- PCR is an artificial technique mimicking natural DNA replication that allows exponential amplification of DNA segments.
- PCR can multiply a specific DNA segment billions of times within a few hours, providing sufficient DNA for various analyses, including forensic and genetic engineering applications.
- Described as the "rocket ship" of DNA replication.
Mechanism of PCR:
- PCR involves exponential amplification of a DNA segment between the 5’ ends of the forward and reverse primers.
- It relies on thermostable DNA polymerase, specifically Taq polymerase, derived from the bacterium Thermus aquaticus, which thrives at high temperatures.
Specificity of PCR:
- The specific target amplification depends on the primer sequences and their orientation.
- PCR is instrumental in forensic analysis to amplify VNTRs, which can be used even when DNA samples are limited or degraded.
DNA Sequencing Through PCR:
- DNA sequencing within PCR utilizes only one primer along with fluorescently labeled dideoxynucleotides, that terminate strand growth at random positions, creating fragments of varying sizes.
- Each fragment ends with a labeled dideoxynucleotide, allowing for identification during subsequent analysis.

1960: Dr. Thomas Brock isolates Thermus aquaticus, which grows at Yellowstone hot springs with an optimal temperature of ~70 °C.
1976: Isolation of Taq DNA polymerase by Chien, Edgar, and Trela published in J. Bacteriology.
1983: Kary Mullis tests his PCR idea.
1993: Kary Mullis awarded the Nobel Prize in Chemistry.
Contributions to PCR Development:
- The development of PCR highlights collaborative scientific efforts, although recognition often goes to a few individuals.

Basic Steps in PCR Cycle:
1. Denaturation: DNA strands are separated by boiling.
2. Annealing: Synthetic primers (oligos) hybridize to template strands.
3. Extension: DNA polymerase synthesizes new DNA strands from the primers.
PCR Cycles:
- PCR involves cycling through the above three steps approximately 30-35 times.
- Within a few cycles, specific DNA products begin to significantly outweigh original template DNA due to exponential amplification.

Amplification Calculation:
- Cycle 3: 8 strands (2³)
- Cycle 4: 16 strands (2⁴)
- Cycle 5: 32 strands (2⁵)
- Cycle 6: 64 strands (2⁶)
- Cycle 30: 1,073,741,824 strands (2³⁰)
- Starting from a single template leads to about 500 million double-stranded products after 30 cycles.

Stepwise PCR Setup:
1. Identify and obtain the target sequence.
2. Design and order forward and reverse primers.
3. Prepare the PCR reaction mix with appropriate components, such as buffer and dNTPs, specific to Taq polymerase.
4. Program the PCR machine for cycling conditions:
- Step 1: 94 °C for 30 seconds (denature)
- Step 2: 60 °C for 30 seconds (anneal)
- Step 3: 72 °C for 60 seconds (extension)
1. Analyze the PCR products on agarose gel after amplification is complete.

Primer Design Example:
1. Identify the VNTR sequence, with variants in individuals.
2. Construct a forward primer (~24 nucleotides) on the 5’ side.
- Example: 5’-TACGTACGTCAGTCCGATGCATCG-3’
1. Construct a complementary reverse primer, writing it backwards to ensure proper binding.
- Example: 5’-GTCACCCATCAGTCGTTACGATCG-3’
Product Size Calculation:
- Product = Length of forward primer (24 bp) + 10 bp (repeat) x Number of Repeats + Length of reverse primer (24 bp) = 149 bp.
Annealing Temperature (Ta):
- Calculated using $T_a = 4(G + C) + 2(A + T) ext{ °C}$ .
- Setting appropriate Ta is critical for the specificity of annealing.

Mismatches at the 3’ end of the primer result in no extension, while mismatches at the 5’ end allow for extension.
Primer Dimers and Self-complementarity:
- If primers can anneal to each other, they form primer dimers, wasting resources and lowering yield.
- Careful design can mitigate self-complementarity problems.

Differentiating Individuals:
- PCR can distinguish between individuals based on unique VNTRs.
- Longer repeats yield longer PCR products; thus, percentage of repeat variation aids identification in forensics.

Use of VNTRs in forensic statistics involves calculating the likelihood of matches based on locus frequencies.
- Combined frequency for 13 loci used by the FBI results in match probabilities of 1 in 100 billion.
- Statistical analysis demonstrates not all individuals are unique, leading to practical considerations in case evaluations.

Hemings-Jefferson Case
- PCR analysis investigates the Y chromosome of descendants to determine paternity through historical lineage, comparing STR profiles across family descendants.
- Results show that lineage likely aligns with Thomas Jefferson based on match evidence, establishing familial connections.

Maternal Lineage Tracing:
- Mitochondrial DNA is inherited exclusively from the mother, which makes it a powerful tool for maternal lineage studies.
- Specific regions, such as the D-loop, are surveyed for polymorphisms that assist in individual identification.

Romanov Case:
- Genetic analysis of mitochondrial DNA played a pivotal role in tracing lineage and identifying remains from historical contexts.

PCR's sensitivity demands careful handling to prevent contamination. Evidence must be managed meticulously from collection to analysis to ensure integrity.

PCR serves as an indispensable tool in modern forensic analysis, demonstrating its wide-ranging applications in genetics, paternity testing, and historical lineage tracing.