FDC DNA Analysis Techniques 5 — Questions and Answers
Question 1: What is the primary function of proteinase K in the DNA extraction process?
- To denature double-stranded DNA into single strands
- To digest proteins and inactivate nucleases that would degrade DNA (Correct answer)
- To precipitate DNA out of solution
- To remove RNA contamination from the sample
Correct answer: To digest proteins and inactivate nucleases that would degrade DNA
Proteinase K digests cellular proteins, including nucleases (DNases) that would otherwise degrade the DNA, helping to release and protect the extracted DNA.
Question 2: In forensic toxicogenomics, what type of variation is most commonly studied to predict drug metabolism phenotypes?
- Mitochondrial heteroplasmy
- Single nucleotide polymorphisms (SNPs) in cytochrome P450 genes (Correct answer)
- Y-STR haplotypes
- Tandem repeat expansions in non-coding regions
Correct answer: Single nucleotide polymorphisms (SNPs) in cytochrome P450 genes
SNPs in cytochrome P450 genes (e.g., CYP2D6, CYP2C19) are the most commonly studied variants for predicting whether individuals are poor, intermediate, or ultra-rapid drug metabolizers.
Question 3: What is 'touch DNA,' and what challenge does it pose for forensic analysis?
- DNA extracted from buccal swabs; challenge is cell lysis efficiency
- Low-quantity DNA shed from skin cells through contact; challenge is stochastic effects and contamination (Correct answer)
- DNA from blood stains on touch-sensitive surfaces; challenge is substrate inhibition
- Mitochondrial DNA from fingerprint residue; challenge is nuclear interference
Correct answer: Low-quantity DNA shed from skin cells through contact; challenge is stochastic effects and contamination
Touch DNA refers to the trace amounts of cellular material shed through skin contact; its low quantity makes it susceptible to stochastic effects (allelic dropout, imbalance) and contamination.
Question 4: Which quality assurance document establishes guidelines for forensic DNA testing laboratories in the US?
- ISO 17025 only
- SWGDAM Guidelines and FBI QAS standards (Correct answer)
- ILAC G19 forensic standards
- NIST SP 800-series publications
Correct answer: SWGDAM Guidelines and FBI QAS standards
The FBI's Quality Assurance Standards (QAS) and SWGDAM guidelines set the requirements for accreditation and quality practices for forensic DNA laboratories in the US.
Question 5: What is the purpose of the denaturation step in PCR thermal cycling?
- To allow primers to bind to template DNA
- To separate double-stranded DNA into single-stranded templates (Correct answer)
- To extend primers and synthesize new DNA
- To cool the reaction and stabilize newly synthesized strands
Correct answer: To separate double-stranded DNA into single-stranded templates
Denaturation (typically ~95°C) breaks the hydrogen bonds between complementary base pairs, separating the double-stranded DNA into single-stranded templates for primer annealing.
Question 6: When a DNA mixture contains contributions from three individuals, which term describes the individual whose DNA is present in the greatest proportion?
- Secondary contributor
- Major contributor (Correct answer)
- Reference donor
- Primary template
Correct answer: Major contributor
In a DNA mixture, the major contributor is the individual whose DNA is present in the highest proportion, typically identified by higher peak heights in the electropherogram.
Question 7: Which extraction method is particularly effective for separating sperm cell DNA from epithelial cell DNA in sexual assault evidence?
- Phenol-chloroform extraction
- Differential extraction (differential lysis) (Correct answer)
- Chelex extraction
- FTA card extraction
Correct answer: Differential extraction (differential lysis)
Differential extraction exploits the resistance of sperm cell heads to lysis by detergents, allowing epithelial cells to be lysed and removed first before the sperm fraction is separately extracted.
FDC DNA Analysis Techniques 5
What is the primary function of proteinase K in the DNA extraction process?