FDC Forensic Laboratory Procedures 3 — Questions and Answers
Question 1: Which safety class of biological hazard does most human blood evidence fall under in a forensic DNA laboratory?
- BSL-1
- BSL-2 (Correct answer)
- BSL-3
- BSL-4
Correct answer: BSL-2
Human blood is classified as a BSL-2 hazard because it may contain bloodborne pathogens such as HIV and hepatitis B/C.
Question 2: What is the primary reason forensic DNA laboratories use positive displacement pipettes for highly viscous or volatile samples?
- To reduce aerosol contamination compared to standard pipettes (Correct answer)
- To increase pipetting speed during high-throughput processing
- Because they are less expensive than air displacement pipettes
- To comply with OSAC requirements for all DNA extractions
Correct answer: To reduce aerosol contamination compared to standard pipettes
Positive displacement pipettes use a disposable piston that contacts the liquid directly, eliminating aerosols that could contaminate samples or expose analysts to hazardous substances.
Question 3: When performing differential extraction for sexual assault evidence, what is the purpose of the first centrifugation step?
- To pellet sperm cells separately from epithelial cells (Correct answer)
- To lyse the epithelial cells while leaving sperm cells intact
- To remove the organic solvent from the aqueous phase
- To concentrate the total DNA prior to quantification
Correct answer: To pellet sperm cells separately from epithelial cells
The first centrifugation pellets all cellular material; differential lysis then lyses epithelial cells first, and a second centrifugation separates the sperm cell fraction from the epithelial cell lysate.
Question 4: A DNA quantification result shows an OD260/OD280 ratio of 1.4. What does this indicate about the sample?
- The sample is pure DNA suitable for STR amplification
- The sample likely contains protein contamination (Correct answer)
- The sample likely contains RNA contamination
- The spectrophotometer requires recalibration
Correct answer: The sample likely contains protein contamination
A 260/280 ratio significantly below 1.8 indicates protein contamination, because proteins absorb strongly at 280 nm.
Question 5: Which of the following best describes a NIST SRM (Standard Reference Material) used in forensic DNA analysis?
- A commercially manufactured STR kit used for crime scene analysis
- A characterized reference material with certified property values used for assay validation (Correct answer)
- A database of allele frequencies compiled from population studies
- An internal size standard added to each capillary electrophoresis run
Correct answer: A characterized reference material with certified property values used for assay validation
NIST SRMs are certified reference materials with well-characterized genetic profiles used to verify assay performance and ensure measurement accuracy.
Question 6: What is the function of formamide in STR capillary electrophoresis sample preparation?
- It acts as an intercalating agent to stain DNA fragments
- It denatures double-stranded DNA and prevents secondary structure formation (Correct answer)
- It increases the ionic strength of the sample buffer
- It serves as a size standard for fragment sizing
Correct answer: It denatures double-stranded DNA and prevents secondary structure formation
Formamide denatures DNA into single strands and destabilizes secondary structures, ensuring consistent migration of STR fragments during capillary electrophoresis.
Question 7: Under SWGDAM guidelines, how frequently must forensic DNA analysts complete proficiency testing?
- Once every five years
- Annually, at minimum (Correct answer)
- Only upon initial certification
- Twice per year at minimum
Correct answer: Annually, at minimum
SWGDAM guidelines and ASCLD/LAB accreditation standards require forensic DNA analysts to complete external proficiency testing at least once per year.
FDC Forensic Laboratory Procedures 3
Which safety class of biological hazard does most human blood evidence fall under in a forensic DNA laboratory?