Ascp Molecular Biology (Mb) Certification Exam
Updated Questions And Correct Answers
DOMAIN 1: Specimen Collection, Processing & Nucleic Acid Extraction
15 Questions | 15% of Examination | Specimen types, pre-analytical variables, extraction methods,
quality assessment, inhibition detection
Question 1 [Specimen Types]
Which of the following collection tubes is most appropriate for molecular diagnostic
testing requiring intact cellular DNA from whole blood?
A. EDTA (lavender-top) tube
B. Sodium heparin (green-top) tube
C. Sodium citrate (light blue-top) tube
D. Serum separator tube (SST)
Rationale: EDTA (lavender-top) tubes are the preferred collection tubes for molecular
diagnostic testing requiring intact cellular DNA. EDTA acts as an anticoagulant by chelating
divalent cations (Mg2+, Ca2+) necessary for nuclease activity, thereby inhibiting DNase and
preserving DNA integrity. Heparin tubes are contraindicated for PCR-based assays because
heparin is a potent polymerase inhibitor that copurifies with nucleic acids and interferes with
amplification. Sodium citrate tubes are primarily used for coagulation studies. Serum separator
tubes lack anticoagulant and allow clot formation, which traps nucleated cells and reduces DNA
yield.
Question 2 [Pre-Analytical Variables]
A whole blood specimen for DNA extraction is received at the molecular laboratory
72 hours after collection. The specimen was stored at room temperature. What is
the most likely effect on nucleic acid quality?
A. DNA remains stable, but RNA is significantly degraded
B. Both DNA and RNA remain stable at room temperature for up to 7 days
C. DNA is fragmented due to residual nuclease activity, and RNA is extensively
degraded
D. Nucleic acids are preserved due to EDTA inhibition of all nucleases
Rationale: While EDTA inhibits many divalent cation-dependent nucleases, prolonged storage
of whole blood at room temperature (72 hours) leads to progressive DNA fragmentation due to
residual nuclease activity and cell lysis. White blood cell membranes become compromised over
time, releasing intracellular nucleases that can partially degrade genomic DNA. RNA is
particularly labile due to ubiquitous RNases that do not require divalent cations for activity.
Best practice dictates processing whole blood specimens within 24-48 hours for DNA extraction
and within 4-6 hours (or immediate stabilization) for RNA analysis.
Question 3 [Extraction Methods]
In the phenol-chloroform organic extraction method, which phase contains the
DNA after phase separation?
, A. Upper aqueous phase
B. Interphase (white layer)
C. Lower organic phase
D. DNA is equally distributed between the aqueous and organic phases
Rationale: In phenol-chloroform extraction, after centrifugation, the mixture separates into
three layers: an upper aqueous phase, a white interphase, and a lower organic phase. DNA,
being hydrophilic and carrying a negative charge at neutral to slightly alkaline pH, partitions
into the upper aqueous phase. Proteins and lipids, being hydrophobic or denatured by the
organic solvent, collect in the lower organic phase and interphase. The key principle is that
phenol denatures proteins, while chloroform improves phase separation and removes lipid
contaminants. Multiple extractions are typically performed to maximize purity.
Question 4 [Extraction Methods]
Which nucleic acid extraction method utilizes the principle that DNA binds to silica
surfaces in the presence of high concentrations of chaotropic salts?
A. Phenol-chloroform extraction
B. Silica column (solid-phase) extraction
C. Magnetic bead-based extraction
D. Cesium chloride density gradient centrifugation
Rationale: Silica column (solid-phase) extraction is based on the principle that DNA binds to
silica (SiO2) surfaces in the presence of high concentrations of chaotropic salts such as
guanidinium thiocyanate or guanidinium hydrochloride. These salts disrupt the hydration shell
around DNA and silica, allowing the negatively charged phosphate backbone to interact
directly with the silica surface. After binding, contaminants are washed away with
ethanolcontaining buffers, and purified DNA is eluted with low-ionic-strength buffer or water.
This method avoids the use of hazardous organic solvents and is amenable to automation.
Question 5 [Extraction Methods]
Magnetic bead-based nucleic acid extraction offers which advantage over traditional
silica column methods?
A. Higher purity of extracted DNA due to multiple wash steps
B. Elimination of centrifugation steps, enabling full automation and faster
processing
C. Greater sensitivity for detection of low-abundance targets
D. No requirement for chaotropic salts during the binding step
Rationale: Magnetic bead-based extraction eliminates the need for centrifugation steps that
are required in silica column methods. Instead of spinning columns to bind, wash, and elute,
magnetic beads are captured by external magnets, allowing supernatant removal by
aspiration. This enables full automation on platforms such as the KingFisher, MagNA Pure, and
QIAsymphony, and reduces hands-on time. Magnetic beads still use chaotropic or
carboxylcoated chemistries for nucleic acid binding. The purity and sensitivity are comparable
to silica columns when protocols are optimized, but the key advantage is automation
compatibility and throughput.
Question 6 [Quality Assessment]
A spectrophotometric A260/A280 ratio of 1.2 for a DNA extract indicates which of
the following?
A. The DNA is pure and free of contamination
B. The sample has significant protein contamination
C. The sample has RNA contamination
, D. The sample has phenol contamination
Rationale: The A260/A280 ratio is used to assess nucleic acid purity. Pure DNA has an
A260/A280 ratio of approximately 1.8, while pure RNA has a ratio of approximately 2.0. A
ratio of 1.2 indicates significant protein contamination because proteins absorb strongly at 280
nm (due to tryptophan and tyrosine residues), which increases the A280 reading and lowers the
A260/A280 ratio. RNA contamination would typically increase the ratio above 1.8. Phenol
contamination typically results in a ratio below 1.0 because phenol absorbs strongly at 270-275
nm and shifts the spectrum. A ratio of 1.2 clearly points to protein carryover from incomplete
purification.
Question 7 [Quality Assessment]
Which method provides the most accurate quantification of double-stranded DNA
for downstream applications such as qPCR?
A. Spectrophotometry (A260 measurement)
B. Fluorometry using dsDNA-binding dyes (e.g., PicoGreen)
C. Agarose gel electrophoresis with ethidium bromide staining
D. Nanodrop spectrophotometric measurement
Rationale: Fluorometry using dsDNA-binding dyes such as PicoGreen or Qubit dsDNA BR/HS
assays provides the most accurate quantification of double-stranded DNA for downstream
applications like qPCR. These intercalating dyes are highly specific for dsDNA and do not bind
single-stranded DNA, RNA, or free nucleotides, thereby avoiding overestimation of DNA
concentration. Spectrophotometry (including Nanodrop) measures total absorbance at 260 nm
from all nucleic acid species, including RNA and free nucleotides, which can lead to inflated
concentration readings. Gel electrophoresis provides qualitative assessment of integrity and
approximate size but is not quantitative enough for precise concentration determination.
Question 8 [FFPE Specimens]
Formalin-fixed, paraffin-embedded (FFPE) tissue presents unique challenges for
molecular testing primarily because:
A. Formalin penetrates tissue too slowly to prevent autolysis
B. Formalin causes cross-linking between proteins and nucleic acids, leading to
fragmentation and chemical modification of DNA
C. Paraffin cannot be completely removed from tissue sections
D. FFPE tissues contain insufficient cellular material for nucleic acid extraction
Rationale: FFPE tissue is widely used in clinical molecular diagnostics but presents significant
challenges due to formalin-induced cross-linking between proteins and nucleic acids. These
methylene bridge cross-links cause DNA fragmentation (typically 100-300 bp fragments) and
chemical modifications, including cytosine deamination (C>T artifacts). The cross-linking also
makes DNA extraction more difficult, requiring prolonged proteinase K digestion. While
paraffin removal is a processing step, it is routinely accomplished with xylene or alternative
deparaffinization reagents. The quantity of material is often sufficient; the primary challenge is
quality degradation from the fixation process itself.
Question 9 [Specimen Transport]
For optimal preservation of RNA in a blood specimen destined for gene expression
analysis, which of the following should be used?
A. Standard EDTA tube transported on ice within 24 hours
B. PAXgene Blood RNA tube with immediate mixing and room-temperature
storage
C. Heparin tube with immediate freezing at -80°C
D. Sodium citrate tube transported at 37°C
, Rationale: PAXgene Blood RNA tubes contain a proprietary reagent that stabilizes
intracellular RNA by immediately lysing cells and inactivating RNases upon blood draw. The
tubes require immediate inversion mixing and can be stored at room temperature for up to 3
days or refrigerated for up to 5 days, making them ideal for transport to reference laboratories.
While EDTA tubes on ice can preserve RNA short-term, RNA degradation begins rapidly after
collection due to ubiquitous RNases. Heparin tubes are contraindicated for PCR-based assays.
Sodium citrate tubes at 37°C would accelerate RNA degradation.
Question 10 [Inhibition Detection]
An internal amplification control (IAC) is included in a PCR assay. The target
amplifies successfully, but the IAC fails to amplify. What is the most likely
interpretation?
A. The specimen is negative for the target organism, and the IAC failure is inconsequential
B. PCR inhibitors are present in the specimen, but the target concentration is
high enough to overcome inhibition
C. The IAC is non-functional and should be redesigned
D. The PCR thermocycling conditions are suboptimal for the IAC but adequate for the target
Rationale: When the target amplifies but the IAC fails, the most likely interpretation is that
PCR inhibitors are present in the specimen at concentrations that partially inhibit amplification.
The target amplifies because it is present at high copy number, which can overcome moderate
inhibition, but the IAC (present at low copy number designed to detect inhibition) is suppressed
below its detection threshold. This is a critical quality indicator suggesting that low-level targets
could be missed (false-negative risk). Appropriate corrective actions include specimen dilution,
additional purification, or addition of amplification facilitators such as BSA or betaine.
Question 11 [Extraction Methods]
When extracting DNA from buccal swab specimens, which pre-treatment step is
critical before proceeding with silica column purification?
A. Proteinase K digestion to lyse epithelial cells and release DNA
B. RNase A treatment to remove contaminating RNA
C. Phenol extraction to remove proteins before column binding
D. Heat inactivation at 95°C for 10 minutes to denature nucleases
Rationale: Buccal swab specimens contain buccal epithelial cells that must be lysed to release
genomic DNA before silica column purification. Proteinase K digestion is the critical
pretreatment step, as it digests cellular and nuclear proteins, including histones and membrane
proteins, liberating DNA into solution. This step also inactivates nucleases that could degrade
DNA during processing. While RNase A treatment may improve purity, it is not critical for most
DNA-based applications. Phenol extraction would be redundant before column purification.
Heat inactivation alone is insufficient because it does not release DNA from the cellular matrix
effectively.
Question 12 [Quality Assessment]
On agarose gel electrophoresis, a high-quality genomic DNA preparation should
display which of the following patterns?
A. A sharp, discrete band at 200-300 bp
B. A single high-molecular-weight band near the well with minimal smearing
C. A diffuse smear spanning from 100 bp to 10,000 bp
D. Two discrete bands corresponding to the leading and lagging strands
Rationale: High-quality genomic DNA should appear as a single, compact, high-
molecularweight band near the top of the gel (close to the well) with minimal smearing,
indicating that the DNA is intact and not significantly fragmented. Smearing down the gel
Updated Questions And Correct Answers
DOMAIN 1: Specimen Collection, Processing & Nucleic Acid Extraction
15 Questions | 15% of Examination | Specimen types, pre-analytical variables, extraction methods,
quality assessment, inhibition detection
Question 1 [Specimen Types]
Which of the following collection tubes is most appropriate for molecular diagnostic
testing requiring intact cellular DNA from whole blood?
A. EDTA (lavender-top) tube
B. Sodium heparin (green-top) tube
C. Sodium citrate (light blue-top) tube
D. Serum separator tube (SST)
Rationale: EDTA (lavender-top) tubes are the preferred collection tubes for molecular
diagnostic testing requiring intact cellular DNA. EDTA acts as an anticoagulant by chelating
divalent cations (Mg2+, Ca2+) necessary for nuclease activity, thereby inhibiting DNase and
preserving DNA integrity. Heparin tubes are contraindicated for PCR-based assays because
heparin is a potent polymerase inhibitor that copurifies with nucleic acids and interferes with
amplification. Sodium citrate tubes are primarily used for coagulation studies. Serum separator
tubes lack anticoagulant and allow clot formation, which traps nucleated cells and reduces DNA
yield.
Question 2 [Pre-Analytical Variables]
A whole blood specimen for DNA extraction is received at the molecular laboratory
72 hours after collection. The specimen was stored at room temperature. What is
the most likely effect on nucleic acid quality?
A. DNA remains stable, but RNA is significantly degraded
B. Both DNA and RNA remain stable at room temperature for up to 7 days
C. DNA is fragmented due to residual nuclease activity, and RNA is extensively
degraded
D. Nucleic acids are preserved due to EDTA inhibition of all nucleases
Rationale: While EDTA inhibits many divalent cation-dependent nucleases, prolonged storage
of whole blood at room temperature (72 hours) leads to progressive DNA fragmentation due to
residual nuclease activity and cell lysis. White blood cell membranes become compromised over
time, releasing intracellular nucleases that can partially degrade genomic DNA. RNA is
particularly labile due to ubiquitous RNases that do not require divalent cations for activity.
Best practice dictates processing whole blood specimens within 24-48 hours for DNA extraction
and within 4-6 hours (or immediate stabilization) for RNA analysis.
Question 3 [Extraction Methods]
In the phenol-chloroform organic extraction method, which phase contains the
DNA after phase separation?
, A. Upper aqueous phase
B. Interphase (white layer)
C. Lower organic phase
D. DNA is equally distributed between the aqueous and organic phases
Rationale: In phenol-chloroform extraction, after centrifugation, the mixture separates into
three layers: an upper aqueous phase, a white interphase, and a lower organic phase. DNA,
being hydrophilic and carrying a negative charge at neutral to slightly alkaline pH, partitions
into the upper aqueous phase. Proteins and lipids, being hydrophobic or denatured by the
organic solvent, collect in the lower organic phase and interphase. The key principle is that
phenol denatures proteins, while chloroform improves phase separation and removes lipid
contaminants. Multiple extractions are typically performed to maximize purity.
Question 4 [Extraction Methods]
Which nucleic acid extraction method utilizes the principle that DNA binds to silica
surfaces in the presence of high concentrations of chaotropic salts?
A. Phenol-chloroform extraction
B. Silica column (solid-phase) extraction
C. Magnetic bead-based extraction
D. Cesium chloride density gradient centrifugation
Rationale: Silica column (solid-phase) extraction is based on the principle that DNA binds to
silica (SiO2) surfaces in the presence of high concentrations of chaotropic salts such as
guanidinium thiocyanate or guanidinium hydrochloride. These salts disrupt the hydration shell
around DNA and silica, allowing the negatively charged phosphate backbone to interact
directly with the silica surface. After binding, contaminants are washed away with
ethanolcontaining buffers, and purified DNA is eluted with low-ionic-strength buffer or water.
This method avoids the use of hazardous organic solvents and is amenable to automation.
Question 5 [Extraction Methods]
Magnetic bead-based nucleic acid extraction offers which advantage over traditional
silica column methods?
A. Higher purity of extracted DNA due to multiple wash steps
B. Elimination of centrifugation steps, enabling full automation and faster
processing
C. Greater sensitivity for detection of low-abundance targets
D. No requirement for chaotropic salts during the binding step
Rationale: Magnetic bead-based extraction eliminates the need for centrifugation steps that
are required in silica column methods. Instead of spinning columns to bind, wash, and elute,
magnetic beads are captured by external magnets, allowing supernatant removal by
aspiration. This enables full automation on platforms such as the KingFisher, MagNA Pure, and
QIAsymphony, and reduces hands-on time. Magnetic beads still use chaotropic or
carboxylcoated chemistries for nucleic acid binding. The purity and sensitivity are comparable
to silica columns when protocols are optimized, but the key advantage is automation
compatibility and throughput.
Question 6 [Quality Assessment]
A spectrophotometric A260/A280 ratio of 1.2 for a DNA extract indicates which of
the following?
A. The DNA is pure and free of contamination
B. The sample has significant protein contamination
C. The sample has RNA contamination
, D. The sample has phenol contamination
Rationale: The A260/A280 ratio is used to assess nucleic acid purity. Pure DNA has an
A260/A280 ratio of approximately 1.8, while pure RNA has a ratio of approximately 2.0. A
ratio of 1.2 indicates significant protein contamination because proteins absorb strongly at 280
nm (due to tryptophan and tyrosine residues), which increases the A280 reading and lowers the
A260/A280 ratio. RNA contamination would typically increase the ratio above 1.8. Phenol
contamination typically results in a ratio below 1.0 because phenol absorbs strongly at 270-275
nm and shifts the spectrum. A ratio of 1.2 clearly points to protein carryover from incomplete
purification.
Question 7 [Quality Assessment]
Which method provides the most accurate quantification of double-stranded DNA
for downstream applications such as qPCR?
A. Spectrophotometry (A260 measurement)
B. Fluorometry using dsDNA-binding dyes (e.g., PicoGreen)
C. Agarose gel electrophoresis with ethidium bromide staining
D. Nanodrop spectrophotometric measurement
Rationale: Fluorometry using dsDNA-binding dyes such as PicoGreen or Qubit dsDNA BR/HS
assays provides the most accurate quantification of double-stranded DNA for downstream
applications like qPCR. These intercalating dyes are highly specific for dsDNA and do not bind
single-stranded DNA, RNA, or free nucleotides, thereby avoiding overestimation of DNA
concentration. Spectrophotometry (including Nanodrop) measures total absorbance at 260 nm
from all nucleic acid species, including RNA and free nucleotides, which can lead to inflated
concentration readings. Gel electrophoresis provides qualitative assessment of integrity and
approximate size but is not quantitative enough for precise concentration determination.
Question 8 [FFPE Specimens]
Formalin-fixed, paraffin-embedded (FFPE) tissue presents unique challenges for
molecular testing primarily because:
A. Formalin penetrates tissue too slowly to prevent autolysis
B. Formalin causes cross-linking between proteins and nucleic acids, leading to
fragmentation and chemical modification of DNA
C. Paraffin cannot be completely removed from tissue sections
D. FFPE tissues contain insufficient cellular material for nucleic acid extraction
Rationale: FFPE tissue is widely used in clinical molecular diagnostics but presents significant
challenges due to formalin-induced cross-linking between proteins and nucleic acids. These
methylene bridge cross-links cause DNA fragmentation (typically 100-300 bp fragments) and
chemical modifications, including cytosine deamination (C>T artifacts). The cross-linking also
makes DNA extraction more difficult, requiring prolonged proteinase K digestion. While
paraffin removal is a processing step, it is routinely accomplished with xylene or alternative
deparaffinization reagents. The quantity of material is often sufficient; the primary challenge is
quality degradation from the fixation process itself.
Question 9 [Specimen Transport]
For optimal preservation of RNA in a blood specimen destined for gene expression
analysis, which of the following should be used?
A. Standard EDTA tube transported on ice within 24 hours
B. PAXgene Blood RNA tube with immediate mixing and room-temperature
storage
C. Heparin tube with immediate freezing at -80°C
D. Sodium citrate tube transported at 37°C
, Rationale: PAXgene Blood RNA tubes contain a proprietary reagent that stabilizes
intracellular RNA by immediately lysing cells and inactivating RNases upon blood draw. The
tubes require immediate inversion mixing and can be stored at room temperature for up to 3
days or refrigerated for up to 5 days, making them ideal for transport to reference laboratories.
While EDTA tubes on ice can preserve RNA short-term, RNA degradation begins rapidly after
collection due to ubiquitous RNases. Heparin tubes are contraindicated for PCR-based assays.
Sodium citrate tubes at 37°C would accelerate RNA degradation.
Question 10 [Inhibition Detection]
An internal amplification control (IAC) is included in a PCR assay. The target
amplifies successfully, but the IAC fails to amplify. What is the most likely
interpretation?
A. The specimen is negative for the target organism, and the IAC failure is inconsequential
B. PCR inhibitors are present in the specimen, but the target concentration is
high enough to overcome inhibition
C. The IAC is non-functional and should be redesigned
D. The PCR thermocycling conditions are suboptimal for the IAC but adequate for the target
Rationale: When the target amplifies but the IAC fails, the most likely interpretation is that
PCR inhibitors are present in the specimen at concentrations that partially inhibit amplification.
The target amplifies because it is present at high copy number, which can overcome moderate
inhibition, but the IAC (present at low copy number designed to detect inhibition) is suppressed
below its detection threshold. This is a critical quality indicator suggesting that low-level targets
could be missed (false-negative risk). Appropriate corrective actions include specimen dilution,
additional purification, or addition of amplification facilitators such as BSA or betaine.
Question 11 [Extraction Methods]
When extracting DNA from buccal swab specimens, which pre-treatment step is
critical before proceeding with silica column purification?
A. Proteinase K digestion to lyse epithelial cells and release DNA
B. RNase A treatment to remove contaminating RNA
C. Phenol extraction to remove proteins before column binding
D. Heat inactivation at 95°C for 10 minutes to denature nucleases
Rationale: Buccal swab specimens contain buccal epithelial cells that must be lysed to release
genomic DNA before silica column purification. Proteinase K digestion is the critical
pretreatment step, as it digests cellular and nuclear proteins, including histones and membrane
proteins, liberating DNA into solution. This step also inactivates nucleases that could degrade
DNA during processing. While RNase A treatment may improve purity, it is not critical for most
DNA-based applications. Phenol extraction would be redundant before column purification.
Heat inactivation alone is insufficient because it does not release DNA from the cellular matrix
effectively.
Question 12 [Quality Assessment]
On agarose gel electrophoresis, a high-quality genomic DNA preparation should
display which of the following patterns?
A. A sharp, discrete band at 200-300 bp
B. A single high-molecular-weight band near the well with minimal smearing
C. A diffuse smear spanning from 100 bp to 10,000 bp
D. Two discrete bands corresponding to the leading and lagging strands
Rationale: High-quality genomic DNA should appear as a single, compact, high-
molecularweight band near the top of the gel (close to the well) with minimal smearing,
indicating that the DNA is intact and not significantly fragmented. Smearing down the gel
