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ABMGG Molecular Genetic Pathology study materials
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ABMGG Molecular Genetic Pathology practice questions

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Question 1Molecular Biology Fundamentals and Nucleic Acid Chemistry

Which enzyme activity is responsible for removing RNA primers and replacing them with DNA during lagging-strand synthesis in eukaryotic replication?

  1. Flap endonuclease 1 combined with DNA polymerase delta Correct answer.
  2. Topoisomerase II decatenation activity
  3. DNA ligase I sealing activity
  4. DNA polymerase alpha primase activity

Explanation

In eukaryotes, polymerase delta extends Okazaki fragments and its associated flap endonuclease removes the displaced RNA primer flap; RNase H also contributes to primer removal. Ligase then seals the nick after replacement. Primase synthesizes rather than removes primers, and topoisomerase relieves supercoiling.

Question 2Molecular Biology Fundamentals and Nucleic Acid Chemistry

In a Sanger sequencing reaction, what is the role of the dideoxynucleotide (ddNTP)?

  1. It lacks a 3'-hydroxyl group, so incorporation terminates chain elongation Correct answer.
  2. It increases the rate of polymerase processivity by stabilizing the template
  3. It provides a fluorescent label only when incorporated at the chain terminus after exonuclease proofreading
  4. It acts as a competitive inhibitor that prevents template binding by polymerase

Explanation

ddNTPs lack the 3'-OH needed for nucleophilic attack by the next incoming nucleotide, so their incorporation halts synthesis and generates a nested set of terminated fragments. The terminating nucleotide is detected by dye or label.

Question 3Molecular Biology Fundamentals and Nucleic Acid Chemistry

Which property of the DNA double helix is most directly responsible for the hyperchromic effect observed upon thermal denaturation?

  1. Hydrogen bonds between strands absorb strongly at 260 nm
  2. Stacked base pairs absorb less UV light than unstacked bases Correct answer.
  3. Phosphate backbone charge changes with temperature
  4. Guanine-cytosine content falls as temperature rises

Explanation

Base stacking quenches UV absorbance; when strands separate, bases become unstacked and absorbance at 260 nm increases (hyperchromicity). The effect depends on base stacking rather than backbone charge or base composition changing with temperature.

Question 4Molecular Biology Fundamentals and Nucleic Acid Chemistry

A restriction endonuclease recognizes the palindromic sequence 5'-GAATTC-3'. Which feature defines this as a Type II enzyme?

  1. It combines methyltransferase and endonuclease activities in one polypeptide
  2. It requires ATP hydrolysis for translocation along DNA
  3. It cleaves DNA at a fixed distance from an asymmetric recognition site
  4. It cleaves within or near a palindromic recognition sequence without ATP Correct answer.

Explanation

Type II restriction enzymes recognize specific, usually palindromic sequences and cleave within or near the site, typically without ATP. Type I and Type III enzymes cut at a distance from the recognition site and require ATP; Type I enzymes also have combined methylase activity.

Question 5Molecular Biology Fundamentals and Nucleic Acid Chemistry

Which primer design feature most reduces the risk of primer-dimer formation in a PCR assay?

  1. Avoiding complementary sequences at the 3' ends of the forward and reverse primers Correct answer.
  2. Maximizing the 3' end complementarity between the two primers
  3. Increasing primer length beyond 45 nucleotides
  4. Choosing primers with self-complementary 3' termini

Explanation

Primer-dimers arise when the 3' ends of primers anneal to each other and are extended by polymerase. Designing primers without 3' complementarity minimizes this. Excessive length does not solve complementarity and may reduce specificity.

Question 6Molecular Biology Fundamentals and Nucleic Acid Chemistry

Which statement correctly describes the difference between the template strand used for transcription and the coding (sense) strand of a gene?

  1. The template strand is always the leading strand of replication
  2. The template strand has the same sequence as the RNA transcript
  3. The coding strand has the same sequence as the RNA transcript except T replaces U Correct answer.
  4. The coding strand is read by RNA polymerase in the 3' to 5' direction

Explanation

The coding strand matches the RNA transcript in sequence with thymine in place of uracil. RNA polymerase reads the template (antisense) strand 3' to 5', synthesizing RNA 5' to 3'. The template strand can be either the leading or lagging strand depending on gene orientation.

Question 7Molecular Biology Fundamentals and Nucleic Acid Chemistry

A DNA sample has a Tm of 78 degrees C in 50 mM NaCl. If the salt concentration is raised to 500 mM, what is the expected effect on Tm?

  1. Tm increases because cations shield phosphate repulsion between strands Correct answer.
  2. Tm is unchanged because salt affects only the backbone and not base pairing
  3. Tm decreases because high salt promotes strand separation
  4. Tm decreases because cations destabilize base stacking

Explanation

Monovalent cations neutralize the negatively charged phosphate backbone, reducing electrostatic repulsion between strands and stabilizing the duplex, thereby raising Tm. Higher ionic strength therefore increases melting temperature.

Question 8Molecular Biology Fundamentals and Nucleic Acid Chemistry

Which feature distinguishes real-time quantitative PCR from conventional endpoint PCR for quantifying target copies?

  1. Use of two primers flanking the target
  2. Use of a thermostable polymerase
  3. Separation of products by agarose gel electrophoresis
  4. Measurement of fluorescence during the exponential phase at the cycle threshold Correct answer.

Explanation

Quantitative PCR measures fluorescence at each cycle and uses the cycle threshold (Ct), taken during exponential amplification, to estimate starting template quantity. Endpoint PCR analyzes final product, which does not reliably reflect input.

Question 9Molecular Biology Fundamentals and Nucleic Acid Chemistry

In a fluorogenic 5' nuclease (TaqMan) assay, what generates the fluorescent signal?

  1. Intercalation of the probe into double-stranded product
  2. Unquenching of the probe upon binding single-stranded template
  3. Cleavage of the probe by the polymerase's 5' exonuclease activity, separating reporter from quencher Correct answer.
  4. Hybridization of two adjacent probes that emit only when ligated

Explanation

The intact probe is quenched because reporter and quencher are close. During extension, the polymerase's 5' exonuclease activity cleaves the annealed probe, releasing the reporter and producing fluorescence proportional to product accumulation.

Question 10Molecular Biology Fundamentals and Nucleic Acid Chemistry

Which type of chemical bond links adjacent nucleotides within a single strand of DNA?

  1. Phosphodiester bond between the 3' hydroxyl and 5' phosphate Correct answer.
  2. Hydrogen bond between complementary bases
  3. Peptide bond between deoxyribose units
  4. Ionic bond between phosphate and sugar

Explanation

Nucleotides in a strand are joined by phosphodiester linkages connecting the 3'-OH of one sugar to the 5'-phosphate of the next. Hydrogen bonds hold the two strands together, while peptide bonds occur in proteins.

Question 11Molecular Biology Fundamentals and Nucleic Acid Chemistry

Which consequence follows from the antiparallel orientation of DNA strands?

  1. The lagging strand cannot be replicated by DNA polymerase
  2. Both strands are synthesized continuously in the same direction
  3. Both strands must be synthesized by separate polymerase enzymes with different active sites
  4. One strand is synthesized continuously and the other discontinuously as Okazaki fragments Correct answer.

Explanation

Because polymerase extends only 5' to 3' while the strands are antiparallel, one strand (leading) is synthesized continuously toward the fork and the other (lagging) is made in short Okazaki fragments. Both strands are replicated by the same core machinery.

Question 12Molecular Biology Fundamentals and Nucleic Acid Chemistry

Which modification of a synthetic oligonucleotide would most improve its resistance to exonuclease degradation in serum?

  1. Labeling with a 3' fluorophore only
  2. Incorporation of phosphorothioate linkages Correct answer.
  3. Addition of a 5' phosphate group
  4. Introduction of a 5' biotin tag

Explanation

Phosphorothioate linkages replace a nonbridging oxygen with sulfur, making the backbone resistant to nuclease attack. A 5' phosphate actually facilitates exonuclease digestion, and terminal labels alone do not confer nuclease resistance.

Question 13Molecular Biology Fundamentals and Nucleic Acid Chemistry

Which feature of bisulfite conversion makes it useful for assessing DNA methylation?

  1. It cleaves DNA at methylated CpG sites
  2. It converts unmethylated cytosine to uracil while leaving methylated cytosine unchanged Correct answer.
  3. It converts methylated cytosine to thymine selectively
  4. It removes methyl groups from all cytosines

Explanation

Bisulfite treatment deaminates unmethylated cytosine to uracil (read as thymine after amplification), whereas 5-methylcytosine resists conversion. Sequencing or PCR-based readout then distinguishes methylated from unmethylated cytosines.

Question 14Molecular Biology Fundamentals and Nucleic Acid Chemistry

A double-stranded DNA molecule contains 30% adenine. What percentage of the bases are guanine?

  1. 40%
  2. 70%
  3. 20% Correct answer.
  4. 30%

Explanation

By Chargaff's rules, adenine equals thymine, so A + T = 60%, leaving G + C = 40%. Since guanine equals cytosine, guanine is 20%.

Question 15Molecular Biology Fundamentals and Nucleic Acid Chemistry

Which property of SYBR green I limits its specificity in quantitative PCR?

  1. It fluoresces only after probe cleavage
  2. It emits signal only at temperatures above the Tm of the product
  3. It requires a quencher covalently attached to the dye
  4. It binds any double-stranded DNA, including primer-dimers and nonspecific products Correct answer.

Explanation

SYBR green fluoresces when bound to double-stranded DNA regardless of sequence, so nonspecific products and primer-dimers contribute signal. Melt-curve analysis or sequence-specific probes are used to confirm specificity.

Question 16Molecular Biology Fundamentals and Nucleic Acid Chemistry

Which statement best describes the proofreading function of the 3' to 5' exonuclease activity of a high-fidelity DNA polymerase?

  1. It removes RNA primers from Okazaki fragments
  2. It adds a 3' adenine overhang to PCR products
  3. It removes mismatched bases from the 3' terminus before extension continues Correct answer.
  4. It excises uracil from DNA

Explanation

The 3' to 5' exonuclease activity acts as a proofreader, excising a misincorporated terminal nucleotide so the correct base can be added. This reduces error rates relative to polymerases lacking proofreading.

Question 17Molecular Biology Fundamentals and Nucleic Acid Chemistry

In a hybridization assay, a 20-nucleotide probe with higher GC content than a second probe of the same length will generally show which behavior?

  1. A higher melting temperature and greater duplex stability Correct answer.
  2. A lower melting temperature and reduced binding stringency
  3. No difference in melting temperature because length is identical
  4. Complete loss of sequence specificity

Explanation

GC base pairs have three hydrogen bonds and greater stacking, so higher GC content increases duplex stability and melting temperature at fixed length. Stringency conditions must be adjusted accordingly.

Question 18Molecular Biology Fundamentals and Nucleic Acid Chemistry

Which feature of the genetic code allows a single point mutation to produce no change in the encoded protein?

  1. The presence of only one tRNA per amino acid
  2. Universal use of uracil in DNA
  3. Degeneracy of the code, in which multiple codons specify the same amino acid Correct answer.
  4. The code being overlapping, so each base belongs to one codon

Explanation

Because the code is degenerate, several codons encode the same amino acid, so a synonymous (silent) substitution may leave the protein sequence unchanged. The code is non-overlapping, and uracil occurs in RNA rather than DNA.

Question 19Molecular Biology Fundamentals and Nucleic Acid Chemistry

Which principle underlies the use of allele-specific oligonucleotide hybridization to detect a single nucleotide variant?

  1. The variant alters the length of the PCR product
  2. A single mismatch at the center of a short probe markedly destabilizes duplex formation under stringent conditions Correct answer.
  3. Allele-specific probes bind only single-stranded RNA
  4. Mismatches increase duplex stability and enhance probe binding

Explanation

A mismatched base disrupts base pairing and reduces duplex stability, especially when centrally located in a short probe. Under appropriately stringent wash conditions, only the perfectly matched probe remains bound, discriminating the allele.

Question 20Molecular Biology Fundamentals and Nucleic Acid Chemistry

A laboratory uses dUTP plus uracil-DNA glycosylase (UNG) in a PCR master mix. What is the primary purpose of this system?

  1. To prevent carryover contamination from previous PCR products Correct answer.
  2. To enable real-time quantification without probes
  3. To increase the yield of long amplicons
  4. To enhance primer annealing specificity

Explanation

Incorporating dUTP marks amplicons as uracil-containing, and UNG treatment before amplification degrades any contaminating carryover product while the native template DNA is unaffected. Newly synthesized DNA lacking uracil is then amplified.

Question 21Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

During the extension step of a Sanger sequencing reaction, incorporation of a dideoxynucleotide causes chain termination because the molecule lacks which feature?

  1. A 3'-hydroxyl group Correct answer.
  2. A fluorescent label
  3. A 5'-phosphate group
  4. A template-binding region

Explanation

Chain termination occurs because ddNTPs lack the 3'-OH needed for the next phosphodiester bond, so no further nucleotide can be added. The 5' phosphate and base are still present, and labeling is a separate modification.

Question 22Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which property of Taq DNA polymerase makes it suitable for PCR cycling protocols that include a high-temperature denaturation step?

  1. Strand-displacement activity
  2. Proofreading 3'-to-5' exonuclease activity
  3. Ability to synthesize RNA from DNA
  4. Thermostability at denaturation temperatures Correct answer.

Explanation

Taq polymerase derives from a thermophilic organism and retains activity after exposure to the near-boiling temperatures used for strand separation. Its lack of proofreading and its DNA-dependent DNA polymerase activity are separate characteristics.

Question 23Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

In a real-time PCR assay using a hydrolysis (TaqMan) probe, fluorescence increases primarily because:

  1. The probe is displaced intact from the template during extension
  2. Intercalating dye is released from double-stranded product
  3. The 5' nuclease activity of the polymerase cleaves the probe, separating reporter from quencher Correct answer.
  4. The probe hybridizes to the target and becomes fluorescent upon binding

Explanation

The hydrolysis probe carries a reporter and quencher held in proximity; cleavage by the polymerase's 5' nuclease activity releases the reporter and generates signal. Hybridization alone does not separate fluor from quencher.

Question 24Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

A laboratory needs to detect a single-nucleotide variant using an allele-specific PCR assay. Which design feature most directly confers allele discrimination?

  1. Addition of a 5' fluorescent tag to the common primer
  2. A longer amplicon size than the wild-type reaction
  3. Use of a lower annealing temperature than the standard PCR
  4. A mismatch placed at or near the 3' end of the allele-specific primer Correct answer.

Explanation

Allele-specific PCR relies on the 3' terminal nucleotide of the primer matching only one allele; a mismatch there impedes extension by the polymerase. Amplicon length, labeling, and annealing temperature affect performance but are not the primary discrimination mechanism.

Question 25Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which step in a typical next-generation sequencing workflow is designed to add sample-specific index sequences so that multiple libraries can be pooled?

  1. Base calling
  2. Cluster generation
  3. Read alignment
  4. Library preparation Correct answer.

Explanation

Indexes (barcodes) are incorporated during library preparation, typically through adapter ligation or index-containing primers, enabling multiplexed sequencing. Cluster generation, base calling, and alignment occur after libraries are pooled.

Question 26Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

In array comparative genomic hybridization, a region of the tumor genome showing a log2 ratio of approximately -1.0 relative to reference DNA most likely represents:

  1. A heterozygous deletion Correct answer.
  2. A single-copy gain
  3. A homozygous deletion
  4. Normal copy number

Explanation

A log2 ratio near -1 corresponds to a halving of copy number relative to reference, consistent with loss of one copy. Homozygous deletion produces a much lower ratio, and gains produce positive ratios.

Question 27Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which enzyme activity is required to convert RNA into complementary DNA during reverse-transcription PCR?

  1. RNA-dependent DNA polymerase Correct answer.
  2. DNA-dependent DNA polymerase
  3. DNA-dependent RNA polymerase
  4. Restriction endonuclease

Explanation

Reverse transcriptase is an RNA-dependent DNA polymerase that synthesizes cDNA from an RNA template. Subsequent amplification uses a DNA-dependent DNA polymerase.

Question 28Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

A laboratory performing a quantitative PCR assay observes that the standard curve has an efficiency of 1.0 across a six-log dilution series. Which statement best describes the expected relationship between input template and quantification cycle (Cq)?

  1. Cq increases by 3.3 cycles for each tenfold decrease in template Correct answer.
  2. Cq is independent of template concentration
  3. Cq increases by one cycle for each tenfold decrease in template
  4. Cq decreases by one cycle for each tenfold decrease in template

Explanation

With perfect doubling (efficiency 1.0), a tenfold dilution corresponds to log2(10) ≈ 3.32 cycles, so Cq shifts by about 3.3 cycles per log. A one-cycle shift would correspond to a twofold change.

Question 29Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which hybridization parameter change would most increase the stringency of a Southern blot wash?

  1. Decreasing the salt concentration of the wash buffer Correct answer.
  2. Increasing the salt concentration of the wash buffer
  3. Increasing the concentration of formamide in the wash buffer
  4. Increasing the duration of the wash at low temperature

Explanation

Lowering salt concentration destabilizes mismatched duplexes and increases stringency, favoring perfect complementarity. Higher salt reduces stringency, and formamide is typically used in hybridization buffer rather than as a wash stringency lever.

Question 30Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which characteristic distinguishes digital PCR from conventional real-time quantitative PCR?

  1. It partitions the reaction into many small volumes and counts positive partitions Correct answer.
  2. It uses intercalating dyes exclusively
  3. It requires no thermal cycling
  4. It relies on melt-curve analysis for quantification

Explanation

Digital PCR divides the sample into thousands of partitions and estimates concentration by counting positive endpoints using Poisson statistics. It still requires thermal cycling and does not depend on melt curves or a single dye class.

Question 31Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

In a multiplex PCR assay, a primer pair produces a strong product in singleplex reactions but fails in the multiplex cocktail. Which is the most likely explanation?

  1. Loss of polymerase thermostability in the combined reaction
  2. Inability of dNTPs to be incorporated in multiplex reactions
  3. Primer-dimer formation or competition among primer pairs in the combined reaction Correct answer.
  4. Failure of the thermal cycler to reach denaturation temperature

Explanation

Multiplex reactions are prone to primer-primer interactions and competition that can suppress specific amplification. Polymerase chemistry and dNTP incorporation are unchanged by multiplexing.

Question 32Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which read-level feature is most characteristic of a sequencing error in Illumina short-read data rather than a true variant?

  1. Presence in both forward and reverse reads at similar allele fractions
  2. High mapping quality and even coverage across the locus
  3. Low base quality at a single position with a strand bias Correct answer.
  4. Consistent allele fraction across multiple independent samples

Explanation

Sequencing errors typically appear as low-quality bases with strand bias and low support, whereas true variants are supported on both strands with balanced quality. Consistent allele fractions across samples and even coverage favor a real variant.

Question 33Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which feature of a microarray probe design most reduces cross-hybridization to related sequences?

  1. Labeling the target with multiple fluorophores
  2. Selecting probes with minimal sequence similarity to paralogs or pseudogenes Correct answer.
  3. Increasing probe length to several kilobases
  4. Using a single probe per gene

Explanation

Specificity depends on probe sequence uniqueness; probes chosen to avoid regions of high similarity reduce cross-hybridization. Longer probes and higher labeling do not correct for sequence similarity.

Question 34Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which mutation type is most likely to be missed by a PCR-based assay that relies on a single primer pair flanking the target region?

  1. A large deletion removing one primer-binding site Correct answer.
  2. A small insertion within the amplicon
  3. A small deletion within the amplicon
  4. A single-nucleotide substitution within the amplicon

Explanation

If a deletion removes a primer-binding site, amplification fails and the allele may be missed (or appear as apparent homozygosity for the other allele). Substitutions and small indels within the amplicon are typically detected.

Question 35Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which statement best describes the purpose of a no-template control (NTC) in a diagnostic PCR run?

  1. It confirms the annealing temperature is optimal
  2. It detects contamination or reagent-derived signal Correct answer.
  3. It verifies that the primers amplify the intended target
  4. It measures the analytical sensitivity of the assay

Explanation

An NTC contains all reagents except template; any signal indicates contamination or nonspecific amplification. Positive controls and dilution series address target verification and sensitivity.

Question 36Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

In capillary electrophoresis Sanger sequencing, the purpose of the size standard is to:

  1. Remove unincorporated dye terminators
  2. Provide a template for the sequencing reaction
  3. Increase the fluorescence intensity of the labeled fragments
  4. Calibrate migration times so that fragment lengths can be assigned Correct answer.

Explanation

The size standard contains fragments of known length whose migration times allow conversion of sample migration times into base positions. It does not template the reaction or remove unincorporated terminators.

Question 37Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which sequencing approach is best suited to detect large structural variants and repeat expansions that short-read sequencing often cannot resolve?

  1. Sanger sequencing of a single exon
  2. Long-read single-molecule sequencing Correct answer.
  3. Allele-specific real-time PCR
  4. Targeted amplicon sequencing with 150-bp reads

Explanation

Long reads span repetitive regions and structural breakpoints that short reads cannot uniquely map. Amplicon and Sanger methods are limited by their read length and targeted scope.

Question 38Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

A quantitative methylation-specific PCR assay uses bisulfite-treated DNA. Which base change underlies the assay's ability to distinguish methylated from unmethylated cytosines?

  1. Methylated cytosine is converted to uracil while unmethylated cytosine is not
  2. Methylated cytosine is converted to adenine
  3. Both methylated and unmethylated cytosine are converted to thymine
  4. Unmethylated cytosine is converted to uracil while methylated cytosine remains cytosine Correct answer.

Explanation

Bisulfite deaminates unmethylated cytosine to uracil (read as thymine after amplification), while methylated cytosine is protected and remains cytosine. This sequence difference allows methylation-specific primers or probes to discriminate.

Question 39Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which characteristic of locked nucleic acid (LNA) probes makes them useful for detecting short or highly related sequences?

  1. They increase the melting temperature of the probe duplex Correct answer.
  2. They eliminate the need for a quencher
  3. They require no target sequence complementarity
  4. They prevent probe binding to the target

Explanation

LNA modifications raise duplex melting temperature and increase binding affinity, allowing short probes with high specificity. They still require complementarity and do not eliminate quenchers.

Question 40Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which is the primary reason for using a hot-start DNA polymerase in a diagnostic PCR assay?

  1. To allow use of RNA as a template
  2. To increase the rate of primer extension
  3. To enable amplification without thermal cycling
  4. To prevent extension of primers before the initial denaturation step Correct answer.

Explanation

Hot-start polymerases are inactive until heated, preventing primer extension and nonspecific products during reaction setup. They do not change the fundamental requirement for thermal cycling or enable RNA templates.

Question 41Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

In a melting-curve analysis performed after real-time PCR with a double-stranded DNA intercalating dye, a heterozygous sample typically shows:

  1. Two distinct peaks corresponding to the two alleles Correct answer.
  2. A single peak at a higher temperature than the homozygous mutant
  3. No peak because intercalating dyes do not fluoresce in double-stranded DNA
  4. A single peak at a lower temperature than the homozygous wild type

Explanation

Heterozygous samples form two homoduplex species with different melting temperatures, producing two peaks (or a broadened, shifted curve). Intercalating dyes fluoresce strongly when bound to double-stranded DNA.

Question 42Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which step in a next-generation sequencing workflow converts fluorescence or electrical signals into nucleotide sequences?

  1. Demultiplexing
  2. Base calling Correct answer.
  3. Library preparation
  4. Variant annotation

Explanation

Base calling translates raw instrument signals into nucleotide sequences. Demultiplexing assigns reads to samples based on indexes, and annotation adds biological context.

Question 43Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

A laboratory performs droplet digital PCR and observes 100 positive droplets out of 20,000 total droplets. Which principle allows estimation of target concentration from these counts?

  1. Melt-curve peak area integration
  2. Linear regression of Cq against template input
  3. Gel densitometry of amplicon bands
  4. Poisson statistics relating positive fraction to mean copies per partition Correct answer.

Explanation

Digital PCR assumes random partitioning described by the Poisson distribution, so the fraction of negative partitions estimates the mean copies per partition. Cq-based regression and melt-curve methods belong to other assay formats.

Question 44Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which design consideration is most important when placing primers for Sanger sequencing of a GC-rich region?

  1. Using the lowest possible annealing temperature
  2. Maximizing primer length to over 60 nucleotides
  3. Selecting primers with a high degree of self-complementarity
  4. Choosing primers that avoid stable secondary structures and homopolymer runs Correct answer.

Explanation

GC-rich templates readily form secondary structures; primers that avoid strong self-complementarity and homopolymers improve specific priming and read quality. Excessively long primers and low annealing temperatures reduce specificity.

Question 45Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which assay format would best enable simultaneous detection of multiple respiratory viral targets in a single reaction with distinct fluorescent channels?

  1. Array CGH
  2. Multiplex real-time RT-PCR with target-specific probes labeled with different fluorophores Correct answer.
  3. Singleplex Sanger sequencing of each target
  4. Southern blotting with a single radiolabeled probe

Explanation

Multiplex real-time RT-PCR with spectrally distinct probes allows simultaneous amplification and detection of multiple targets in one tube. Sanger sequencing, Southern blotting, and array CGH are not designed for same-tube multiplex detection.

Question 46Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which enzyme is most commonly used in PCR to synthesize DNA from a DNA template?

  1. DNA ligase
  2. RNA polymerase
  3. Taq DNA polymerase Correct answer.
  4. Reverse transcriptase

Explanation

Taq DNA polymerase is a thermostable enzyme that synthesizes DNA from a DNA template during PCR. Reverse transcriptase synthesizes DNA from RNA, DNA ligase joins DNA fragments, and RNA polymerase synthesizes RNA from DNA.

Question 47Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which step in a typical PCR cycle is responsible for separating the DNA strands?

  1. Annealing
  2. Extension
  3. Denaturation Correct answer.
  4. Ligation

Explanation

Denaturation, typically at 94–98°C, breaks hydrogen bonds between complementary bases, separating double-stranded DNA into single strands. Annealing allows primers to bind, and extension allows polymerase to synthesize new DNA.

Question 48Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

A laboratory uses a quantitative PCR assay to measure gene expression. The standard curve has a slope of -3.32. What is the amplification efficiency of the assay?

  1. 50%
  2. 100% Correct answer.
  3. 80%
  4. 90%

Explanation

Amplification efficiency is calculated as E = 10^(-1/slope) - 1. For a slope of -3.32, E = 10^(1/3.32) - 1 ≈ 1.0, corresponding to 100% efficiency.

Question 49Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

Which next-generation sequencing platform relies on detecting hydrogen ions released during nucleotide incorporation?

  1. Illumina sequencing by synthesis
  2. Oxford Nanopore sequencing
  3. PacBio single-molecule real-time sequencing
  4. Ion Torrent semiconductor sequencing Correct answer.

Explanation

Ion Torrent sequencing measures pH changes caused by hydrogen ions released when nucleotides are incorporated. Illumina uses fluorescently labeled reversible terminators, PacBio detects fluorescent nucleotides in real time, and Nanopore measures changes in ionic current as DNA passes through a pore.

Question 50Molecular Techniques and Assay Design (amplification, hybridization, sequencing, microarrays)

A laboratory performs microarray analysis and observes a log2 ratio of -0.5 for a genomic region. Which interpretation is most appropriate?

  1. Single-copy loss Correct answer.
  2. Single-copy gain
  3. Homozygous deletion
  4. Normal copy number

Explanation

A log2 ratio of -0.5 corresponds to a copy number ratio of 2^(-0.5) ≈ 0.707, which is consistent with a heterozygous deletion (single-copy loss). Homozygous deletion would show a much lower ratio (log2 ratio approaching negative infinity), while gains show positive log2 ratios.