Which bases are purines?
Show answer
Answer: A — Adenine and guanine
Purines (A, G) have two rings. Pyrimidines (C, T, U) have one ring. Thymine is found in DNA and uracil replaces it in RNA.
Reference: Thompson & Thompson Genetics in Medicine
Exam Prep
Molecular Genetics practice questions.
Nucleic acids, mutations and inheritance, extraction, electrophoresis, PCR, sequencing and flow cytometry.
Independent practice questions written by Labologics. Not affiliated with DHA, DOH (formerly HAAD), SCFHS, Prometric or any exam body. Not recalled exam questions.
Nucleic acid structure
How many hydrogen bonds join guanine and cytosine?
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Answer: B — Three
G-C pairs have three hydrogen bonds; A-T (DNA) and A-U (RNA) pairs have two. DNA rich in G-C therefore needs a higher temperature to separate (higher melting temperature).
Reference: Thompson & Thompson Genetics in Medicine
In RNA, adenine pairs with:
Show answer
Answer: C — Uracil
RNA uses uracil instead of thymine. RNA also contains ribose (with a 2'-OH group) instead of deoxyribose.
Reference: Thompson & Thompson Genetics in Medicine
The two strands of a DNA double helix are:
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Answer: D — Antiparallel and complementary
One strand runs 5' to 3' and the other 3' to 5'. Bases pair by hydrogen bonds; the sugar-phosphate backbone is joined by phosphodiester bonds.
Reference: Thompson & Thompson Genetics in Medicine
A double-stranded DNA sample contains 30% adenine. What percentage is guanine?
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Answer: A — 20%
A = T = 30%, so A + T = 60%. G + C = 40%, and G = C = 20%.
Reference: Thompson & Thompson Genetics in Medicine
A segment of DNA that codes for a functional product such as a protein is called a:
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Answer: B — Gene
A codon is three bases coding one amino acid; a chromosome carries many genes; a nucleosome is DNA wrapped around histones.
Reference: Thompson & Thompson Genetics in Medicine
The end product of purine breakdown in humans is:
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Answer: C — Uric acid
Purines (A, G) are broken down to uric acid. Pyrimidines are broken down to soluble products such as beta-alanine.
Reference: Tietz Fundamentals of Clinical Chemistry and Molecular Diagnostics
Replication & gene expression
Which enzyme unwinds the DNA double helix during replication?
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Answer: D — Helicase
Helicase separates the strands. Primase lays down RNA primers, DNA polymerase extends them, and ligase seals the gaps between Okazaki fragments.
Reference: Thompson & Thompson Genetics in Medicine
DNA polymerase synthesizes new DNA in which direction?
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Answer: A — 5' to 3' only
New nucleotides are added to the 3'-OH end, so synthesis is 5' to 3' while the template is read 3' to 5'. This is why the lagging strand is made in Okazaki fragments.
Reference: Thompson & Thompson Genetics in Medicine
Which enzyme joins Okazaki fragments on the lagging strand?
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Answer: B — DNA ligase
DNA ligase forms the final phosphodiester bond. Topoisomerase relieves supercoiling ahead of the replication fork.
Reference: Thompson & Thompson Genetics in Medicine
Making messenger RNA from a DNA template is called:
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Answer: C — Transcription
Transcription (by RNA polymerase) happens in the nucleus. Translation of mRNA into protein happens on ribosomes in the cytoplasm.
Reference: Thompson & Thompson Genetics in Medicine
Which enzyme makes DNA from an RNA template?
Show answer
Answer: D — Reverse transcriptase
Reverse transcriptase is used by retroviruses such as HIV and in the lab to make cDNA, the first step of RT-PCR.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Mature eukaryotic mRNA has a poly(A) tail. In the lab, this is used to:
Show answer
Answer: A — Prime cDNA synthesis with oligo(dT) primers
Oligo(dT) primers bind the poly(A) tail, so reverse transcriptase copies mRNA selectively into cDNA.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
A codon consists of:
Show answer
Answer: B — Three nucleotides
Each codon of three bases codes one amino acid or a stop signal. AUG is the start codon (methionine).
Reference: Thompson & Thompson Genetics in Medicine
Mutations & chromosomes
A single base change that creates a premature stop codon is a:
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Answer: C — Nonsense mutation
Nonsense mutations truncate the protein. Missense changes one amino acid (e.g., the sickle mutation); silent mutations do not change the amino acid; frameshifts come from insertions or deletions not divisible by three.
Reference: Thompson & Thompson Genetics in Medicine
The sickle cell mutation (Glu6Val in HBB) is an example of a:
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Answer: D — Missense point mutation
A single A-to-T change in codon 6 (GAG to GTG) replaces glutamic acid with valine.
Reference: Thompson & Thompson Genetics in Medicine
The karyotype 47,XX,+21 describes:
Show answer
Answer: A — Down syndrome in a female
47 chromosomes with an extra chromosome 21. Turner syndrome is 45,X and Klinefelter syndrome is 47,XXY.
Reference: ISCN: International System for Human Cytogenomic Nomenclature
For a conventional karyotype, cell division is stopped at metaphase by adding:
Show answer
Answer: B — Colchicine (Colcemid)
Colcemid blocks spindle formation, so chromosomes are held at their most condensed stage. G-banding with Giemsa then shows the banding pattern.
Reference: ISCN: International System for Human Cytogenomic Nomenclature
Autosomes are:
Show answer
Answer: C — The 22 pairs of non-sex chromosomes
Humans have 22 pairs of autosomes plus one pair of sex chromosomes (46 total).
Reference: Thompson & Thompson Genetics in Medicine
DiGeorge syndrome (22q11.2 deletion) is most commonly confirmed by:
Show answer
Answer: D — FISH or chromosomal microarray
The deletion is too small to be seen reliably on a routine karyotype. FISH with a 22q11.2 probe or a chromosomal microarray detects it.
Reference: Thompson & Thompson Genetics in Medicine
Which test detects small deletions and duplications (copy number variants) across the whole genome in one assay?
Show answer
Answer: A — Chromosomal microarray
Microarrays detect CNVs far smaller than a karyotype can show, but they cannot detect balanced translocations.
Reference: Thompson & Thompson Genetics in Medicine
Inheritance
One parent is affected by an autosomal dominant disorder (heterozygous) and the other is unaffected. The chance that each child is affected is:
Show answer
Answer: C — 50%
Autosomal dominant: one abnormal copy causes disease, and an affected heterozygous parent passes it on with 50% probability each pregnancy.
Reference: Thompson & Thompson Genetics in Medicine
Both parents are carriers of an autosomal recessive disease. The chance that each child is affected is:
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Answer: A — 25%
Carrier x carrier gives 25% affected, 50% carriers and 25% unaffected non-carriers.
Reference: Thompson & Thompson Genetics in Medicine
The father carries sickle cell trait (HbAS) and the mother is normal (HbAA). Which is correct for each child?
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Answer: B — 0% affected, 50% carrier
AS x AA gives 50% AA and 50% AS. No child can be SS because the mother has no S allele.
Reference: Thompson & Thompson Genetics in Medicine
A mother carries an X-linked recessive disorder such as hemophilia A; the father is unaffected. Which is correct?
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Answer: A — Each son has a 50% chance of being affected
Each son has a 50% chance of inheriting the affected X and being affected; each daughter has a 50% chance of being a carrier.
Reference: Thompson & Thompson Genetics in Medicine
Mitochondrial DNA disorders are inherited from:
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Answer: B — The mother only
Mitochondria in the embryo come from the egg, so mitochondrial DNA is maternally inherited.
Reference: Thompson & Thompson Genetics in Medicine
The observable characteristics of an individual are called the:
Show answer
Answer: C — Phenotype
The genotype is the genetic makeup (e.g., AO); the phenotype is what is expressed (e.g., blood group A).
Reference: Thompson & Thompson Genetics in Medicine
The mother is group O and the father is group AB. Which blood groups are possible in their children?
Show answer
Answer: D — A or B only (50% each)
OO x AB gives AO (group A) or BO (group B), 50% each. The children cannot be O or AB.
Reference: Thompson & Thompson Genetics in Medicine
The mother is group O, the father is group B, and their child is group O. What is the father's genotype?
Show answer
Answer: A — BO
The child (OO) must inherit an O allele from each parent, so the group B father must be heterozygous BO.
Reference: Thompson & Thompson Genetics in Medicine
Both parents are group A and both are heterozygous (AO). What is the chance that a child is group O?
Show answer
Answer: B — 25%
AO x AO gives 25% AA, 50% AO (both group A) and 25% OO (group O). If either parent were AA, no child could be group O.
Reference: Thompson & Thompson Genetics in Medicine
Extraction & specimens
Which blood collection tube is preferred for DNA extraction and PCR?
Show answer
Answer: C — EDTA (lavender top)
EDTA chelates magnesium and inhibits DNases. Heparin inhibits PCR polymerases and should be avoided.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Which is a standard method of DNA extraction?
Show answer
Answer: D — Phenol-chloroform (organic) extraction
Common methods are organic (phenol-chloroform), solid-phase (silica columns or magnetic beads) and chelating resin (e.g., Chelex).
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
A DNA sample has an A260/A280 ratio of about 1.8. This indicates:
Show answer
Answer: A — Reasonably pure DNA
Pure DNA gives about 1.8 and pure RNA about 2.0. A lower ratio suggests protein or phenol contamination.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
A fresh tissue sample is received for nucleic acid testing. What is the best handling?
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Answer: B — Process it immediately or freeze it (e.g., -80 C)
Nucleases degrade DNA and especially RNA quickly. Prolonged formalin fixation fragments nucleic acids.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
RNA is more difficult to handle than DNA mainly because:
Show answer
Answer: C — RNases are everywhere and very stable, and RNA is chemically less stable
Use RNase-free reagents, gloves and tubes, and process or stabilize samples quickly.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Electrophoresis
Electrophoresis separates molecules mainly according to their:
Show answer
Answer: D — Charge and size
Charged molecules move in an electric field. For nucleic acids, which have a uniform charge-to-mass ratio, the gel separates mainly by size.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
In agarose gel electrophoresis, DNA moves towards the:
Show answer
Answer: A — Anode (positive electrode)
The phosphate backbone makes DNA negatively charged. Smaller fragments move faster and further.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Polyacrylamide gel (PAGE) is preferred over agarose for:
Show answer
Answer: B — Proteins and very small DNA fragments
PAGE has smaller pores and higher resolution; SDS-PAGE separates proteins by molecular weight. Agarose suits larger DNA fragments.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Which dye is commonly used to visualize DNA in an agarose gel under UV light?
Show answer
Answer: C — Ethidium bromide or SYBR Green
These dyes bind nucleic acids and fluoresce. Ethidium bromide is a mutagen, so safer alternatives are often used. Coomassie and Ponceau stain proteins.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Which blotting technique detects specific RNA?
Show answer
Answer: D — Northern blot
Southern = DNA, Northern = RNA, Western = protein.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
PCR
What is the correct order of steps in one PCR cycle?
Show answer
Answer: A — Denaturation, annealing, extension
Denaturation at about 94-95 C separates the strands, annealing at about 50-65 C binds the primers, and extension at about 72 C lets Taq polymerase copy the template.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Taq polymerase is used in PCR because it:
Show answer
Answer: B — Is heat-stable and survives repeated denaturation steps
It comes from the thermophilic bacterium Thermus aquaticus and survives repeated heating to about 95 C.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
In PCR, primers:
Show answer
Answer: C — Are short single-stranded oligonucleotides that define the region to be amplified
A forward and a reverse primer bind opposite strands and flank the target, giving polymerase a 3'-OH starting point.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Which component is an essential cofactor for DNA polymerase in a PCR reaction?
Show answer
Answer: D — Magnesium (Mg2+)
Mg2+ concentration affects enzyme activity, specificity and yield. Too little gives no product; too much gives non-specific bands.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Starting from one double-stranded DNA target, how many copies exist after 10 ideal PCR cycles?
Show answer
Answer: C — 1,024
Copies double each cycle: 2^n. After 10 cycles, 2^10 = 1,024. After 30 cycles, about one billion.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
A PCR no-template control (NTC) shows a band. This most likely means:
Show answer
Answer: B — Contamination of reagents or the work area
Any amplification in the NTC means contamination, so patient results cannot be reported. Separate pre- and post-PCR areas with one-way workflow and use dUTP/UNG to reduce carryover.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
In real-time (quantitative) PCR, a LOWER Ct (cycle threshold) value means:
Show answer
Answer: C — More target in the sample
The more starting template there is, the fewer cycles are needed to cross the fluorescence threshold.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Which statement about SYBR Green real-time PCR is correct?
Show answer
Answer: D — It binds any double-stranded DNA, so a melt curve is used to check specificity
SYBR Green also detects primer-dimers and non-specific products. Hydrolysis probes (TaqMan) add sequence specificity.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
RT-PCR is needed to detect which type of target?
Show answer
Answer: A — RNA, for example RNA viruses or gene expression
Reverse transcriptase first converts RNA into cDNA, which PCR then amplifies.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Sequencing & methods
Sanger sequencing works by:
Show answer
Answer: B — Chain termination using dideoxynucleotides (ddNTPs)
A ddNTP lacks the 3'-OH group, so DNA synthesis stops. Fluorescent labels identify the last base, read by capillary electrophoresis.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
The main advantage of next-generation sequencing (NGS) is:
Show answer
Answer: C — Massively parallel sequencing of many genes and samples in one run
Barcodes (indexes) allow many samples to be pooled in one run. NGS is used for gene panels, exomes, genomes, tumor profiling and pathogen identification.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Restriction endonucleases cut DNA at:
Show answer
Answer: D — Specific, often palindromic, recognition sequences
Bacteria use them as defense. In the lab they are used in cloning and RFLP analysis.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
FISH (fluorescence in situ hybridization) is used to:
Show answer
Answer: A — Detect specific DNA sequences on chromosomes, e.g., BCR::ABL1 fusion or HER2 amplification
Fluorescent probes bind target sequences in cells or tissue, detecting deletions, translocations and amplifications.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Computer-based analysis of sequence and other biological data is called:
Show answer
Answer: B — Bioinformatics
Bioinformatics pipelines align NGS reads, call variants and interpret them against databases.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Flow cytometry
In flow cytometry, forward scatter (FSC) mainly reflects:
Show answer
Answer: C — Cell size
Side scatter (SSC) reflects internal complexity or granularity. Fluorescent antibody labels detect CD markers.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
The three main systems of a flow cytometer are:
Show answer
Answer: D — Fluidics, optics and electronics
Fluidics uses hydrodynamic focusing to pass cells one at a time through the laser; optics collects scattered and fluorescent light; electronics converts signals into data.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
A common clinical use of flow cytometry is:
Show answer
Answer: A — Immunophenotyping leukemias and lymphomas with CD markers
Other uses include CD4 counts in HIV, PNH testing (CD55/CD59, FLAER), and CD34 stem cell counts.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Clinical applications
Which test confirms MERS-CoV or SARS-CoV-2 infection?
Show answer
Answer: B — RT-PCR (nucleic acid amplification test)
Both are RNA viruses, so RT-PCR on respiratory samples is the confirmatory method.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Which test confirms active hepatitis C infection after a reactive anti-HCV result?
Show answer
Answer: C — HCV RNA by RT-PCR
Anti-HCV shows exposure; HCV RNA shows current infection. Quantitative HCV RNA is also used to check treatment response.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Hepatitis B is a DNA virus. Its viral load is measured by:
Show answer
Answer: D — Real-time PCR for HBV DNA
HBV DNA quantitation guides treatment and monitoring. RT-PCR is used for RNA viruses such as HCV and HIV.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
A tissue biopsy shows large cells with 'owl's eye' intranuclear inclusions. Which test best confirms the likely virus?
Show answer
Answer: A — CMV PCR (or immunohistochemistry)
Owl's eye inclusions are typical of cytomegalovirus. CMV PCR on blood or tissue confirms and quantifies infection.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Which molecular test supports the diagnosis of polycythemia vera?
Show answer
Answer: B — JAK2 V617F mutation testing
About 95% of PV have JAK2 V617F, usually detected by allele-specific or real-time PCR; most of the rest have JAK2 exon 12 mutations.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
The response of chronic myeloid leukemia to tyrosine kinase inhibitors is monitored by:
Show answer
Answer: C — Quantitative RT-PCR for BCR::ABL1 transcripts
Results are reported on the International Scale; a major molecular response is BCR::ABL1 of 0.1% or less.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Short tandem repeats (STRs) used in paternity and forensic testing are:
Show answer
Answer: D — Repeats of 2-6 base pairs, analyzed by PCR and capillary electrophoresis
STR loci are highly variable between people. Testing many loci together gives a very high power of discrimination.
Reference: Buckingham: Molecular Diagnostics - Fundamentals, Methods and Clinical Applications
Matching donors and recipients for bone marrow or stem cell transplantation relies mainly on:
Show answer
Answer: A — HLA typing
HLA genes (MHC on chromosome 6) are typed at high resolution, usually by sequencing-based methods.
Reference: Thompson & Thompson Genetics in Medicine
Cystic fibrosis is caused by mutations in which gene?
Show answer
Answer: B — CFTR
CFTR encodes a chloride channel. F508del is the most common mutation worldwide. Sweat chloride testing supports the diagnosis.
Reference: Thompson & Thompson Genetics in Medicine