Molecular Biology Lab Quiz
PCR setups, gel electrophoresis, cloning, sequencing, and genetic modification controls.
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Molecular Biology Lab Quiz: PCR, Gel Electrophoresis, Cloning, Sequencing, and Genetic Modification
Molecular biology transformed microbiology. Before the development of PCR, Southern blotting, and DNA sequencing, identifying an organism or understanding its genetic makeup required indirect methods, large amounts of material, and enormous amounts of time. Today, a PCR assay can detect a single copy of a target DNA sequence in a clinical sample within hours. Whole-genome sequencing of a bacterial isolate for outbreak investigation takes less than a day. These tools have changed what is possible in both research and clinical practice.
This quiz is built for molecular biology students, genomics and diagnostics laboratory technicians, research scientists, and anyone who works with or is learning nucleic acid-based techniques. The questions cover PCR principles and the major PCR variants, gel electrophoresis including how to set up and interpret a gel, DNA cloning using restriction enzymes and ligation, sequencing from Sanger to next-generation sequencing (NGS), and the controls that make genetic modification experiments valid.
Core Topics
PCR: Principles and the Major Variants
The polymerase chain reaction (PCR), developed by Kary Mullis in 1983 and awarded the Nobel Prize in Chemistry in 1993, is the most important molecular biology technique ever developed. PCR amplifies a specific region of DNA exponentially by repeating a cycle of three steps: denaturation, annealing, and extension. During denaturation, the double-stranded DNA template is heated (typically to 94 to 98 degrees Celsius) to separate the two strands. During annealing, short DNA fragments called primers bind to complementary sequences flanking the target region. During extension, a thermostable DNA polymerase (most commonly Taq polymerase from Thermus aquaticus, a hot spring bacterium) extends from each primer, synthesising a new DNA strand complementary to the template. Repeating this cycle 25 to 40 times produces millions of copies of the target sequence from as little as a single template molecule.
Conventional (endpoint) PCR generates a visible band on a gel at the end of the reaction. Quantitative PCR (qPCR, also called real-time PCR) measures fluorescence during each cycle, allowing the starting quantity of template DNA to be calculated from the cycle at which the fluorescence signal crosses a threshold (the cycle threshold, or Ct value). Reverse transcriptase PCR (RT-PCR) first converts RNA into complementary DNA (cDNA) using the enzyme reverse transcriptase, then amplifies the cDNA by PCR. This is how RNA viruses like SARS-CoV-2 and HIV are detected in clinical and research settings. Digital droplet PCR (ddPCR) partitions the reaction into tens of thousands of tiny droplets, allowing absolute quantification of target copies without the need for a standard curve.
Gel Electrophoresis
Agarose gel electrophoresis separates DNA fragments by size. DNA is negatively charged (due to its phosphate backbone) and migrates through an agarose gel matrix towards the positive electrode when an electric current is applied. Smaller fragments move faster and travel further from the well. The gel is stained with a fluorescent dye (most commonly ethidium bromide or safer alternatives like SYBR Safe) and visualised under UV light. A DNA ladder (a mixture of fragments of known sizes) is run alongside the samples to allow the sizes of experimental bands to be determined.
Reading a gel correctly is an important skill. The presence of a band at the expected size for your PCR product confirms amplification of the correct target. Multiple bands may indicate non-specific amplification due to primer dimers or the primers binding at multiple sites. A smear rather than discrete bands often indicates DNA degradation. No visible bands may indicate a failed PCR due to poor primer design, incorrect annealing temperature, inhibitors in the sample, or absent template.
DNA Cloning
DNA cloning inserts a fragment of DNA into a vector (typically a plasmid) so it can be propagated in a bacterial host and produced in large quantities. The process uses restriction enzymes, which are bacterial proteins that cut double-stranded DNA at specific recognition sequences (for example, EcoRI cuts at GAATTC). Cutting the insert DNA and the vector plasmid with the same restriction enzyme creates complementary sticky ends. The enzyme DNA ligase then joins the insert to the vector. The recombinant plasmid is introduced into competent bacterial cells (most commonly E. coli) by transformation. Bacteria that have taken up the recombinant plasmid are selected on antibiotic plates (the vector carries an antibiotic resistance gene) and the insert is confirmed by colony PCR or restriction digest analysis, then sequenced.
Sanger Sequencing vs. Next-Generation Sequencing
Sanger sequencing (chain termination sequencing), developed by Frederick Sanger in 1977, reads DNA sequences by incorporating dideoxynucleotides (ddNTPs) that terminate chain elongation at specific bases. It produces reads of 500 to 1000 base pairs with very high accuracy and remains the gold standard for confirming single-gene sequences and for sequencing cloned inserts. Its limitation is throughput: it produces one sequence at a time.
Next-generation sequencing (NGS) platforms (including Illumina, Oxford Nanopore, and PacBio) sequence millions of DNA fragments simultaneously, enabling whole-genome sequencing (WGS) of bacterial isolates within hours and comprehensive metagenomic analysis of entire microbial communities. Illumina short-read sequencing dominates clinical and public health genomics due to its low error rate and high throughput. Long-read platforms from Oxford Nanopore and PacBio can sequence through repetitive regions and resolve structural variants that short-read platforms miss.
Frequently Asked Questions
What are the three steps of PCR?
The three steps of each PCR cycle are denaturation, annealing, and extension. During denaturation, the reaction is heated to 94 to 98 degrees Celsius to separate the double-stranded DNA template into two single strands. During annealing, the temperature is lowered (typically to 50 to 65 degrees Celsius depending on the primer sequences) to allow the primers to bind to their complementary sequences on the template. During extension, the temperature is raised to 72 degrees Celsius for Taq polymerase to synthesise a new DNA strand from each primer, using the template strand as a guide.
What is a DNA ladder in gel electrophoresis?
A DNA ladder (also called a molecular weight marker) is a mixture of DNA fragments of precisely known sizes, run alongside experimental samples on a gel. When visualised, the ladder produces a series of bands at known positions on the gel. The positions of experimental bands are compared to the ladder bands to determine their approximate size in base pairs. Different ladders cover different size ranges, so choosing the right ladder for the expected product size matters.
What is the difference between PCR and RT-PCR?
Standard PCR amplifies DNA. RT-PCR (reverse transcriptase PCR) is used to detect and amplify RNA targets. A reverse transcriptase enzyme first converts the RNA into complementary DNA (cDNA), and then standard PCR amplifies the cDNA. RT-PCR is used to detect RNA viruses (including influenza, SARS-CoV-2, and HIV), to study gene expression levels from mRNA, and in molecular diagnostics where the target is an RNA molecule.
What is a restriction enzyme?
A restriction enzyme (also called a restriction endonuclease) is a bacterial enzyme that cuts double-stranded DNA at a specific recognition sequence, typically a palindromic sequence of 4 to 8 base pairs. Bacteria produce restriction enzymes as a defence against bacteriophage DNA. In molecular biology, restriction enzymes are used to cut DNA at precise, predictable locations for cloning, restriction mapping, and analysis of genetic constructs.
What is qPCR used for?
Quantitative PCR (qPCR) is used to measure the amount of a specific DNA (or, combined with reverse transcription, RNA) sequence present in a sample. It measures fluorescence during each amplification cycle and determines the cycle threshold (Ct) at which the fluorescence signal rises above background. A lower Ct value means more starting template was present. qPCR is used in clinical diagnostics to quantify viral load (for example HIV or hepatitis B viral copies per mL), to measure gene expression levels, and to detect pathogen DNA in food or environmental samples.
How does DNA ligation work?
DNA ligation is performed by the enzyme DNA ligase, which joins two DNA fragments by forming a phosphodiester bond between the 3’-hydroxyl end of one fragment and the 5’-phosphate end of the adjacent fragment. In cloning, DNA ligase joins the insert fragment to the linearised vector after both have been cut with the same restriction enzyme (creating complementary sticky ends that hold the two fragments together while the ligase seals the nick). T4 DNA ligase, derived from bacteriophage T4, is the most commonly used ligase in molecular biology.
What is a vector in cloning?
A vector is a DNA molecule used as a vehicle to carry a foreign DNA insert into a host cell. The most commonly used vectors are plasmids, circular extrachromosomal DNA molecules that replicate independently of the bacterial chromosome. A cloning vector typically contains an origin of replication (allowing it to replicate in the host), a selectable marker (usually an antibiotic resistance gene), and a multiple cloning site (MCS, a short region containing recognition sequences for multiple restriction enzymes). Other vector types include bacteriophage-based vectors, cosmids, bacterial artificial chromosomes (BACs), and expression vectors designed to produce protein from the inserted gene.
What is the difference between Sanger sequencing and NGS?
Sanger sequencing reads one DNA fragment at a time by chain termination, producing reads of 500 to 1000 base pairs with very high accuracy. It is the method of choice for confirming single-gene sequences, sequencing cloned inserts, and targeted analysis of specific regions. NGS (next-generation sequencing) sequences millions of DNA fragments simultaneously, enabling whole-genome sequencing, transcriptomics, metagenomics, and other large-scale analyses at a fraction of the cost per base pair. NGS produces shorter reads (typically 150 to 300 base pairs for Illumina, up to tens of kilobases for long-read platforms) and requires bioinformatic analysis pipelines to assemble and interpret the data.
What is a primer in PCR?
A primer is a short single-stranded DNA oligonucleotide, typically 18 to 25 nucleotides long, that is complementary to a sequence flanking the target region to be amplified. Two primers (a forward primer and a reverse primer) are used in each PCR reaction, flanking the region to be amplified on opposite strands of the DNA. Primers define the location and length of the PCR product. Good primer design avoids sequences that can form hairpin structures or primer dimers, ensures adequate GC content for stable binding, and avoids off-target binding sites in the genome.
What is CRISPR and how is it different from cloning?
CRISPR-Cas9 is a genome editing tool derived from a natural bacterial immune system. It uses a guide RNA to direct the Cas9 protein to a specific location in the genome, where it cuts the DNA. The cell’s own repair mechanisms then either disrupt the gene (through error-prone non-homologous end joining) or introduce a specific change (through homology-directed repair using a supplied template). Cloning is the process of copying and propagating a DNA sequence by inserting it into a vector and replicating it in bacterial cells. The key difference is that CRISPR edits the genome of a living organism in place, while cloning copies a specific DNA sequence for study or protein production.