301. Purified DNA is incubated with the correct restriction endonuclease, but the reaction mixture is maintained under conditions unsuitable for the enzyme's activity. Which observation is most likely?
ⓐ. The DNA will necessarily be completely digested because enzyme identity alone determines cleavage.
ⓑ. Digestion may be poor or absent despite use of the correct enzyme.
ⓒ. The enzyme will automatically act as DNA ligase.
ⓓ. The DNA will be converted directly into recombinant protein.
Correct Answer: Digestion may be poor or absent despite use of the correct enzyme.
Explanation: Restriction digestion requires not only the correct enzyme and recognition sites but also suitable reaction conditions that permit the enzyme to function effectively. If those conditions are inappropriate, the expected DNA cleavage may be reduced or absent even though the correct restriction endonuclease was selected. This is why restriction digestion is described as incubation of purified DNA with the selected enzyme under conditions appropriate for its activity. A correct enzyme cannot compensate for an unsuitable reaction environment.
302. Replicate samples of the same DNA are completely digested with the same restriction endonuclease under identical suitable conditions. Agarose electrophoresis repeatedly produces the same set of fragment bands in each replicate. Which conclusion is best supported?
ⓐ. Restriction endonucleases cut DNA randomly, but random cuts always produce identical fragments.
ⓑ. The gel pattern shows that DNA ligase has joined all restriction fragments back together.
ⓒ. Sequence-specific cleavage at defined recognition sites can produce a reproducible fragment pattern.
ⓓ. Reproducible bands show that fragment size is unrelated to restriction-enzyme action.
Correct Answer: Sequence-specific cleavage at defined recognition sites can produce a reproducible fragment pattern.
Explanation: Restriction endonucleases recognise particular DNA sequences and cleave at characteristic positions associated with those sites. When the same DNA molecule is completely digested with the same enzyme under comparable conditions, cleavage occurs at the same recognition sites, so a reproducible set of fragments is expected. Agarose electrophoresis then separates those fragments by size. Reproducibility therefore reflects sequence-specific cutting rather than random breakage or ligase activity.
303. Source DNA and vector DNA are both cut with the same suitable restriction enzyme, producing complementary sticky ends. Before ligase is added, what molecular event is most likely to occur?
ⓐ. The insert gains antibiotic resistance simply by contacting the opened vector.
ⓑ. The opened vector begins protein synthesis before recombinant DNA is formed.
ⓒ. The complementary sticky ends become selectable antibiotic-resistance markers.
ⓓ. Complementary sticky ends can hydrogen-bond and align the insert with the vector.
Correct Answer: Complementary sticky ends can hydrogen-bond and align the insert with the vector.
Explanation: Cutting source DNA and vector DNA with the same suitable restriction enzyme can generate compatible sticky ends. The exposed complementary bases on those ends can pair through hydrogen bonds, bringing the insert and vector into alignment. This association is an important intermediate step, but it does not create complete covalent continuity in the DNA backbone. DNA ligase is still required to seal the aligned molecules and form a stable recombinant DNA construct.
304. A cloning reaction contains a correctly opened vector and a purified insert with compatible sticky ends. The two molecules associate, but after treatment the recombinant construct remains unstable because breaks in the sugar-phosphate backbone persist. Which step failed?
ⓐ. DNA staining
ⓑ. DNA ligation
ⓒ. Cell-wall disruption
ⓓ. Primer annealing
Correct Answer: DNA ligation
Explanation: Compatible sticky ends can bring vector and insert together through complementary base pairing, but those hydrogen-bond interactions do not fully seal the DNA backbone. DNA ligase is required to form the covalent phosphodiester bonds that stabilise the joined molecule. Persistent backbone breaks therefore identify a failure of ligation rather than a problem with staining, cell disruption or PCR. Recombinant DNA formation depends on both end compatibility and successful covalent joining.
305. A PCR is performed with the same template, nucleotides and thermostable polymerase in two reactions. Reaction P uses primers flanking a short target region, whereas reaction Q uses primers that anneal farther apart on the same template. Assuming both primer pairs anneal specifically, which prediction is most appropriate?
ⓐ. Both reactions must amplify DNA fragments of identical length because the template molecule is the same.
ⓑ. Primer positions affect only DNA visibility after electrophoresis and cannot influence the amplified region.
ⓒ. The amplified region is defined by the primer positions, so Q can produce a longer target fragment than P.
ⓓ. Moving primer sites farther apart prevents thermostable polymerase from extending DNA in any PCR.
Correct Answer: The amplified region is defined by the primer positions, so Q can produce a longer target fragment than P.
Explanation: PCR primers do more than provide free ends for DNA synthesis; their complementary binding positions define the boundaries of the region that is repeatedly copied. If a second primer pair binds farther apart on the same template, the segment between those sites is correspondingly longer, provided annealing and extension remain suitable. The template sequence has not changed, but the selected target region has. This illustrates how primer design gives PCR its target specificity rather than treating primers as interchangeable reaction components.
306. Two primer pairs bind successfully to the same DNA template:
Pair P flanks a relatively short region.
Pair Q binds farther apart and flanks a larger region.
If both reactions proceed successfully, which statement is most appropriate?
ⓐ. Both primer pairs must produce exactly the same amplified segment because primer location is irrelevant.
ⓑ. The amplified region is determined in part by the positions at which the primers bind.
ⓒ. Pair Q cannot function because PCR primers must always be adjacent.
ⓓ. Primer position affects only colony colour after transformation.
Correct Answer: The amplified region is determined in part by the positions at which the primers bind.
Explanation: PCR primers are complementary to regions associated with the boundaries of the DNA segment to be amplified. Their binding positions therefore help define which portion of the template is preferentially copied. If a second valid primer pair binds farther apart, the region enclosed by those positions differs from the region defined by the first pair. Primer specificity is therefore both a sequence-recognition property and a determinant of target boundaries.
307. During PCR, denaturation and primer annealing occur correctly, but extension is repeatedly interrupted because free nucleotides are absent. Why does amplification fail?
ⓐ. The primers supply all nucleotide bases needed to complete synthesis of new strands.
ⓑ. Nucleotides are required only to separate the double-stranded template during denaturation.
ⓒ. DNA polymerase lacks the nucleotide building blocks required to synthesise new DNA strands.
ⓓ. Lack of nucleotides prevents template DNA from entering a bacterial host cell.
Correct Answer: DNA polymerase lacks the nucleotide building blocks required to synthesise new DNA strands.
Explanation: DNA polymerase extends the annealed primers by adding free nucleotides complementary to the template strand. If those nucleotide substrates are missing, the enzyme cannot construct complete new DNA molecules even when denaturation and primer annealing occur correctly. The primers provide only the starting points for synthesis; they do not supply all of the bases required to build the target sequence. PCR therefore depends on supplied nucleotides as essential raw material for each extension stage.
308. A PCR system is designed for many repeated thermal cycles without adding fresh polymerase after every cycle. Why is a polymerase derived from Thermus aquaticus especially useful in this design?
ⓐ. The polymerase is protected because the denaturation stage does not expose enzymes to high temperature.
ⓑ. The polymerase is regenerated from the DNA template after each denaturation stage.
ⓒ. The polymerase is required only in the first cycle, after which primers alone synthesise new DNA.
ⓓ. Its thermostability allows the same enzyme preparation to survive repeated denaturation steps and function in later extensions.
Correct Answer: Its thermostability allows the same enzyme preparation to survive repeated denaturation steps and function in later extensions.
Explanation: PCR repeatedly exposes the reaction mixture to a high-temperature denaturation stage. A strongly heat-sensitive polymerase would lose activity and would have to be replaced repeatedly. The polymerase obtained from Thermus aquaticus is thermostable, so it can remain functional through successive heating cycles and catalyse primer extension again when the reaction reaches the extension stage. This property makes repeated cyclic amplification practical without replenishing the enzyme after every denaturation step.
309. A graph plots relative amount of a selected DNA segment on the vertical axis against successive PCR cycling on the horizontal axis. Under a successful amplification condition, which qualitative trend is most appropriate?
ⓐ. Target DNA remains essentially at its starting copy number throughout repeated PCR cycling.
ⓑ. Target DNA progressively accumulates to a very high copy number during repeated PCR cycling.
ⓒ. Target DNA progressively decreases in copy number after each extension stage of repeated cycling.
ⓓ. Target DNA is progressively translated into protein, so its amount falls during repeated cycling.
Correct Answer: Target DNA progressively accumulates to a very high copy number during repeated PCR cycling.
Explanation: PCR repeatedly synthesises new copies of the selected target segment through successive cycles of denaturation, primer annealing and extension. As these cycles continue successfully, the amount of target DNA can increase dramatically, reaching approximately \(1\) billion copies under effective repeated amplification. The important interpretation is qualitative amplification over repeated cycles rather than an unsupported exact mathematical relationship between cycle number and copy number.
310. Which sequence best links PCR with subsequent cloning?
ⓐ. amplify DNA \(\to\) vector preparation \(\to\) insert-vector ligation \(\to\) recombinant DNA entry into host
ⓑ. introduce host into PCR \(\to\) purify protein \(\to\) prepare primers \(\to\) perform transformation
ⓒ. perform downstream processing \(\to\) denature protein \(\to\) create vector ori \(\to\) select transformants
ⓓ. select transformants \(\to\) create target by electrophoresis \(\to\) isolate DNA \(\to\) perform ligation
Correct Answer: amplify DNA \(\to\) vector preparation \(\to\) insert-vector ligation \(\to\) recombinant DNA entry into host
Explanation: PCR can first generate many copies of a selected DNA segment in vitro. That amplified fragment can then be incorporated into a suitable vector through restriction-based preparation and ligation. The recombinant DNA is subsequently introduced into a recipient host for biological propagation or expression. PCR and cloning are therefore connected sequentially but are not the same process: PCR amplifies DNA outside cells, whereas vector-based cloning maintains and multiplies the DNA inside a living host.
311. After an attempted transformation with a plasmid carrying ampR, the same bacterial suspension is spread on two plates:
| Plate | Medium | Observation |
|---|
| P | No ampicillin | Many colonies |
| Q | Ampicillin present | A smaller set of colonies |
The original recipient strain is ampicillin-sensitive. Which conclusion is best supported?
ⓐ. Colonies on plate Q prove that every plasmid carries the desired foreign insert.
ⓑ. Every colony on plate P contains the plasmid because no ampicillin is present.
ⓒ. Cells failing on plate Q prove that the introduced plasmid cannot replicate.
ⓓ. P confirms viable cells; Q selects cells that acquired ampicillin resistance.
Correct Answer: P confirms viable cells; Q selects cells that acquired ampicillin resistance.
Explanation: The non-selective plate allows viable recipient cells to grow whether or not they acquired the plasmid, so growth there does not identify transformants. Ampicillin creates the selective condition: because the original strain is sensitive, survival on the antibiotic plate is consistent with uptake and function of the vector-associated ampR marker. This selection establishes a transformant phenotype, but it does not by itself prove that the desired foreign insert is present. A separate recombinant-screening step is required for that distinction.
312. Two bacterial cultures contain the same cloned foreign gene. Culture P maintains many vector copies per cell but produces no detectable encoded protein, whereas culture Q contains fewer vector copies but produces the protein. Which inference is most appropriate?
ⓐ. A higher vector copy number necessarily produces more protein because every cloned gene copy must be expressed equally.
ⓑ. Vector copy number and gene expression are separate outcomes, so abundant cloned DNA need not produce detectable protein.
ⓒ. Protein production in culture Q indicates that its vector must replicate successfully even without an origin of replication.
ⓓ. Failure to detect protein in culture P shows that the foreign DNA must be absent from all of its vector molecules.
Correct Answer: Vector copy number and gene expression are separate outcomes, so abundant cloned DNA need not produce detectable protein.
Explanation: Vector copy number describes how many copies of the vector and its linked foreign DNA are maintained inside a cell. Gene expression is a different process involving production of RNA and, for a protein-coding gene, translation into the encoded protein. Culture P shows that successful multiplication of cloned DNA does not by itself demonstrate expression of that gene. Culture Q produces the protein despite carrying fewer vector copies, showing that DNA abundance and protein production cannot be treated as identical outcomes. Recombinant-protein production therefore requires appropriate expression of the cloned gene in addition to its maintenance and replication.
313. A recombinant protein is detected in a laboratory culture, but total yield remains very low despite correct gene expression. Which stage should be improved next?
ⓐ. Generate additional sticky ends on the recombinant plasmid before scaling up culture.
ⓑ. Identify additional restriction-enzyme recognition sequences in the donor DNA.
ⓒ. Repeat genomic-DNA isolation from the original donor before further production.
ⓓ. Scale up cultivation of the host already expressing the recombinant protein.
Correct Answer: Scale up cultivation of the host already expressing the recombinant protein.
Explanation: Detection of the desired protein shows that gene cloning and expression have already succeeded. If the remaining problem is insufficient total quantity, the next logical step is scale-up of the production culture rather than repeating earlier DNA-manipulation steps. Large-scale cultivation in an appropriately controlled bioreactor can increase biomass and total recombinant-product yield while maintaining conditions favourable for the production cells.
314. A continuous culture is maintained with balanced fresh-medium input and spent-medium removal. If spent-medium removal stops while fresh medium continues to enter, which general problem may eventually arise?
ⓐ. Culture conditions may deteriorate because continuous-culture flow is unbalanced.
ⓑ. The plasmid origin of replication disappears immediately from every cell.
ⓒ. Restriction digestion begins automatically throughout the unbalanced culture.
ⓓ. Product purification finishes automatically without downstream operations.
Correct Answer: Culture conditions may deteriorate because continuous-culture flow is unbalanced.
Explanation: Continuous culture relies on coordinated addition of fresh medium and removal of spent medium to help maintain cells in an active physiological state. If spent-medium removal stops, unwanted metabolic products and excess culture material can accumulate while fresh medium continues to enter. The controlled balance that supports sustained growth is therefore disturbed. The problem is physiological and process-related, not a direct change in recombinant DNA structure.
315. Two production cultures contain the same recombinant organism:
| Culture | Growth state | Biomass accumulation | Product output |
|---|
| P | Active log phase maintained | High | High |
| Q | Active growth not maintained | Lower | Lower |
Which relationship is best supported?
ⓐ. Continuous culture removes the need for expression of the cloned gene.
ⓑ. Product output remains independent of growth state and cellular physiology.
ⓒ. High biomass proves that every cell carries a different recombinant plasmid.
ⓓ. Active growth can support greater biomass and higher product output.
Correct Answer: Active growth can support greater biomass and higher product output.
Explanation: The comparison shows that the culture maintained in an active log or exponential phase accumulates more biomass and gives greater product output. This is consistent with the purpose of continuous culture, where fresh medium supply and removal of spent medium help maintain favourable conditions for active growth. The genetic construct provides the capacity to produce the recombinant product, while culture physiology strongly influences how much biomass and product are ultimately generated.
316. A stirred-tank bioreactor has normal pH and temperature but very poor mixing. Which consequence is most reasonable?
ⓐ. Poor mixing can create uneven distributions of cells, nutrients and oxygen.
ⓑ. Poor mixing automatically increases the specificity of PCR amplification.
ⓒ. Poor mixing necessarily increases DNA ligase activity throughout the culture.
ⓓ. Poor mixing makes the culture sterile by reducing agitation.
Correct Answer: Poor mixing can create uneven distributions of cells, nutrients and oxygen.
Explanation: Agitation in a stirred-tank bioreactor helps mix the culture contents and distribute cells, nutrients and gases more uniformly. If mixing becomes inadequate, local regions of the vessel can experience different nutrient or oxygen conditions even when the overall measured pH and temperature appear satisfactory. Efficient large-scale biological production therefore depends on physical mixing as well as chemical control variables.
317. In a sparged stirred-tank bioreactor, sterile-air bubbles are introduced but the impeller is functioning poorly. Which statement is most accurate?
ⓐ. Aeration continues, but poor agitation can reduce uniform oxygen distribution.
ⓑ. Sterile-air bubbles completely replace all mixing functions of the impeller.
ⓒ. Impeller failure affects restriction digestion but not culture mixing.
ⓓ. Sparging guarantees perfectly uniform oxygen availability despite poor agitation.
Correct Answer: Aeration continues, but poor agitation can reduce uniform oxygen distribution.
Explanation: Sparging and agitation contribute differently to reactor performance. Air bubbles increase the gas-liquid interface and support oxygen transfer, whereas the impeller mixes the culture and distributes gases, nutrients and cells throughout the vessel. If the impeller performs poorly, the reactor can still receive air but may not distribute oxygen and other components uniformly. Efficient operation therefore depends on the coordinated function of aeration and mixing systems.
318. A bioreactor contains adequate substrate, salts and vitamins, but both pH and temperature drift outside their suitable ranges. Which conclusion is most appropriate?
ⓐ. Adequate nutrients guarantee normal production even when pH and temperature are unsuitable.
ⓑ. Productivity can fall because several culture conditions must remain suitable together.
ⓒ. Unsuitable pH and temperature cause the vector to acquire a new origin of replication.
ⓓ. Downstream purification corrects poor growth conditions while cells are still producing.
Correct Answer: Productivity can fall because several culture conditions must remain suitable together.
Explanation: Bioreactor productivity depends on a coordinated set of conditions, including temperature, pH, nutrients, salts, vitamins and oxygen where required. Adequate substrate alone cannot compensate for major deviations in pH and temperature because cellular metabolism and enzyme activity depend on the overall physiological environment. Large-scale production therefore requires simultaneous control of multiple variables rather than optimisation of only one component.
319. The following operations are listed for a biotechnology product:
P. Recombinant cells synthesise the desired protein.
Q. The product is separated from the culture material.
R. The product is purified.
S. The purified product is formulated with suitable components.
Which boundary correctly separates biosynthesis from downstream processing?
ⓐ. Downstream processing starts before recombinant cells synthesise the product in P.
ⓑ. P, Q, R and S are all stages of genetic engineering before production.
ⓒ. Only final formulation in S is part of downstream processing.
ⓓ. Downstream processing starts with separation at Q, after biosynthesis at P.
Correct Answer: Downstream processing starts with separation at Q, after biosynthesis at P.
Explanation: Production of the desired molecule by recombinant cells represents the biosynthetic stage. Once the product has been made, downstream processing begins with its recovery or separation from the culture material. Purification and formulation then follow as appropriate. The distinction is temporal and functional: biosynthesis creates the biological product, whereas downstream operations recover, refine and prepare that product for its intended use.
320. Two biotechnology products have completed purification and formulation. Product P is intended for therapeutic administration and has passed product-quality tests but has not undergone appropriate clinical evaluation. Product Q is a non-therapeutic product and has passed the quality assessment required for its intended use. Which interpretation is most appropriate?
ⓐ. Product P is ready for therapeutic use because purification, formulation and product-quality testing are sufficient for release.
ⓑ. Products P and Q require identical clinical evaluation because both were produced through biotechnology-based processes.
ⓒ. Product P still requires appropriate clinical evaluation, whereas product Q should meet the requirements relevant to its own intended use.
ⓓ. Neither product requires any further assessment because purification and formulation complete all downstream requirements.
Correct Answer: Product P still requires appropriate clinical evaluation, whereas product Q should meet the requirements relevant to its own intended use.
Explanation: Downstream requirements depend partly on the nature and intended use of the final product. A therapeutic preparation requires appropriate clinical evaluation before therapeutic use even when purification, formulation and product-quality testing have been completed successfully. A non-therapeutic biotechnology product still requires suitable quality assessment, but it should not automatically be assigned the same clinical pathway as a therapeutic product. The correct interpretation therefore distinguishes product-specific evaluation from the general downstream operations of recovery, purification and formulation.
