Quiz: PCR and Plasmid Cloning — 20 questions

1. What is the primary purpose of the Polymerase Chain Reaction?

A. To exponentially amplify a selected DNA target through repeated heating and cooling

B. To determine the amino acid sequence of a newly synthesized protein

C. To visualize chromosomes during different stages of cell division

D. To separate proteins according to their molecular weights through an electric field

Answer: (A)

Explanation

PCR produces many copies of a selected DNA region by repeating temperature-controlled cycles. Protein separation, protein sequencing, and chromosome visualization are different laboratory procedures.

2. Which combination contains the essential components of a basic PCR reaction?

A. DNA template, Taq polymerase, primers, dNTPs, buffer, MgCl2, and a PCR machine

B. Protein template, reverse transcriptase, lipids, glucose, salts, and a spectrophotometer

C. DNA template, restriction enzymes, agarose, fluorescent dyes, antibodies, and a microscope

D. RNA template, ribosomes, amino acids, ATP, antibodies, and a centrifuge

Answer: (A)

Explanation

A basic PCR requires target-containing DNA, heat-resistant Taq polymerase, two primers, dNTPs, buffer, bivalent cations such as MgCl2, and a thermal cycler. The other combinations list components associated with translation, protein analysis, or DNA visualization rather than standard PCR.

3. Which application is a recognized use of PCR?

A. Measuring blood pressure through electrical signals

B. Separating intact cells by their buoyant density

C. Producing antibodies directly from purified lipids

D. Detecting a mutation in a patient's DNA

Answer: (D)

Explanation

PCR can amplify target sequences for mutation detection and other applications such as genotyping, cloning, diagnostics, and sequencing. The other choices describe unrelated physiological or cell-separation procedures.

4. Which sequence correctly describes the three temperature stages of one PCR cycle?

A. Annealing at 95°C, extension at 50–65°C, and denaturation at 72°C

B. Denaturation at 72°C, extension at 50–65°C, and annealing at 95°C

C. Denaturation at 95°C, annealing at 50–65°C, and extension at 72°C

D. Extension at 95°C, denaturation at 50–65°C, and annealing at 72°C

Answer: (C)

Explanation

PCR cycles use high heat to separate DNA strands, a lower temperature for primer binding, and 72°C for polymerase-mediated extension. Annealing does not separate strands, and extension does not occur at the denaturation temperature.

5. Under efficient PCR conditions, what can happen to one or a few starting copies of a target region?

A. They remain at roughly the original quantity because DNA is not copied

B. They are degraded into nucleotides during each heating step

C. They are converted into proteins without requiring any additional molecular steps

D. They can be amplified into billions of copies after repeated cycles

Answer: (D)

Explanation

Repeated PCR cycles can produce billions of copies from one or a few starting target molecules under efficient conditions. DNA is copied rather than maintained, translated, or degraded as the intended outcome of the reaction.

6. What does the melting temperature of DNA represent?

A. The temperature at which half of the nucleotides are removed from DNA

B. The temperature at which half of the DNA duplex molecules separate into single strands

C. The temperature used to extend primers by Taq polymerase

D. The temperature at which all primers begin synthesizing new DNA strands

Answer: (B)

Explanation

DNA melting temperature, or Tm, is defined as the temperature at which 50% of duplex DNA dissociates into single strands. It is distinct from the annealing temperature, which is used for primer binding during PCR.

7. Why is the melting temperature of PCR primers important?

A. It reflects duplex stability and strongly influences the specificity of PCR

B. It determines the concentration of dNTPs needed for DNA synthesis

C. It controls whether the DNA template contains a mutation

D. It identifies which buffer can replace magnesium ions in the reaction

Answer: (A)

Explanation

Primer Tm provides information about duplex stability, and PCR specificity depends strongly on primer Tm values. Primer Tm does not determine dNTP concentration, replace magnesium requirements, or reveal the presence of a mutation.

8. A primer contains 6 G or C bases and 14 A or T bases. Using $Tm = 4(G+C) + 2(A+T)$, what is its calculated melting temperature before the PCR-machine adjustment?

A. 64°C

B. 44°C

C. 40°C

D. 52°C

Answer: (D)

Explanation

Substituting the base counts gives $Tm = 4(6) + 2(14) = 24 + 28 = 52\,\text{°C}$. The 5°C subtraction is applied when setting up the PCR machine, so it is not included in this requested value.

9. Which primer design meets the recommended melting-temperature relationship for a PCR pair?

A. One primer at 68°C and the other at 74°C

B. One primer at 50°C and the other at 57°C

C. One primer at 55°C and the other at 63°C

D. One primer at 62°C and the other at 66°C

Answer: (D)

Explanation

Primer Tm values should generally fall between 55°C and 70°C and differ by no more than 5°C; 62°C and 66°C satisfy both conditions. The other pairs either differ by more than 5°C or include a value outside the recommended range.

10. Which type of DNA can serve as a PCR template for further amplification?

A. A purified restriction enzyme

B. A free nucleotide mixture

C. A previously generated PCR product

D. A bacterial ribosome

Answer: (C)

Explanation

A PCR product can be used as template DNA for another amplification reaction, along with genomic DNA, complementary DNA, and plasmid DNA. A restriction enzyme, ribosome, or nucleotide mixture does not provide the DNA sequence being copied.

11. What is the likely effect of using an excessively high amount of template DNA in a PCR reaction?

A. Increased nonspecific amplification

B. Complete loss of polymerase activity

C. Formation of compatible plasmid ends

D. Reduced primer annealing temperature

Answer: (A)

Explanation

Higher template concentrations increase the risk that unintended sequences will also be amplified. Reduced template amounts are more associated with lower PCR yield, while the other choices describe unrelated processes.

12. A PCR reaction contains inhibitors and produces a weak product despite suitable primers and template; what adjustment may improve amplification but also increase nonspecific products if excessive?

A. Replacing the template with nucleotides

B. Increasing the DNA polymerase amount

C. Using blunt-ended DNA fragments

D. Removing the bacterial origin of replication

Answer: (B)

Explanation

Adding more polymerase can help overcome inhibitors, although excessive enzyme may promote nonspecific products. The other changes do not address polymerase limitation in an inhibited PCR reaction.

13. What is the primary purpose of a plasmid vector in bacterial molecular cloning?

A. To digest inserted DNA at every recognition site

B. To synthesize proteins without entering bacterial cells

C. To carry DNA fragments generally smaller than 10,000 base pairs

D. To replace the bacterial chromosome during cell division

Answer: (C)

Explanation

Plasmid vectors are extrachromosomal cloning vehicles designed to carry relatively small DNA fragments in bacteria. The bacterial chromosome is not a plasmid replicon, and plasmids do not inherently digest DNA or function outside host cells.

14. Which pair of features is expected in a bacterial plasmid vector?

A. A bacterial origin of replication and an antibiotic-resistance gene

B. A eukaryotic nucleus and a mitochondrial targeting sequence

C. A bacterial chromosome and a membrane transport channel

D. A restriction enzyme and a ribosomal protein gene

Answer: (A)

Explanation

Bacterial plasmid vectors contain an origin of replication and an antibiotic-resistance marker, such as ampicillin, kanamycin, or tetracycline resistance. The other pairs do not describe the core structural features required for plasmid propagation and selection.

15. Why can a plasmid maintain replication independently of bacterial chromosome replication?

A. Its cloning site replaces the bacterial chromosome

B. Its antibiotic-resistance gene copies the plasmid

C. Its origin of replication initiates plasmid replication

D. Its inserted DNA supplies cellular replication enzymes

Answer: (C)

Explanation

The plasmid origin of replication permits plasmid DNA to replicate independently from the bacterial chromosome. Antibiotic resistance provides selection, while cloning sites and inserted DNA do not initiate plasmid replication.

16. What happens when an antibiotic corresponding to a plasmid resistance gene is added to a bacterial culture?

A. All bacteria increase plasmid copy number

B. The antibiotic inserts DNA into the plasmid

C. Bacteria without plasmids become resistant

D. Bacteria lacking the plasmid are killed

Answer: (D)

Explanation

The corresponding antibiotic selects for bacteria carrying the plasmid because cells lacking the resistance gene are killed. Antibiotic selection does not itself insert DNA or guarantee an increased plasmid copy number.

17. What does a restriction enzyme do when it encounters its recognition site?

A. It removes all single-stranded regions from DNA

B. It copies the recognition sequence into a plasmid

C. It joins unrelated DNA fragments through translation

D. It recognizes a short specific sequence and cleaves DNA there

Answer: (D)

Explanation

Restriction enzymes identify particular short DNA sequences and cut the DNA at those sites, producing defined ends. They do not copy sequences, translate DNA, or universally remove single-stranded regions.

18. Which type of DNA end lacks a single-stranded overhang?

A. A 5' cohesive end

B. A 3' cohesive end

C. A blunt end

D. A staggered cohesive end

Answer: (C)

Explanation

Blunt ends have no single-stranded overhang, whereas 5' and 3' cohesive ends contain overhanging single-stranded DNA. A staggered cut generally produces cohesive rather than blunt ends.

19. What must occur before a DNA fragment can be inserted into a linearized plasmid during molecular cloning?

A. The DNA must be translated into a protein

B. The fragment must first replace the host chromosome

C. The plasmid must lose its origin of replication

D. Their ends must be compatible for joining by phosphodiester bonds

Answer: (D)

Explanation

Compatible ends allow the DNA fragment and linearized plasmid to align, after which phosphodiester bonds form the recombinant plasmid. Chromosome replacement, loss of the plasmid origin, and translation are not prerequisites for ligation.

20. What is the role of the host organism after a recombinant DNA molecule is introduced during molecular cloning?

A. It replicates the recombinant DNA molecule

B. It prevents the recombinant molecule from being copied

C. It converts the DNA into a restriction enzyme

D. It removes the vector before DNA replication

Answer: (A)

Explanation

Molecular cloning uses a host organism in which the recombinant DNA molecule is replicated. The host does not normally eliminate the vector or prevent copying, and conversion into a restriction enzyme is unrelated to cloning.