Part 6: Methods of Selection & Screening (25 MCQs)
Part 6: Methods of Selection & Screening – Antibiotic resistance, Reporter genes, Blue-white selection (25 MCQs)
Q126. The purpose of selection in rDNA technology is:
a) To identify recombinant clones
b) To cut DNA
c) To ligate DNA
d) To precipitate DNA
Answer: a
- a) Correct: Selection helps differentiate recombinants from non-recombinants.
Q127. Selectable markers are usually:
a) Restriction sites
b) Antibiotic resistance genes
c) Structural proteins
d) Ribosomal RNA
Answer: b
- b) Correct: Ampicillin/tetracycline resistance genes used as markers.
Q128. pBR322 plasmid has resistance to:
a) Ampicillin and tetracycline
b) Streptomycin and rifampicin
c) Chloramphenicol and kanamycin
d) Only tetracycline
Answer: a
- a) Correct: pBR322 has amp^R and tet^R genes.
Q129. Recombinant plasmids can be identified by:
a) Growth on selective medium
b) Insertional inactivation
c) Reporter gene activity
d) All of the above
Answer: d
- d) Correct: Multiple methods used for identifying recombinants.
Q130. Insertional inactivation works by:
a) Activating resistance gene
b) Inactivating a marker gene after DNA insertion
c) Inserting DNA in promoter region
d) None
Answer: b
- b) Correct: Insertion disrupts a gene → loss of activity → identification possible.
Q131. In pBR322, insertion into tet^R gene makes bacteria:
a) Resistant to tetracycline
b) Sensitive to tetracycline
c) Resistant to ampicillin
d) None
Answer: b
- b) Correct: Insertion disrupts tet^R → sensitive to tetracycline.
Q132. In pBR322, recombinants can be screened by:
a) Replica plating on antibiotic media
b) Heat shock
c) Gel electrophoresis
d) All of the above
Answer: a
- a) Correct: Replica plating distinguishes resistant vs. sensitive colonies.
Q133. The principle of blue-white screening is based on:
a) Antibiotic sensitivity
b) Disruption of lacZ gene
c) ORI mutation
d) Protein secretion
Answer: b
- b) Correct: lacZ gene (β-galactosidase) disrupted by DNA insertion.
Q134. Non-recombinants in blue-white screening appear:
a) White
b) Blue
c) Green
d) Red
Answer: b
- b) Correct: Non-recombinants = blue (intact lacZ hydrolyzes X-gal).
Q135. Recombinants in blue-white screening appear:
a) White colonies
b) Blue colonies
c) Yellow colonies
d) Red colonies
Answer: a
- a) Correct: Recombinants = white (lacZ disrupted).
Q136. The substrate used in blue-white screening is:
a) ONPG
b) X-gal
c) Maltose
d) IPTG
Answer: b
- b) Correct: X-gal gives blue color when cleaved by β-galactosidase.
Q137. IPTG is used in blue-white screening because it:
a) Is an inducer of lac operon
b) Cuts DNA
c) Acts as antibiotic
d) Prevents recombination
Answer: a
- a) Correct: IPTG induces lac operon expression → β-galactosidase activity.
Q138. A recombinant plasmid inserted in lacZ gene is detected because:
a) Colonies remain white on X-gal medium
b) Colonies become blue on X-gal
c) Colonies die
d) Colonies fluoresce
Answer: a
- a) Correct: Recombinants disrupt lacZ → white colonies.
Q139. Reporter genes are used because they:
a) Destroy foreign DNA
b) Help in visual identification of recombinants
c) Act as enzymes in ligation
d) Produce antibiotics
Answer: b
- b) Correct: Reporter genes (GFP, lacZ) indicate recombinants by color/fluorescence.
Q140. Example of a commonly used reporter gene is:
a) GFP (green fluorescent protein)
b) lacZ (β-galactosidase)
c) CAT (chloramphenicol acetyltransferase)
d) All of the above
Answer: d
- d) Correct: All are reporter genes for screening.
Q141. A recombinant identified by insertional inactivation is usually:
a) Resistant to both antibiotics
b) Sensitive to one antibiotic
c) Unable to metabolize a substrate
d) Both b and c
Answer: d
- d) Correct: Recombinants often lose resistance or reporter activity.
Q142. Which method is fastest for screening recombinants?
a) Replica plating
b) Blue-white screening
c) Antibiotic sensitivity tests
d) All are equal
Answer: b
- b) Correct: Blue-white screening gives quick visible results.
Q143. Which one is NOT a selectable marker?
a) Ampicillin resistance gene
b) GFP
c) lacZ gene
d) Taq polymerase
Answer: d
- d) Correct: Taq polymerase is not a marker.
Q144. Positive selection of recombinants means:
a) Only recombinants survive on selective medium
b) Recombinants die on selective medium
c) Both recombinants and non-recombinants survive
d) None
Answer: a
- a) Correct: Positive selection = only desired recombinants grow.
Q145. Negative selection of recombinants means:
a) Recombinants killed
b) Non-recombinants killed
c) Both survive
d) None
Answer: a
- a) Correct: In some systems, recombinants lose survival ability.
Q146. Blue-white screening requires which plasmid vectors?
a) pBR322
b) pUC18/pUC19
c) Ti plasmid
d) λ phage
Answer: b
- b) Correct: pUC vectors have lacZ gene → used in blue-white selection.
Q147. In insertional inactivation, the foreign DNA disrupts:
a) ORI
b) A marker or reporter gene
c) RNA polymerase gene
d) Restriction enzyme
Answer: b
- b) Correct: Recombinant DNA disrupts marker/reporter gene.
Q148. An ideal selectable marker should:
a) Distinguish recombinants from non-recombinants
b) Not affect host viability
c) Be easy to detect
d) All of the above
Answer: d
- d) Correct: All are essential for good marker design.
Q149. In pBR322, which antibiotic resistance gene is lost when foreign DNA is inserted in tet^R site?
a) Ampicillin resistance
b) Tetracycline resistance
c) Kanamycin resistance
d) Streptomycin resistance
Answer: b
- b) Correct: tet^R gene disrupted → loss of tetracycline resistance.
Q150. Recombinant clones in blue-white screening do NOT produce:
a) β-galactosidase
b) Blue colonies
c) White colonies
d) Antibiotic resistance
Answer: a
- a) Correct: Recombinants lack β-galactosidase activity → appear white.
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