Weekly chapter test: CBSE Class 12

1. Why must the recognition sequence of a restriction enzyme be palindromic for the enzyme to produce identical sticky ends on both cut fragments?

• A. Because palindromic sequences allow the enzyme to cut only one strand
• B. Because both strands have the same sequence when read 5′→3′, so the same overhangs are generated on each fragment
• C. Because non-palindromic sequences would produce blunt ends instead
• D. Because palindromic sequences are always six base pairs long

2. A researcher starts with 10 copies of a target DNA segment and performs 15 cycles of PCR. How many copies of the target segment will be present at the end of the amplification?

• A. 150 copies
• B. 32768 copies
• C. 327680 copies
• D. 1048576 copies

3. If two different restriction enzymes produce compatible sticky ends, fragments cut by these enzymes can ligate together. What happens to the hybrid junction site after ligation?

• A. It can be cut by either of the original enzymes
• B. It may not be recognised by either original enzyme because the complete recognition sequence is altered
• C. It will always create a new unique restriction site
• D. Ligation cannot occur between fragments from different enzymes

4. The sequence 5′-GCATGC-3′ is found on one strand of DNA. What is the sequence of the complementary strand when read in the 5′→3′ direction?

• A. 5′-CGTACG-3′
• B. 5′-GCATGC-3′
• C. 5′-CATGCG-3′
• D. 5′-TACGTA-3′

5. A bioreactor is essential for large-scale production of recombinant proteins. What is the primary role of a bioreactor in biotechnology?

• A. To cut and ligate DNA fragments under sterile conditions
• B. To provide optimal and controlled conditions for growing large volumes of cells carrying recombinant DNA
• C. To separate proteins from cell debris using gel electrophoresis
• D. To amplify DNA using repeated heating and cooling cycles

6. A researcher performs PCR but uses a DNA polymerase from a mesophilic bacterium instead of Taq polymerase. After 30 cycles, almost no amplification is observed. What is the most likely reason?

• A. Mesophilic polymerase cannot recognise the primers used
• B. Mesophilic polymerase is denatured at 94–98 °C and must be replenished after every denaturation step
• C. Mesophilic polymerase synthesises DNA only in the 3′→5′ direction
• D. Mesophilic polymerase requires RNA primers instead of DNA primers

7. In blue-white screening, if a researcher forgets to add X-gal to the medium but IPTG is present, what outcome would occur?

• A. All colonies would appear blue
• B. All colonies would appear white, making recombinants indistinguishable from non-recombinants
• C. Only recombinant colonies would grow
• D. No colonies would grow at all

8. The naming convention for restriction enzymes follows a specific rule. Based on this, what information does the name 'HindIII' convey?

• A. Third enzyme isolated from Haemophilus influenzae strain d
• B. Enzyme that produces three sticky ends from Helicobacter species
• C. Enzyme that recognises a three-nucleotide sequence in human DNA
• D. Third type of ligase enzyme from Haemophilus genus

9. In gel electrophoresis, DNA fragments migrate towards the positive electrode. A mixture contains fragments of 500 bp, 1000 bp and 2000 bp. Which fragment will be closest to the well after electrophoresis?

• A. 500 bp
• B. 1000 bp
• C. 2000 bp
• D. All fragments travel equal distance

10. In a transformation experiment, 200 µg of plasmid DNA is mixed with competent E. coli cells treated with CaCl₂. After heat shock at 42 °C and plating on selective medium, 500 colonies appear. If only 1 in 10000 cells takes up the plasmid, approximately how many competent cells were used in the experiment?

• A. 50000 cells
• B. 500000 cells
• C. 5000000 cells
• D. 50000000 cells