1. Host controlled restriction is a phenomenon related to ________
a) Bacteria
b) Virus
c) Plasmid
d) Gene of interest
Explanation: The initial observation that led to the discovery of restriction endonucleases was made in early 1950s when it was shown that some strains of bacteria are immune to bacteriophage infection.
2. Why does the restriction phenomenon in bacteria naturally occur?
a) For efficient cloning
b) Bacteria produce an enzyme
c) Destruction of bacterium’s own DNA
d) For survival
Explanation: Restriction occurs because the bacterium produces an enzyme that degrades phage DNA before it has time to replicate and direct synthesis of new phage particles.
3. Which type of restriction endonucleases is used most in genetic engineering?
a) Type I
b) Type II
c) Type III
d) Type IV
Explanation: Type I and Type III are complex and have only a limited role in genetic engineering. Type II restriction endonucleases are used mostly as the cutting enzymes in gene cloning
4. The restriction endonuclease PvuI (isolated from Proteus Vulgaris) cuts DNA at which position?
a) Hexanucleotide CGATCG
b) Random position
c) Towards the end
d) Hexanucleotide CAGCTG
Explanation: A particular enzymes cleaves DNA at the recognition sequence and nowhere else. PvuI cuts DNA at hexanucleotide sequence CGATCG.
5. The restriction endonuclease AluI is isolated from which microbe?
a) Proteus Vulgaris
b) Staphylococcus Aureus
c) Arthrobacter Luteus
d) Haemophilus Influenzae
Explanation: AluI restriction endonuclease derived from Arthrobacter Luteus, cleaves a DNA molecule at nucleotide long site- AGCT. There are other endonucleases that cleave at bigger sites.
6. EcoR1 has which of the following hexanucleotide recognition sequence?
a) GAATTC
b) GGATCC
c) AGATCT
d) GCAGCA
Explanation: E.coli isolated from the gram-negative bacterium Escherichia coli. A recognition sequence is the one where an enzyme cleaves.
7. Which of the following is an example of flush end cutter?
a) PvuI
b) PvuII
c) EcoR1
d) SfiI
Explanation: Many restriction endonucleases like PvuII, AluI, HaeIII simply cleave the double stranded DNA producing bunt or flush ends.
8. Which of the following does not produce cohesive ends upon cleavage on their recognition sequences?
a) EcoRI
b) PvuII
c) BamHI
d) HindIII
Explanation: PvuII produces blunt ends whereas all others produce sticky ends. Some endonucleases cleave the DNA strand in a staggered way usually by two or four nucleotides, so that the resulting DNA fragments have short single-stranded overhangs at each end.
9. What is dithiothreitol?
a) Reducing agent
b) Restriction endonuclease
c) Ligase
d) Exonuclease
Explanation: Most endonuclease function at pH 7.4, but different enzymes vary in their requirements for ionic strength and concentration. It is advisable to add a reducing agent like DTT which stabilizes the enzyme and prevents its deactivation. Before adding the enzyme, solution containing DNA must be adjusted to provide the correct conditions for maximum enzyme activity.
10. Restriction digests with which enzyme needs to be kept at a higher temperature than the usual 37 degree temperature?
a) TaqI
b) EcoRI
c) BamHI
d) HindIII
Explanation: Most endonucleases work best at 37 degrees, TaqI enzyme from Thermus Aquaticus has a high working temperature like Taq DNA polymerase.