MCQs on Exonucleases
1 - Question
Which of the following statements is correct with respect to exonuclease?
a) They only act on single stranded DNA molecules
b) They only act on double stranded DNA molecules
c) They remove a single nucleotide base at a time
d) They remove nucleotide bases from the middle of a polynucleotide chain
Explanation: Exonuclease is responsible for removal of a single nucleotide base at a time, from the end of the polynucleotide sequence. They can act on both single and double stranded DNA molecules.
2 - Question
How many approaches are there which can be used for exonucleolytic activity in double stranded DNA molecules?
Explanation: There are 2 approaches which can be used to carry out exonucleolytic activity in double stranded DNA. The two approaches which are used as a separation of both the strands together and separation of both the strands separately
3 - Question
What is the mode of action of exonuclease III?
a) Exonuclease III acts on single stranded DNA in 3’-5’ direction
b) Exonuclease III acts on double stranded DNA in 5’-3’ direction
c) Exonuclease III acts on single stranded DNA in 5’-3’ direction
d) Exonuclease III acts on double stranded DNA in 3’-5’ direction
Explanation: It is an enzyme which is having exonuclease activity in 3’-5’ direction and only on double stranded DNA. It doesn’t act on single stranded molecules.
4 - Question
Which of the following statements is correct regarding S1 nuclease?
a) It acts on double stranded DNA
b) It acts on single stranded DNA
c) It acts on both types of strands
d) It is obtained from E. coli
Explanation: S1 nuclease is obtained from Aspergillus orzyae and it acts on single stranded DNA molecules. It is having exonucleolytic activity
5 - Question
What happens if a DNA molecule is treated by first Exonuclease III and then followed by treatment with S1 nuclease?
a) The molecule is shortened only from 3’ end
b) The molecule is shortened only from 5’ end
c) The molecule is shortened from both the ends
d) Only Exonuclease acts and S1 doesn’t acts
Explanation: The molecule is shortened from both the ends. It is so because as firstly Exonuclease III acts, single stranded ends are produced on both the sides. These are further acted upon by S1 nuclease and thus the molecule is shortened from both the sides.
6 - Question
How can one end be protected from the action of Exonuclease III, so that the molecule is not shortened from both the ends?
a) By using Phosphorothioate nucleotide analogue
b) By making both the ends double stranded in nature
c) By labelling one end with a radioactive compound
d) By increasing the time of exposure of the DNA molecule to the enzyme
Explanation: Phosphorothioate nucleotide analogues are used to replace some of the nucleotides at the ends with it. It doesn’t allows the action of the enzyme on it and thus the molecule won’t be shortened from that end on which replacement is done.
7 - Question
Bal31 is also an enzyme which is used. Which of the following statements hold true in its context?
a) It is having only 3’-5’ exonuclease activity and no endonuclease activity
b) It is having only an endonuclease activity
c) It is having 5’-3’ exonuclease activity
d) It leads to shortening from both the ends
Explanation: Bal31 is having an exonuclease activity in 3’-5’ direction and also an endonuclease activity. Thus, after having 3’-5’ exonuclease activity, the endonuclease activity takes place and the molecule is shortened from both the ends
8 - Question
The extent of deletions can be manipulated by controlling which of them?
a) Time of incubation only
b) Amount of nuclease added
c) They both have a role to play in the extent of the deletion
d) The amount of deletion which can be carried out for a particular amount of DNA is fixed regardless of the amount of nuclease and incubation time
Explanation: The extent of deletion can be varied by varying the incubation time and the amount of nuclease added. Thus, they both play a role in controlling the extent of deletion.
9 - Question
What is the function of methylase?
a) Addition of methyl groups to DNA
b) Removal of methyl groups from DNA
c) Both in removal and addition of methyl groups from DNA
d) It is used in production of methane gas
Explanation: Methylase is used for the addition of methyl groups on DNA. It is done via placing the methyl groups which are taken from S-adenosyl methionine.
10 - Question
Methylase is useful in cloning experiments.
Explanation: It is useful in cloning experiments because as it methylates DNA, that DNA is protected from that group of restriction enzyme. It means that if we carry out methylation by EcoRI methylase, the DNA won’t be cleaved by the restriction enzyme EcoRI. In cloning experiments, sometimes it is necessary to protect the DNA from cleavage by a particular enzyme.