1. Which of the following enzyme is said as reverse transcriptase?
a) DNA dependent DNA polymerase
b) RNA dependent RNA polymerase
c) RNA dependent DNA polymerase
d) DNA dependent RNA polymerase
Explanation: RNA dependent DNA polymerases are said as reverse transcriptase and the function follows it name. The function is to reverse the phenomenon of general transcription, in which RNA is synthesized from DNA. But here it is reversed and DNA is synthesized by using RNA as a template.
2. Why reverse transcriptase enzymes are having comparatively high error rates than other polymerases?
a) Because they are not having 3’-5’ proofreading exonucleolytic activity
b) Because they are not having 5’-3’ proofreading exonucleolytic activity
c) Because they are having slow rates of exonucleolytic activity
d) It is difficult to synthesize DNA from RNA
Explanation: Reverse transcriptase enzymes are not having proof reading exonuclease activity in 3’-5’ direction. Hence, when the synthesis is done in 5’-3’ direction it is not checked in the 3’-5’ direction and thus the errors inculcated won’t be removed
3. Which polymerase can be used in conjunction with appropriate phage promoters in order to have high levels of specific transcription?
a) RNA dependent DNA polymerase
b) DNA dependent RNA polymerase
c) RNA dependent RNA polymerase
d) DNA dependent DNA polymerase
Explanation: DNA dependent RNA polymerases are very specific for phage promoters and thus they are used for carrying out highly specific transcription. They are often isolated from T3, T7 phages.
4. Template independent polymerases are the enzymes which add nucleotide bases without a template. Which of the following statements is correct with respect to these?
a) They only add a single nucleotide
b) They only add a string of nucleotides and not a single nucleotide
c) Terminal transferase adds a series of nucleotides at the 3’ end
d) Taq polymerase adds a single nucleotide at the 5’ end of the PCR product
Explanation: Template independent polymerases can either add a single nucleotide or a series of nucleotides; it is based on the template. Terminal transferase adds a series of nucleotides at the 3’ end, which generates a single stranded tail. Whereas, Taq polymerase adds a single A base at 3’ end of the PCR product
5. 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.
6. How many approaches are there which can be used for exonucleolytic activity in double stranded DNA molecules?
a) 1
b) 2
c) 3
d) 4
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.
7. 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.
8. 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.
9. 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.
10. 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