MCQs on Polymerases
1 - Question
Basic classification of polymerases includes how many types?
Explanation: The types are- DNA dependent DNA polymerase, DNA dependent RNA polymerase, RNA dependent DNA polymerase, template independent polymerase and RNA dependent RNA polymerase. Out of these, RNA dependent RNA polymerases are not focused so much in comparison to other polymerases.
2 - Question
Polymerase can be defined as ______________
a) an enzyme used to synthesize a new DNA or RNA strand on the basis of pre-existing strand or at times without a pre-existing strand
b) an enzyme used for removal of nucleotides from the DNA or RNA strand
c) an enzyme which can synthesize only a new DNA strand, not an RNA strand
d) an enzyme which can synthesize either a new DNA or an RNA strand but only when a strand is there
Explanation: The synthesis of a new DNA or RNA strand can be done either on the basis of a pre-existing strand or without it. Those which don’t require a pre-existing strand are known as template free or template independent.
3 - Question
Which of the following activity is not possible in the case of DNA polymerase I?
a) 3’-5’ exonuclease
b) 5’-3’ exonuclease
c) 5’-3’ DNA synthesis
d) 3’-5’ DNA synthesis
Explanation: DNA polymerase is having the ability to synthesize DNA strand in 5’-3’ direction but not in 3’-5’ direction. Also, it is having exonuclease activity in both the directions, which means that it can remove bases in either of the direction but can synthesize only in 5’-3’ direction.
4 - Question
The E. coli DNA polymerase enzyme gives different domains with different activities on cleaving with protease subtilisin. Which of the following statements is correct with respect to the fragments generated?
a) The smaller fragment is having C terminal and the larger fragment is having N terminal
b) The smaller fragment is named as Klenow fragment and the intact molecule is called as Kornberg fragment
c) The smaller fragment is having 5’-3’ exonuclease activity whereas the larger fragment is having 5’-3’ polymerase and 3’-5’ exonuclease activity
d) Both the fragments are having 5’-3’ polymerase and 3’-5’ exonuclease activity
Explanation: The two fragments are generated of size 35kDa and 76kDa, with the N terminal in the smaller fragment. The larger fragment, which is of 76kDa is known as Klenow fragment and the intact molecule is called Kornberg fragment. The smaller fragment is having 5’-3’ exonuclease activity whereas the larger fragment is having 5’-3’ polymerase and 3’-5’ exonuclease activity.
5 - Question
Removal of single stranded portions produced due to exonucleolytic activity and due to polymerase activity are termed as _____________
a) polishing and end filling respectively
b) end filling and polishing respectively
c) polishing in both the cases
d) end filling in both the cases
Explanation: The single stranded ends generated due to exonucleolytic activity can either be removed by generating their complementary strand or removing the single stranded overhang. In both cases, it is termed as polishing. Whereas removal of single stranded regions generated because of polymerase activity is termed as end-filling.
6 - Question
Thermostable DNA polymerases are very important in PCR. How are they obtained?
a) They are obtained by heating the bacteria manually over high temperatures
b) They are isolated from extremely stable thermophilic bacteria which are often found growing in oceanic vents
c) They are found everywhere in nature
d) They are obtained by genetically modifying the E. coli bacteria with thermal stability property
Explanation: Thermostable DNA polymerases are found naturally in thermophilic bacteria which can be found growing in oceanic vents. They are having high stability and their DNA polymerases can function effectively even at high temperatures in-vitro
7 - Question
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
8 - Question
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.
9 - Question
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.
10 - Question
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.