PASSAGE
Read the sentence and answer the questions.
The process of replication requires a set of catalyst (enzymes). The main enzyme is referred to as DNA dependent DNA Polymerase. E. coli that has only 4.6 × 106 bp (compare it with human whose diploid content is 6.6 × 106 bp), completes the process of replication within 18 minutes, that means the average rate of polymerisation has to be approximately 2000 bp per second these polymerase also have catalyse the reaction with high degree of accuracy. Deoxyribonucleoside triphosphates serve dual purposes. In addition to DNA dependent DNA polymerases, many additional enzymes are required to complete the process of replication with high degree of accuracy. For long DNA molecules, since the two strands of DNA cannot be separated in its entire length (due to very high energy requirement), the replication occur with in a small opening of the DNA helix, referred to as replication fork. The DNA-dependent DNA polymerase catalyse polymerisation only in one direction, that is 5′→3′. This creates some additional complications at the replicating fork. Consequently, on one strand (the template with polarity 3′→5′) the replication is continuous, which on the others (the template with polarity 5′→3′), it is the discontinuous. The non-continuously synthesized fragments are later joined by the enzyme DNA ligase.
In E. coli the main enzyme required to catalyze the polymerisation of deoxynucleotide is ________
Answer & explanation
Correct answer: option 1
The correct answer is Option (1) → DNA-Dependent DNA Polymerase
A polymerase is named by its template and its product: an “X-dependent Y polymerase” reads a template of X and builds a chain of Y.
Option (1) DNA-Dependent DNA Polymerase — Correct. It reads a DNA template and joins deoxynucleotides into a new DNA strand. This is the main enzyme of replication, as the passage says.
Option (2) DNA-Dependent RNA Polymerase — Incorrect. It reads DNA but builds RNA. This is RNA polymerase, the enzyme of transcription.
Option (3) RNA-Dependent DNA Polymerase — Incorrect. It builds DNA on an RNA template. This is reverse transcriptase, used by retroviruses such as HIV.
Option (4) RNA-Dependent RNA Polymerase — Incorrect. It reads an RNA template and builds RNA. Some RNA viruses use it to copy their RNA genome; it plays no part in DNA replication.
Note 1: The passage is taken from NCERT. NTA has put lines of the NCERT text together to make it. Most CUET (UG) questions come straight from NCERT, so study the book line by line, including figures and their titles.
Note 2: It seems that there is a mistake in the original NTA paper, which prints option (4) the same as option (3), ‘RNA-Dependent DNA Polymerase’. Option (4) has been changed to ‘RNA-Dependent RNA Polymerase’ for the benefit of students.