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.
Which of the following statements are correct?
A. DNA dependent DNA polymerase is a slow catalyst
B. Deoxyribonucleoside triphosphate provide energy for polymerisation
C. Replication fork is a small opening of the DNA helix
D. The replication is discontinuous on DNA template with polarity 3′→5′
E. The replication is continuous on DNA template with polarity 3′→5′
Choose the correct answer from the options given below;
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → C and E only
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.
A. DNA dependent DNA polymerase is a slow catalyst — False. In E. coli it adds about 2000 bp per second. It is a very fast enzyme.
B. Deoxyribonucleoside triphosphate provide energy for polymerisation — True. Each dNTP is both a building block and the energy source. Its two end phosphates are high-energy bonds, as in ATP.
C. Replication fork is a small opening of the DNA helix — True. The two strands of a long DNA cannot be opened along the whole length, because that needs too much energy. Replication goes on in a small opening called the replication fork.
D. The replication is discontinuous on DNA template with polarity 3′→5′ — False. The polymerase builds only in the 5′→3′ direction. On the 3′→5′ template the new strand grows towards the fork, so it is made in one piece. The discontinuous strand is made on the 5′→3′ template, and its pieces are joined by DNA ligase.
E. The replication is continuous on DNA template with polarity 3′→5′ — True. This is the same rule from the other side: on the 3′→5′ template replication is continuous.
Note 2: Statements B, C and E are all correct, and no option lists all three. So Option (2) B and C only is also correct, as both its statements are true.
NCERT Extract:
The Machinery and the Enzymes – “… In living cells, such as E. coli, the process of replication requires a set of catalysts (enzymes). The main enzyme is referred to as DNA-dependent DNA polymerase, since it uses a DNA template to catalyse the polymerisation of deoxynucleotides. These enzymes are highly efficient enzymes as they have to catalyse polymerisation of a large number of nucleotides in a very short time. E. coli that has only 4.6 × 106 bp (compare it with human whose diploid content is 6.6 × 109 bp), completes the process of replication within 18 minutes; that means the average rate of polymerisation has to be approximately 2000 bp per second. Not only do these polymerases have to be fast, but they also have to catalyse the reaction with high degree of accuracy. Any mistake during replication would result into mutations. Furthermore, energetically replication is a very expensive process. Deoxyribonucleoside triphosphates serve dual purposes. In addition to acting as substrates, they provide energy for polymerisation reaction (the two terminal phosphates in a deoxynucleoside triphosphates are high-energy phosphates, same as in case of ATP).”