DNA replication
Read the following passage carefully and answer the given questions.
In addition to DNA-dependent DNA polymerases, many additional enzymes are required to complete the process of replication with high degree of accuracy. Since the two strands of DNA cannot be separated in its entire length (due to very high energy requirement), the replication occur within a small opening of the DNA helix, referred to as replication fork. The DNA-dependent DNA polymerases 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, while on the other (the template with polarity 5′ → 3′), it is discontinuous. The discontinuously synthesised fragments are later joined by the enzyme DNA ligase. After the completion of replication, each DNA molecule would have one parental and one newly synthesised strand. This scheme was termed as semiconservative DNA replication. In eukaryotes replication of DNA takes place at S-Phase of cell cycle.
E. coli has 4.6×106 bp and completes the process of replication within 18 minutes. So the average rate of polymerisation is approximately -
Answer & explanation
Correct answer: option 3
The correct answer is Option (3) → 2000 bp per second
Note 1: The given answer is as per the official NTA Answer Key, and it is the value NCERT gives. However, a plain division of the numbers in the question gives about 4260 bp per second, which is nearer to Option (4). NCERT’s figure of about 2000 bp per second is the rate at each of the two replication forks, as explained below.
DNA polymerases have to be very fast. E. coli copies its whole DNA of 4.6 × 106 bp in only 18 minutes.
\( \displaystyle \text{Time} = 18 \times 60 = 1080\ \text{s} \)
\( \displaystyle \text{Rate for the whole DNA} = \frac{4.6 \times 10^{6}}{1080} \approx 4260\ \text{bp per second} \)
But the DNA of E. coli is not copied from one end only. Replication starts at a single origin and moves out in both directions, so two replication forks work at the same time. Each fork copies half of the DNA.
\( \displaystyle \text{Rate at each fork} = \frac{4260}{2} \approx 2130 \approx 2000\ \text{bp per second} \)
So the average rate of polymerisation by the enzyme is about 2000 bp per second, as NCERT states.
Note 2: 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.
NCERT Extract:
The Machinery and the Enzymes – “… 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.”