Question: 135. Which of the following statement is correct regarding the process of replication in E.coli?
(1) The DNA dependent DNA polymerase catalyses polymerization in one direction that is 3^{\prime} \rightarrow 5^{\prime}
(2) The DNA dependent RNA polymerase catalyses polymerization in one direction, that is 5^{\prime} \rightarrow 3^{\prime}
(3) The DNA dependent DNA polymerase catalyses polymerization in 5^{\prime} \rightarrow 3^{\prime} as well as 3^{\prime} \rightarrow 5^{\prime} direction
(4) The DNA dependent DNA polymerase catalyses polymerization in 5^{\prime} \rightarrow 3^{\prime} direction.
Answer: Option (4)
Explanation:
During DNA replication in E. coli, the enzyme DNA dependent DNA polymerase is responsible for the synthesis of the new DNA strand.
DNA polymerase can add nucleotides only to the free 3^{\prime}-OH end of the growing DNA strand.
Therefore, synthesis of DNA always proceeds in the 5^{\prime} \rightarrow 3^{\prime} direction.
The enzyme cannot catalyse polymerization in the 3^{\prime} \rightarrow 5^{\prime} direction, making option (1) incorrect.
Option (2) refers to DNA dependent RNA polymerase, which is involved in transcription, not replication,
so it is not correct in this context.
Option (3) is incorrect because DNA polymerase cannot synthesize DNA in both directions.
Hence, the correct statement is that DNA dependent DNA polymerase catalyses polymerization only in the 5^{\prime} \rightarrow 3^{\prime} direction.