DNA Replication Sequence in E. coli

DNA replication involves several critical steps orchestrated by specific enzymes. Understanding the correct order is essential for comprehending the process in organisms like E. coli.

Correct Sequence of DNA Replication Steps

The ordered sequence of events in DNA replication in E. coli is as follows:

1. Helicase Unwinds DNA Helix

   Step B: The replication process initiates with the unwinding of the DNA double helix. Enzymes like Helicase break the hydrogen bonds between base pairs. Topoisomerase (e.g., DNA Gyrase) manages the resulting supercoiling.

2. RNA Primer Synthesis

   Step D: DNA polymerases require a starting point (a 3'-OH group) to synthesize new DNA. The enzyme Primase, a type of RNA polymerase, synthesizes a short RNA primer complementary to the template DNA strand.

3. Okazaki Fragment Formation

   Step A: DNA synthesis proceeds using DNA polymerase. On the lagging strand, replication is discontinuous due to the 5' to 3' directionality of DNA polymerase. This results in the formation of short DNA segments called Okazaki fragments, typically 1000-2000 nucleotides long in prokaryotes.

4. Primer Removal and DNA Gap Filling

   Step C: After the Okazaki fragments are synthesized, the RNA primers must be replaced with DNA. Enzymes such as RNase H or the 5' to 3' exonuclease activity of DNA Polymerase I remove the RNA primers. DNA Polymerase I then fills the gaps with DNA nucleotides.

5. Fragment Joining by DNA Ligase

   Step E: The final step involves sealing the nicks between the newly synthesized DNA fragments. The enzyme DNA Ligase catalyzes the formation of phosphodiester bonds, joining the fragments into a continuous DNA strand.

Therefore, the correct sequence is B, D, A, C, E.