DNA Replication – The Machinery and the Enzymes -NCERT-NEET-‘‘It has not escaped our notice that the specific pairing we have postulated immediately suggests a possible copying mechanism for the genetic material’’(Watson and Crick, 1953).

Step	Enzymes Involved	Details	Prokaryotic vs Eukaryotic Differences
Initiation	Helicase, Primase	Helicase unwinds the DNA helix at the origin of replication. Primase synthesizes RNA primers to provide a starting point for DNA synthesis. Replication occurs at the replication fork, a small opening in the DNA helix.	Prokaryotes: Single origin of replication. Eukaryotes: Multiple origins of replication.
Elongation	DNA Polymerase III (prokaryotes), DNA Polymerase δ and ε (eukaryotes)	DNA polymerase synthesizes the new DNA strand by adding nucleotides to the 3′ end of the primer. Leading strand is synthesized continuously. Lagging strand is synthesized in short fragments called Okazaki fragments. Replication is highly accurate and efficient to prevent mutations. Deoxyribonucleoside triphosphates act as substrates and provide energy for polymerization.	Prokaryotes: DNA polymerase III is the main enzyme. Eukaryotes: DNA polymerases δ and ε are the main enzymes.
Primer Removal	DNA Polymerase I (prokaryotes)	RNA primers are removed and replaced with DNA.	Prokaryotes: DNA polymerase I removes RNA primers and fills in the gaps.
Ligase Activity	DNA Ligase	DNA ligase seals the nicks between Okazaki fragments to create a continuous DNA strand.	Prokaryotes: Single type of DNA ligase. Eukaryotes: Multiple types of DNA ligases with specialized functions.
Termination	Topoisomerase	In E. coli, the process of replication is highly efficient, with the average rate of polymerization being approximately 2000 bp per second.	Prokaryotes: Tus protein binds to termination sequences to stop replication. Eukaryotes: Termination occurs when replication forks meet, and telomerase may be involved in replicating telomere ends. Replication occurs during the S-phase of the cell cycle in eukaryotes and must be coordinated with cell division to prevent polyploidy.
Time Discrepancy	N/A	Cell Division Time (20 minutes): Time it takes for a bacterial cell to divide into two daughter cells under optimal conditions. DNA Replication Time (38 minutes): Time required to replicate the entire E. coli genome bidirectionally.	Overlapping Replication Cycles: E. coli can begin a new round of DNA replication before the previous round is complete, allowing the cell to prepare for the next division. Bidirectional Replication: DNA replication in E. coli starts at a single origin and proceeds in two directions simultaneously, splitting the replication time.