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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).

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).

/ Molecular Basis of Inheritance, NCERT LINE BY LINE, pre-class / By Prof. Siddharth Sanghvi

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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.
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