DNA Replication

• The Basics of DNA Structure

  • Double-helix structure
  • Two complementary strands
  • Nucleotides: Adenine (A), Thymine (T), Cytosine (C), and Guanine (G)
  • Base pairing: A-T and C-G
• The Replication Process


  1. Initiation
  2. Elongation
  3. Termination
• Initiation


  DNA replication begins at specific sites called origins of replication. Here, the enzyme helicase unwinds the DNA double helix, creating a replication fork.
• 1. Elongation


  As the DNA unwinds, several key players come into action:

  • Primase: Synthesizes short RNA primers
  • DNA polymerase III: The main replication enzyme, adds nucleotides to the growing strand
  • DNA polymerase I: Replaces RNA primers with DNA
  • Ligase: Joins Okazaki fragments on the lagging strand
• Termination


  Replication continues until the entire chromosome is duplicated. The process ends when replication forks meet.
• Key Concepts of DNA Replications

  • Semiconservative replication: Each new DNA molecule contains one original strand and one new strand.
  • Leading strand: Synthesized continuously in the 5' to 3' direction.
  • Lagging strand: Synthesized discontinuously in short fragments (Okazaki fragments) in the 5' to 3' direction.
• Producing Exact Copies

  DNA replication is the process of creating two identical replicas of DNA from one original DNA molecule. This is crucial for:

  1. Reproduction: Ensuring genetic information is passed accurately to offspring.
  2. Growth: Allowing new cells to have the same genetic information as the parent cell.
  3. Tissue Replacement: Enabling the replacement of old or damaged cells with genetically identical new ones.
• Semi-Conservative Replication:


  • Each new DNA molecule contains one original strand and one newly synthesized strand.
  • This was proven by the Meselson-Stahl experiment in 1958.
• Complementary Base Pairing:


  • Adenine (A) always pairs with Thymine (T)
  • Cytosine (C) always pairs with Guanine (G)
• Template-Driven Synthesis: 


  The existing strand acts as a template, ensuring the correct bases are added to the new strand.
• Error Checking: 


  DNA polymerase can "proofread" the newly added bases, removing and replacing any incorrect ones.
• Hydrogen Bonding: 


  The specific hydrogen bonds between complementary bases (two for A-T, three for C-G) ensure correct pairing.
• The high fidelity of DNA replication is crucial for genetic stability. However, the rare errors that do occur can lead to mutations, which are the basis for genetic variation and evolution.
• Helicase


  Helicase plays a crucial role in the initiation and progression of DNA replication:

  1. Unwinding: Helicase separates the two strands of the DNA double helix.
  2. Breaking Hydrogen Bonds: It breaks the hydrogen bonds between complementary base pairs.
• DNA Polymerase


  DNA polymerase is the main enzyme responsible for synthesizing new DNA strands:

  1. Nucleotide Addition: It adds nucleotides to the growing DNA strand in the 5' to 3' direction.
  2. Proofreading: It checks for and corrects errors in the newly synthesized strand.