DNA replication

DNA Replication Process

  1. Purpose:

    • Ensure each daughter cell receives a complete copy of genetic information before cell division.

  2. Models of Replication:

    • Semi-conservative model confirmed by Meselson and Stahl experiment.

    • Each new DNA strand is half old (parent template) and half new.

  3. Anti-parallel Strands:

    • DNA strands run in opposite directions, creating a challenge for replication.

  4. Bonding in DNA:

    • Covalent phosphodiester bonds and hydrogen bonds are key for DNA structure and replication.

Enzymes and Steps in DNA Replication

  1. Unwinding DNA:

    • Helicase: Unwinds the DNA helix.

    • Single-stranded binding proteins: Stabilize the unwound DNA.

    • DNA gyrase: Prevents tangling of the DNA.

  2. RNA Primase:

    • Adds RNA primers to the template DNA, allowing DNA polymerase III to start building the new strand.

  3. Building New DNA Strand:

    • DNA Polymerase III: Adds new complementary bases to the growing DNA strand.

  4. Replacing RNA Primers:

    • DNA Polymerase I: Replaces RNA primers with DNA nucleotides.

    • DNA Ligase: Joins Okazaki fragments on the lagging strand, sealing nicks in the DNA backbone.

Replication Fork Dynamics

  • Leading Strand:

    • Synthesized continuously in the 5' to 3' direction.

  • Lagging Strand:

    • Synthesized discontinuously, creating Okazaki fragments, which are joined by DNA ligase.