DNA Structure and Replication

DNA Structure and Replication Theories

  • DNA Replication Models

    • Three theories on how DNA copies itself:

      • Semiconservative:

        • Proposed by Watson and Crick.

        • DNA splits in half and new strands are synthesized on each side.

      • Conservative:

        • The original DNA remains intact while making a complete copy.

      • Dispersive:

        • A mix of both old and new DNA in each strand, combining elements of conservative and semiconservative theories.

        • Histone proteins play a role in packaging the DNA.

Meselson-Stahl Experiment

  • Used two nitrogen isotopes: nitrogen-14 (light) and nitrogen-15 (heavy).

  • E. coli cultured in nitrogen-15 until all DNA was labeled.

  • Transferred to nitrogen-14 broth:

    • First generation produced DNA with an intermediate density, suggesting semiconservative replication, not conservative.

    • Further generations confirmed the findings, supporting the semiconservative model.

Understanding DNA Structure

  • Components of DNA:

    • Each nucleotide contains:

      • Sugar: Deoxyribose

      • Nitrogenous Base

      • Phosphate Group

  • Antiparallel Strands:

    • DNA strands run in opposite directions:

      • Recognized by carbon numbering in the sugar (1'-5').

DNA Replication Process

  • Role of Enzymes in DNA Replication:

    • Helicase: Unwinds the double helix into single strands.

    • Single-Strand Binding Proteins: Stabilize unwound DNA structures.

    • DNA Polymerase: Adding new nucleotides; operates only in the 3' direction.

      • Leading Strand: Continuous synthesis in the 5' to 3' direction.

      • Lagging Strand: Discontinuous synthesis, creating Okazaki fragments.

  • Lagging Strand Synthesis:

    • Requires an RNA primer initiated by RNA primase.

    • DNA Polymerase adds nucleotides after primer placement.

    • DNA Ligase: Connects fragments where RNA and DNA meet, sealing the gaps.

Origin of DNA Replication

  • Types of Cells:

    • Prokaryotic:

      • Single circular DNA loop; replication begins at one origin and proceeds bidirectionally.

    • Eukaryotic:

      • Larger, linear DNA requiring multiple origins of replication.

      • Replication proceeds in both directions from multiple sites until chromosomes are duplicated.

  • Significance of Replication Forks:

    • Areas where DNA splits and new strands are formed during the replication process.