In-Depth Notes on DNA Replication

Structure of DNA Replication

  • Directionality of DNA Strands
    • DNA strands have a direction, marked as 5' (five prime) and 3' (three prime) ends.
    • The leading strand is synthesized continuously in the 5' to 3' direction.
    • The lagging strand is synthesized in small fragments called Okazaki fragments and requires multiple primers.

Leading vs. Lagging Strands

  • Leading Strand

    • Synthesized continuously starting from a single primer.
    • Can follow the unwinding of the DNA without needing to stop.

  • Lagging Strand

    • Synthesized in fragments because it runs in the opposite direction (3' to 5').
    • Requires multiple RNA primers to initiate synthesis of each fragment.
    • The fragments (Okazaki fragments) are linked later.

Okazaki Fragments

  • Definition: Short DNA fragments synthesized on the lagging strand.
  • Enzymatic Activity:
    • Each fragment requires a new RNA primer.
    • Repeatedly, DNA polymerase III synthesizes until it hits the next primer, then falls off and goes back to continue synthesis at another primer.

Key Enzymes in DNA Replication

  • DNA Polymerase III

    • Main enzyme that synthesizes new DNA strands by adding nucleotides (A, T, C, G) in a 5' to 3' direction.

  • DNA Polymerase I

    • Responsible for replacing RNA primers with DNA bases after the lagging strand fragments have been synthesized.

  • Ligase

    • Links the Okazaki fragments together by sealing gaps where RNA primers were removed.

The Role of Telomeres

  • Telomeres

    • Non-coding sequences at the ends of chromosomes that protect genes from being eroded during DNA replication.
    • Shortening of telomeres is associated with aging.

  • Telomerase

    • An enzyme that extends telomeres and is primarily active in germ cells (ovaries and testes).
    • Helps maintain the integrity of genetic information by preventing chromosome shortening during cell replication.

Summary of the Replication Process

  1. Unwinding: DNA helicase unwinds the DNA double helix.
  2. Primer Synthesis: RNA primers are laid down to provide a starting point for DNA synthesis.
  3. Synthesis:
    • Leading strand is synthesized continuously; lagging strand synthesized in fragments.
  4. Replacing Primers: DNA Polymerase I replaces RNA primers with DNA bases.
  5. Ligation: Ligase links Okazaki fragments to form a continuous strand.
  6. Completion: Ends of strands (telomeres) are maintained by telomerase, preserving genetic information.