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.