DNA Replication: The Semi-Conservative Model

The Semi-Conservative Model

  • DNA replication produces two double-stranded DNA molecules, each containing one original (parental) strand and one newly synthesized strand.
  • This mechanism ensures the accurate transfer of genetic information across cell generations.

Structural Arrangement

  • DNA is an anti-parallel double helix where the two strands run in opposite directions (5′5' to 3′3' and 3′3' to 5′5').
  • The strands consist of a deoxyribose-phosphate backbone and nitrogenous bases (AdenineAdenine, ThymineThymine, GuanineGuanine, and CytosineCytosine).

Timing and Cell Cycle

  • Replication occurs exclusively during the Synthesis phase, or "S phase," of the cell cycle.
  • The genome must be completely duplicated before the cell enters mitosis (MphaseM phase).

Key Enzymes and Unwinding

  • Origins of Replication: Specific chromosomal locations where replication starts.
  • Helicase: Unwinds the double helix by breaking hydrogen bonds between base pairs (A−TA-T and G−CG-C), forming a "replication fork."
  • Single-Strand Binding (SSB) proteins: Stabilize the separated DNA strands to prevent re-joining or degradation.
  • Topoisomerase: Relieves physical tension and supercoiling ahead of the replication fork by making temporary cuts in the backbone.

Synthesis Mechanism

  • Primase: Creates a short RNA primer to provide a free 3′−OH3'-OH group, which is required for synthesis to begin.
  • DNA Polymerase: Adds nucleotides complementary to the template strand; it only synthesizes in the 5′5' to 3′3' direction.
  • Leading Strand: Synthesized continuously toward the replication fork.
  • Lagging Strand: Synthesized discontinuously away from the fork in short segments called Okazaki fragments.
  • DNA Ligase: Joins the disconnected Okazaki fragments together by forming covalent bonds.

Accuracy and Proofreading

  • DNA Polymerase possesses a 3′3' to 5′5' exonuclease activity that allows it to detect, remove, and correct mismatched nucleotides.

Telomeres and Aging

  • Telomeres: Caps at the ends of linear human chromosomes consisting of repetitive, non-coding sequences (TTAGGGTTAGGG).
  • End Replication Problem: A small portion of DNA is lost during each division because the final gap on the lagging strand cannot be filled.
  • Senescence: Cell division stops once telomeres become critically short.
  • Telomerase: An enzyme that can add lost sequences back to telomeres; it is typically inactive in normal adult cells but highly active in cancer cells.

Clinical Relevance

  • Acyclovir: An antiviral drug for Herpes Simplex Virus that mimics a nucleotide but lacks a 3′−OH3'-OH group, resulting in "chain termination" and stopping viral replication.