Biology · MU Sofia

9. DNA replication.

Covers the principles of DNA replication, starting with why the double helix suggests a copying mechanism and the semi-conservative model confirmed by the Meselson-Stahl experiment. It then walks through the mechanism and 5' to 3' directionality, followed by initiation, elongation and termination, including leading and lagging strands with Okazaki fragments. Three alternative modes (theta, rolling circle and D-loop) are described, and a table lists the key enzymes and proteins with their functions. The note closes with chromatin behaviour, the end replication problem and telomerase, and gives a mnemonic for the enzymes.

Inside: 8 sections · 6 labelled pictures · 1 table · 20 flashcards · 4 min

Topic 10 of 105 in the MU Sofia syllabus for Biology (Medicine, Year 1). The free AceMed note covers it in 8 sections, about 527 words (4 min of reading), with 20 flashcards.

In this note

  1. DNA replication.
  2. Mechanism:
  3. Directionality of Synthesis:
  4. Phases of Replication:
  5. Modes of Replication:
  6. Key Enzymes and Proteins:
  7. Replication and Chromatin:
  8. End Replication Problem (Linear DNA):

DNA replication.

  • DNA replication is essential for life, allowing organisms to reproduce at molecular, cellular, and organismal levels.
  • It involves template-based synthesis, where the complementary nature of nucleotides enables accurate copying.
  • The double helix structure of DNA naturally suggests a copying mechanism.
  • Replication is semi-conservative—each new double helix has one old (template) strand and one new strand.
  • The Meselson-Stahl experiment (1958) confirmed this model using isotopic labeling of nitrogen in bacterial DNA.

Mechanism:

  1. 1. Strands separate at the origin.
  2. 2. Free nucleotides spontaneously base-pair with exposed bases.
  3. 3. DNA polymerase forms phosphodiester bonds, linking nucleotides into a new strand.
  4. 4. Result: two identical DNA molecules.

Directionality of Synthesis:

  • DNA is synthesized 5’ to 3’, using triphosphate nucleotides.
  • The template is read 3’ to 5’.
  • DNA polymerase adds nucleotides by attaching the new base to the 3’ OH of the growing strand, releasing pyrophosphate.

Phases of Replication:

1. Initiation:

  • Begins at replication origins (ori)—usually AT-rich palindromic sequences.
  • Prokaryotes: usually one ori. Eukaryotes: multiple origins per chromosome.
  • DNA polymerase cannot start a new strand due to its proofreading mechanism.
  • Primase, an RNA polymerase, synthesizes short RNA primers (~10 nt) to initiate synthesis.
  • Eukaryotic primase is part of a complex with DNA polymerase alpha, which extends the primer to ~40 nt (RNA + DNA).
  • The origin is first recognized by a replication initiator protein (e.g., ORC in eukaryotes), which recruits:
  • Helicase (unwinds DNA),
  • Primase and polymerase,
  • Forming a replisome (functional replication unit).
  • Helicase is loaded by a helicase loader and becomes part of both the pre-replicative complex and the replisome.

2. Elongation:

  • DNA synthesis occurs at the replication fork, a Y-shaped structure.
  • Leading strand: synthesized continuously toward the fork.
  • Lagging strand: synthesized discontinuously away from the fork in Okazaki fragments (1000–2000 nt in prokaryotes, 100–200 nt in eukaryotes).

3. Termination:Involves primer removal, fragment joining, and resolving end replication problems (for linear DNA).

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