DNA Replication Lecture Notes

Overview of DNA Structure and Replication Models

• DNA Composition: Nucleotides

  • DNA is made up of nucleotides attached to a single strand.
  • Nucleotides come together to form the structure of DNA.

• Replication Models Proposed

  • Semiconservative: The parental strand divides to make two new DNA strands; each new strand has one original and one new strand.
  • Conservative: The parental strand duplicates entirely; a new double-stranded DNA is produced, leaving the original intact.
  • Dispersive: The parental strand mixes with new strands to form two new DNA strands.

• Current Understanding: The semiconservative model is the correct method of DNA replication.

DNA Replication Process

• Initiation

  • Begins at the origin of replication on a specific gene sequence.
  • Special proteins target the origin and start unwinding the DNA.
  • The area where DNA unwinding occurs is called the replication fork.

• Directionality

  • DNA strands have directionality: one strand is 5' to 3', and the opposite is 3' to 5'.
  • DNA is synthesized in the 5' to 3' direction.

Key Proteins and Their Functions

• Helicase

  • Unwinds DNA by breaking hydrogen bonds between bases.

• Single-Stranded Binding Proteins (SSBPs)

  • Prevent hairpins formation by binding to single-stranded DNA.

• DNA Primase/RNA Primase

  • Synthesizes RNA primers to initiate DNA synthesis.

• DNA Polymerase

  • Adds nucleotides in the 5' to 3' direction.
  • DNA Polymerase III (Pol III) synthesizes new DNA strands.
  • DNA Polymerase I (Pol I) converts RNA primers to DNA.

• Okazaki Fragments

  • Short DNA fragments synthesized on the lagging strand.

• DNA Ligase

  • Seals nicks between DNA fragments, ensuring a continuous DNA strand.

Leading and Lagging Strands

• Leading Strand

  • Synthesized continuously in the 5' to 3' direction.

• Lagging Strand

  • Synthesized discontinuously in fragments (Okazaki fragments).
  • Requires complex process involving multiple proteins.

Prokaryotic vs. Eukaryotic DNA Replication

• Prokaryotic Cells

  • Helicase: Unwinds DNA.
  • Pol III: Builds DNA strands.
  • SSBPs: Prevents hairpin formation.
  • Primase: Makes RNA primers.
  • Pol I: Converts RNA primers to DNA.
  • Ligase: Seals nicks.

• Eukaryotic Cells

  • Helicase: Same function.
  • Pol Delta: Equivalent to Pol III, builds DNA.
  • Replication Factor A (RPA): Similar to SSBPs.
  • Pol Alpha: Equivalent to Primase, makes RNA primers.
  • MF1: Equivalent to Pol I, converts RNA primers to DNA.
  • Ligase: Same function.

Summary

• Replication Fork: Site of DNA unwinding and synthesis.
• Direction: New strands synthesized in 5' to 3' direction.
• Key Proteins: Helicase, SSBPs, Primase, Pol III, Pol I, Ligase.
• Leading vs. Lagging Strand: Continuous vs. discontinuous synthesis.
• Prokaryotic vs. Eukaryotic: Similar processes with different protein names.

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