11.2 DNA Replication - Microbiology | OpenStax

Learning Objectives

• Explain semiconservative DNA replication.
• Discuss bidirectional DNA replication with leading and lagging strands.
• Explain the formation of Okazaki fragments.
• Describe the DNA replication process and enzyme functions.
• Identify differences between DNA replication in bacteria and eukaryotes.
• Explain rolling circle replication.

Key Concepts

Semiconservative DNA Replication

• Double helix structure by Watson and Crick (1953) implies how DNA is copied.
• Semiconservative model suggests each strand acts as a template.
• Competing models: conservative, dispersive.
• Meselson and Stahl's experiment supported semiconservative replication:
  • Used E. coli with 15N and 14N to track DNA replication.
  • Proved each DNA molecule includes one parental and one new strand.

DNA Replication in Bacteria

• Studied in E. coli with 4.6 Mbp circular chromosome.
• Process is rapid (1000 nucleotides/second) with minimal errors.
• Involves numerous proteins and enzymes:
  • DNA polymerases (pol I, II, III): major player is DNA pol III.
  • Helicase, Ligase, Primase: key enzymes in replication process.
  • Single-stranded binding proteins, sliding clamp: stabilize replication process.
  • Topoisomerase II (DNA gyrase): relaxes supercoiling.
  • Topoisomerase IV: resolves concatenated chromosomes.

Initiation

• Begins at origin of replication (oriC in bacteria), rich in AT sequences.
• Involves binding proteins, topoisomerase, helicase.

Elongation

• DNA pol III synthesizes in 5' to 3' direction.
• Leading strand: continuous synthesis.
• Lagging strand: Okazaki fragments, discontinuous.
• Sliding clamp holds polymerase in place.
• DNA pol I replaces RNA primers; DNA ligase seals gaps.

Termination

• Completion of chromosome ends replication.
• Bacterial topoisomerase IV resolves concatenated DNA.

DNA Replication in Eukaryotes

• Larger, more complex genomes with multiple linear chromosomes.
• Multiple origins of replication.
• Rate: 100 nucleotides/second.
• Essential steps similar to prokaryotes.
• Different polymerases (pol α, pol δ) for leading and lagging strands.
• Telomeres with telomerase maintain chromosomal ends.

Rolling Circle Replication

• Used by plasmids and some viruses.
• Involves enzymatic nicking and unidirectional replication.
• Replication at the double-stranded origin (dso) site.
• Displaces a single strand, which can recircularize.

Check Your Understanding

• Meselson and Stahl's experiment: If two bands were found after the first generation, it might suggest a different replication model.
• Enzymes involved:
  • Helicase: unwinds DNA.
  • DNA ligase: seals gaps in lagging strand.
  • RNA primer removal in bacteria by DNA pol I.
• Differences in origins:
  • Prokaryotes: single origin.
  • Eukaryotes: multiple origins.
• Telomeres and telomerase: Found at chromosome ends to prevent degradation.