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DNA Replication: Mechanisms and Enzymes in Prokaryotes and Eukaryotes

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DNA Replication

Overview of DNA Replication

DNA replication is a fundamental process that ensures genetic information is accurately transmitted during cell division. It occurs in all living organisms and is essential for growth, development, and reproduction.

  • Semiconservative replication: Each daughter DNA molecule consists of one parental strand and one newly synthesized strand.

  • Accuracy and completeness: DNA must replicate exactly once per cell cycle to maintain genome integrity.

  • Genome size variation: Organisms have widely varying genome sizes, from viruses to mammals.

Key Terms and Definitions

  • Origin of replication (ori): Specific DNA sequence where replication begins.

  • Replication bubble: Region where the DNA double helix is unwound and replication occurs.

  • Replication fork: Y-shaped structure formed during DNA replication where new strands are synthesized.

  • Consensus sequence: The most common sequence found at a particular functional site, such as the origin of replication.

Mechanism of DNA Replication

Initiation of Replication

Replication begins at the origin of replication, where specific proteins recognize and bind to consensus sequences to start the process.

  • Prokaryotes: Typically have a single origin of replication (e.g., oriC in E. coli).

  • Eukaryotes: Have multiple origins of replication per chromosome.

  • Initiator proteins (e.g., DnaA): Bind to the origin and facilitate unwinding of DNA.

  • Helicase (e.g., DnaB): Unwinds the DNA helix, extending the replication bubble.

  • Single-stranded binding proteins (SSB): Stabilize separated DNA strands and prevent reannealing.

Elongation Phase

During elongation, new DNA strands are synthesized by DNA polymerases using the parental strands as templates.

  • Primase (DnaG): Synthesizes short RNA primers required for DNA polymerase to begin synthesis.

  • DNA polymerase III: Main enzyme for DNA synthesis in prokaryotes; extends primers to form new DNA strands in the 5' to 3' direction.

  • Leading strand: Synthesized continuously toward the replication fork.

  • Lagging strand: Synthesized discontinuously away from the fork in short segments called Okazaki fragments.

Termination

Replication ends when the entire DNA molecule has been copied. Specific termination sequences and proteins may be involved, especially in prokaryotes.

DNA Polymerases and Their Functions

Types of DNA Polymerases in Prokaryotes

Prokaryotes have several DNA polymerases, each with distinct roles in replication and repair.

Polymerase

Structure

Exonuclease Activity

Function

DNA pol I

1 polypeptide

5'→3' (RNA), 3'→5' (DNA)

Primer removal during replication, DNA repair

DNA pol II

1 polypeptide

3'→5' (DNA)

DNA repair

DNA pol III

Many polypeptides

3'→5' (DNA, proofreading)

DNA replication

DNA pol IV

1 polypeptide

no

DNA repair

DNA pol V

1 polypeptide

no

DNA repair

Enzymatic Activities of DNA Polymerases

  • 5'→3' polymerase activity: Adds nucleotides to the 3' end of a growing DNA strand.

  • 3'→5' exonuclease activity: Proofreads and removes incorrectly paired nucleotides (increases fidelity).

  • 5'→3' exonuclease activity: Removes RNA primers and repairs DNA.

Substrates for DNA Replication

  • dNTPs (deoxyribonucleoside triphosphates): Building blocks for DNA synthesis.

  • Primed template: DNA strand with an RNA primer to provide a free 3' hydroxyl group for extension.

DNA Synthesis Reaction

DNA polymerases catalyze the formation of phosphodiester bonds between the 3' OH of the growing strand and the 5' phosphate of the incoming nucleotide.

  • Reaction:

  • Beta and gamma phosphates are released as pyrophosphate ().

Initiation of DNA Replication in Prokaryotes

Origin of Replication (oriC in E. coli)

The oriC site is approximately 245 base pairs and contains specific sequences recognized by initiator proteins.

  • 13-mer AT-rich region: Site where DNA strands separate during replication.

  • 9-mer recognition site: Site for initiator protein binding.

Bacterial Origin-of-Replication Consensus Sequences

Species

9-mer Sequence

Escherichia coli

TTATCCACA

Salmonella typhimurium

TTATCCACA

Proteus vulgaris

TTATCCACA

Vibrio cholerae

TTATCCACA

Consensus sequence

TTATCCACA

Key Proteins in Initiation

  • DnaA: Binds to 9-mer sequences, initiates strand separation at 13-mer AT-rich region.

  • DnaB (helicase): Unwinds DNA, extends replication bubble.

  • DnaC: Helicase loader, helps DnaB bind to DNA.

  • SSB (single-stranded binding protein): Stabilizes single-stranded DNA, prevents reannealing.

Primer Synthesis and Extension

  • Primase (DnaG): Synthesizes short RNA primers on single-stranded DNA.

  • DNA polymerase III: Extends RNA primers to synthesize new DNA strands.

Additional Info

  • DNA polymerases cannot initiate synthesis de novo; they require a primer with a free 3' OH group.

  • RNA polymerases (primase) can initiate synthesis without a primer.

  • Okazaki fragments are short DNA segments synthesized on the lagging strand.

  • DNA pol I removes RNA primers and fills gaps with DNA.

  • Replication is bidirectional from the origin, forming two replication forks.

Example: DNA Replication in E. coli

  • Initiation at oriC by DnaA binding.

  • Helicase (DnaB) unwinds DNA, SSB stabilizes single strands.

  • Primase synthesizes RNA primers, DNA pol III extends primers.

  • DNA pol I removes primers, fills gaps; DNA ligase seals nicks.

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