Eukaryotic & Prokaryotic DNA Replication Machinery
| Enzyme / Protein | Primary Physiological Function | Key Clinical / Pharmacological Target |
|---|---|---|
| Helicase (DnaB in prokaryotes) |
Unwinds the DNA double helix at the replication fork by breaking hydrogen bonds. Requires ATP. Mutant **BLM gene** causes **Bloom Syndrome** (defective helicase; presents with growth retardation, facial erythema, immunodeficiency). | Defects lead to extreme chromosomal instability and sister chromatid exchange. |
| Topoisomerases (Gyrase in prokaryotes) |
Creates transient single (Type I) or double (Type II) strand breaks to alleviate positive supercoiling ahead of the replication fork. | Fluoroquinolones (inhibit bacterial Gyrase/Topo IV). Etoposide/Teniposide (inhibits eukaryotic Topo II). Irinotecan/Topotecan (inhibits eukaryotic Topo I). |
| DNA Polymerase III (Prokaryotic) |
Primary prokaryotic replicative enzyme. Elongates the leading strand continuously and the lagging strand discontinuously (**Okazaki fragments**). Synthesis occurs **5′ → 3’**. Possesses **3′ → 5′ exonuclease activity** for proofreading. | High fidelity engine. Loss of 3′ → 5′ proofreading leads to a massive accumulation of replication mutations. |
| DNA Polymerase I (Prokaryotic) |
Excises RNA primers synthesized by Primase and replaces them with DNA. Unique among prokaryotic polymerases for possessing **5′ → 3′ exonuclease activity** to systematically degrade the forward RNA primer. | Essential for Okazaki fragment maturation before ligation. |
| DNA Ligase | Catalyzes the formation of a **phosphodiester bond** between the 3′ hydroxyl group of one DNA fragment and the 5′ phosphate group of an adjacent fragment. Seals the nicks between Okazaki fragments. | Requires ATP (eukaryotes) or NAD+ (prokaryotes) to drive bond condensation. |
| Telomerase (Eukaryotic) |
An **RNA-dependent DNA polymerase** (Reverse Transcriptase). Contains an intrinsic RNA template sequence (3′-AAUCCC-5′) used to synthesize TTAGGG hexanucleotide repeats at the 3′ ends of eukaryotic chromosomes, preventing end-replication shortening. | Upregulated in **malignant tumor cells**, conferring replicative immortality. Absent or restricted in normal somatic tissue, leading to cellular senescence. |
High-Yield Core Realities:
- The Absolute Directionality Constraint: All DNA polymerases synthetically read the parental template strand in the 3′ → 5′ direction and construct the new nascent strand exclusively in the 5′ → 3′ direction. The requirement for a free 3′-OH group means DNA synthesis cannot initiate de novo; it requires an RNA primer laid down by Primase (RNA Polymerase).
- Eukaryotic Polymerase Equivalences:
- Pol α: Contains intrinsic primase activity; synthesizes the initial RNA-DNA hybrid primer.
- Pol δ: Replicates the lagging strand; possesses 3′ → 5′ proofreading exonuclease activity.
- Pol ε: Replicates the leading strand; highly processive with 3′ → 5′ proofreading capabilities.
- Pol γ: Replicates and proofreads **mitochondrial DNA**. Can be inhibited by Nucleoside Reverse Transcriptase Inhibitors (NRTIs) like zidovudine, inducing clinical mitochondrial myopathies.
- Prokaryotic Origin of Replication (oriC): Replication begins at a singular, specific sequence rich in A-T base pairs (three 13-bp repeats and four 9-bp repeats). The lower thermodynamic stability of two hydrogen bonds in A-T pairs (versus three bonds in G-C pairs) allows local strand separation and denaturation by DnaA initiator proteins. Eukaryotes utilize **multiple origins of replication** to ensure timely replication of large linear genomes.
- The Single-Stranded Stabilization (SSB / RPA): Once helicase breaks the hydrogen bonds, the newly exposed single-stranded template strands are vulnerable to re-annealing or hairpin creation. Single-Stranded Binding Proteins (SSBs) in prokaryotes and **Replication Protein A (RPA)** in eukaryotes coat the strands, stabilizing them without covering the chemical bases required for polymerase reading.
- Mismatch Repair Coupling: Replicative proofreading by the 3′ → 5′ exonuclease catches mistakes immediately. Errors that escape are recognized post-replication by mismatch repair enzymes (e.g., MutS/MutL homologs, **MSH2, MLH1**). In prokaryotes, the template strand is identified by its **methylated adenines** within GATC sequences via Dam methylase. In eukaryotes, the lagging-strand template identification relies on recognizing unsealed single-strand nicks before ligation. Defects in eukaryotic mismatch repair trigger **Lynch Syndrome (HNPCC)**, characterized by microsatellite instability.