DNA Replication in Prokaryotes vs Eukaryotes
Concept Name
DNA Replication
Genetic Loci
E. coli: dnaA, dnaB (helicase), dnaE (DNA pol III α subunit). Human: ORC1 (1p32), MCM2‑7 (various), PCNA (20p12), POLA1 (Xp22.1‑p21.3).
Intracellular Cascade
Initiation: recognition of OriC by DnaA → unwinding by DnaB helicase → priming by primase → elongation by DNA pol III (leading strand) and pol I/ligase (lagging strand). CDK/cyclin complexes regulate eukaryotic replication licensing.
Required Cofactors
Requires Mg²⁺ for polymerase activity; ATP for helicase and ligase; NAD⁺ for DNA ligase in bacteria (ATP in eukaryotes). Single‑strand binding proteins stabilize ssDNA.
Histology Stains
BrdU (bromodeoxyuridine) incorporation labels cells in S‑phase; detected by anti‑BrdU antibodies. PCNA immunohistochemistry marks proliferating cells.
EM Findings
Replication forks appear as Y‑shaped structures on electron microscopy. Replication bubbles can be visualized in early S‑phase nuclei.
Knockout Phenotype
Knockout of MCM2 in mice is early embryonic lethal (pre‑implantation). Hypomorphic mutations in ORC1 cause Meier‑Gorlin syndrome (microcephalic primordial dwarfism).
Specific Toxins
Aphidicolin inhibits eukaryotic DNA polymerases (α, δ, ε). Cytarabine (ara‑C) is a chain‑terminating nucleoside analog. Ciprofloxacin inhibits bacterial DNA gyrase (topoisomerase II).