The replisome is a set of enzymes, and enzyme kinetics are set by their environment. Temperature controls the fork rate two ways at once: below the ~37 °C optimum, catalysis simply slows (a roughly two-fold rate drop for every 10 °C, the enzyme's Q10), while above it the polymerase's own protein fold starts to denature, and above ~42–45 °C it collapses and the fork stalls outright — the same reason a 39–40 °C fever measurably slows cell division and a sustained 42 °C+ is lethal to human cells.
Mg²⁺ is not just a helper — it is the catalytic cofactor DNA polymerase uses to position the incoming dNTP and the primer's 3'-OH for the bond-forming reaction, so too little Mg²⁺ (below ~0.5 mM) starves catalysis and the fork slows. But more is not better: physiological free Mg²⁺ sits near 1–2 mM for a reason — excess Mg²⁺ stabilizes mismatched (non-Watson-Crick) base pairs in the active site and also inhibits the polymerase's built-in 3'→5' exonuclease proofreading domain, so error rate rises sharply once Mg²⁺ climbs past a few mM even though the fork itself may run faster.
Baseline polymerase fidelity (no proofreading) is about one error per 10⁴–10⁵ bases; proofreading exonuclease activity cuts that another 100-fold to roughly one in 10⁶–10⁷, and mismatch repair after replication tightens it further to near one in 10⁹ — this simulation shows only the polymerase + proofreading stage. The mutagen-pulse button forces a mismatched base into the fork; whether it gets excised or becomes a fixed mutation depends entirely on how favorable the current temperature and Mg²⁺ are for proofreading at that instant.
fork rate = base_rate x Q10_factor(T) x denaturation_factor(T)
error rate = baseline x mismatch_stabilization(Mg) / proofreading_efficiency(T, Mg)
optimum: T ~ 37C, Mg2+ ~ 1-2 mM -> fastest fork AND lowest error rate together