DNA pol III is the principal replication enzyme of E. coli. The enzyme is highly complex and has more than 10 types of subunits. These subunits are α, ε, θ, τ, δ, δ’, β, χ, ψ, γ. The α, ε and θ subunits combine to form the core polymerase. The core polymerase has a limited processivity. The two core polymerases are linked in a complex by a dimer of τ subunits. Two γ subunits, one δ, one δ’, one χ and one ψ subunit combine to form a single clamp-loading complex. The dimeric core polymerase and the clamp-loading complex (14 subunits of 9 types) combine to form the DNA pol III enzyme. The α and ε subunits carry out the replication and proofreading activities, respectively. DNA pol III has a low processivity, but addition of β subunits increases the processivity required to replicate the E. coli chromosome. The 4 β subunits associate with the dimeric core and the clamp-loading complex to make the DNA pol III holoenzyme. The β subunits associate in pairs to form a donut-shaped structure. This encircles the DNA and acts like a clamp. This slides along the DNA as replication proceeds and prevents the dissociation of DNA pol III from DNA.