F

Thymidylate dUMP synthetase dTDP

dTMP

Tetrahydrofolate ilate

(THF) ^ Dihydrofolate (DHF)

Dihydrofolate reductase

DNA Polymerase Elongates DNA Strands 111

FIGURE 5.11 DNA Polymerase III—Sliding Clamp

The sliding clamp, made of two similar ß subunits, encircles the DNA helix. It is loaded onto the DNA with help from the clamp-loading complex.

DNA Polymerase Elongates DNA Strands 111

FIGURE 5.11 DNA Polymerase III—Sliding Clamp

The sliding clamp, made of two similar ß subunits, encircles the DNA helix. It is loaded onto the DNA with help from the clamp-loading complex.

DNA polymerase is kept attached to the DNA by a doughnut shaped protein ring that slides up and down the DNA strand.

DNA polymerase not only synthesizes new DNA, but also checks for mismatched base pairs and corrects any errors it has made.

DNA Polymerase Elongates DNA Strands

DNA polymerase III (Pol III) is the major enzyme involved in elongating DNA during chromosome replication. DNA polymerase III has several components. First, the "sliding clamp" is shaped like a doughnut and is made from two semicircular subunits of DnaN protein. It slides up and down like a curtain ring on the template strand of DNA (Fig. 5.11) and requires several accessory proteins (HolA, B, C and D plus the g subunit) known as the clamp-loading complex to load the sliding clamp onto the DNA—a process requiring energy.

The "sliding clamp" binds the very large and complex core enzyme to the DNA. The core enzyme synthesizes DNA and consists of three subunits, DnaE (a-subunit; polymerization), DnaQ (e-subunit;proofreading) and HolE (8-subunit; function uncertain). Two core assemblies, one to make each new strand of DNA, are held together by a pair of tau subunits. A single clamp loader complex is shared by the two core assemblies (Fig. 5.12).

Although hydrogen bonding alone would match bases correctly, the great majority of the time this is not accurate enough for replication of the genome. Consequently, many DNA polymerases possess kinetic proofreading ability (Fig. 5.13). This refers to clamp-loading complex Group of proteins that loads the sliding clamp of DNA polymerase onto the DNA

core enzyme The part of DNA or RNA polymerase that synthesizes new DNA or RNA (i.e. lacking the recognition and/or attachment subunits) DNA polymerase III (Pol III) Enzyme that makes most of the DNA when bacterial chromosomes are replicated kinetic proofreading Proofreading of DNA that occurs during the process of DNA synthesis sliding clamp Subunit of DNA polymerase that encircles the DNA, thereby holding the core enzyme onto the DNA

FIGURE 5.12 DNA Polymerase III—Assembly of Subunits

A single core subunit is shown above and its assembly into a dimeric unit is shown below. The dimeric subunit contains only one clamp loader, which is associated with the lagging strand synthetic unit. The two core proteins are bound together by the tau subunit. The components of DNA polymerase III are the b subunits forming the sliding clamp, the epsilon and theta subunits and the clamp loader. When the DNA polymerase assembles, tau subunits join two nearly identical complexes together.

Sliding clamp

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