The Replication Fork

DNA replication requires not only an enzymatic mechanism for adding nucleotides to the growing chains but also a means of unwinding the parental double helix. These are distinct processes, and the unwinding of the helix is closely related to the initiation of synthesis of precursor fragments. 

The pol III holoenzyme cannot unwind the helix. In order for unwinding to occur, hydrogen bonds and 

Inhibitors of DNA synthesis are used in the laboratory and in treatment of bacterial, viral, and neoplastic diseases. Successful treatment of these conditions requires careful attention to dosage and the fine difference between drug effectiveness and toxicity. 

Inhibitors of DNA synthesis can be divided into three main classes  

Those that prevent or reduce the synthesis of precursors (bases, nucleotides), 

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Lagging strand (Section 28.10) In DNA replication, the strand that grows away from the replication fork. 

Figure 36-17. The generation of "replication bubbles" during the process of DNA synthesis. The bidirectional replication and the proposed positions of unwinding proteins at the replication forks are depicted.

DNA synthesis occurs in both directions at each of the rep-licating forks. Once a DNA strand has been primed, synthesis toward the replicating fork can be visualized as continuous. Growth of the opposite,... 

CPTs kill cells in S-phase by the mechanism involving the replication fork collision model. It is known that reversible TOPO-1 CPT DNA cleavable complexes are in themselves nonlethal and that collisions with an advancing replication fork cause cell death (8). Several biochemical events follow collision with the replication fork the formation of double-strand breaks, irreversible arrest of the replication fork, and the formation of TOPO-1-linked DNA break at the site of collision. These mechanisms account for the S-phase-specific cytotoxicity seen at relatively low doses of camptothecin. 

Once the replication fork is established, other proteins begin assembling the functional DNA replication complex. 

C. Topoisomerases are responsible for relieving supercoils in the dsDNA that occur by twisting and fold-back as the DNA is unwound ahead of the replication fork. 

If supercoils or superhelices were not removed, they would eventually block movement of the replication fork by preventing further DNA unwinding. 

Slipped mispairing at the replication fork can cause repeated copying of some sequences within the tract and thus lead to expansion of trinucleotide repeat (TNR) tracts at the 5 ends of certain genes. 

Elongation The elongation phase of replication includes two distinct but related operations leading strand synthesis and lagging strand synthesis. Several enzymes at the replication fork are important to the synthesis of both strands. Parent DNA is first unwound by DNA helicases, and the resulting topological stress is relieved by topo-isomerases. Each separated strand is then stabilized by... 

Yet another polymerase, DNA polymerase e, replaces DNA polymerase S in some situations, such as in DNA repair. DNA polymerase e may also function at the replication fork, perhaps playing a role analogous to that of the bacterial DNA polymerase I, removing the primers of Okazaki fragments on the lagging strand. 

DNA is synthesized in the 5 —>3 direction by DNA polymerases. At the replication fork, the leading strand is synthesized continuously in the same direction as replication fork movement the lagging strand is synthesized discontinuously as Okazaki fragments, which are subsequently ligated. 

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