DNA topology during replication

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Figure 5.22: An assay for direction of helicase tracking along a single stranded region.

Figure 5.22B shows the results of the tracking assay for a helicase called PriA. In what direction does it track along the single-stranded DNA?

Figure 5.23. Changes in topology of the DNA during replication. The ATP-dependent untwisting (negative DT) catalyzed by DNA helicases causes a compensatory change in writhing (positive DW), which is relieved by the action of a topoisomerase II, such as DNA gyrase. Gyrase catalyzes an ATP-dependent, three-step reaction, cleaving the two strands of DNA, passing another part of the DNA duplex through the break and re-sealing the break. The action of gyrase generates a negative DW to balance the positive DW from the action of helicase. X and Y mark two different regions of the DNA molecule. A gray arrow indicates the direction of duplex movement through the break. Gyrase is a tetramer, and is shown as four pink balls.