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tions, but this is known not to occur. How, then, can the other strand be synthesized?

The resolution of the problem was provided by the demonstration that only one of the two daughter strands, called the leading strand, is synthesized continuously in the overall direction of fork movement, from the 5' to the 3' end (see Figure 3). The second daughter strand, called the lagging strand, is made discontinuously in small segments, called Okazaki fragments in honor of their discoverer. Each Okazaki fragment is made in the 5' to 3' direction, by a DNA polymerase whose direction of synthesis is backwards compared to the overall direction of fork movement. These fragments are then joined together by an enzyme called DNA ligase.

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