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Point ofjoining Lagging strand

Okazaki fragment Short RNA primer

Leading strand

Point ofjoining Lagging strand

Okazaki fragment Short RNA primer

Leading strand

▲ FIGURE 4-33 Schematic diagram of leading-strand and lagging-strand DNA synthesis at a replication fork.

Nucleotides are added by a DNA polymerase to each growing daughter strand in the 5'n3' direction (indicated by arrowheads). The leading strand is synthesized continuously from a single RNA primer (red) at its 5' end. The lagging strand is synthesized discontinuously from multiple RNA primers that are formed periodically as each new region of the parental duplex is unwound. Elongation of these primers initially produces Okazaki fragments. As each growing fragment approaches the previous primer, the primer is removed and the fragments are ligated. Repetition of this process eventually results in synthesis of the entire lagging strand.

Because growth of the lagging strand must occur in the 5'n3' direction, copying of its template strand must somehow occur in the opposite direction from the movement of the replication fork. A cell accomplishes this feat by synthesizing a new primer every few hundred bases or so on the second parental strand, as more of the strand is exposed by unwinding. Each of these primers, base-paired to their template strand, is elongated in the 5'n3' direction, forming discontinuous segments called Okazaki fragments after their discoverer Reiji Okazaki (see Figure 4-33). The RNA primer of each Okazaki fragment is removed and replaced by DNA chain growth from the neighboring Okazaki fragment; finally an enzyme called DNA ligase joins the adjacent fragments.

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