Because a new DNA strand can be elongated only at the 3′ end, the two daughter strands are synthesized in quite different ways (Fig. 12.5). The daughter strand shown at the bottom of Fig. 12.5 has its 3′ end pointed toward the replication fork, so that as the parental double helix unwinds, nucleotides can be added onto the 3′ end and this daughter strand can be synthesized as one long, continuous polymer. This daughter strand is called the leading strand.

252

The situation is different for the daughter strand shown at the top in Fig. 12.5. Its 5′ end is pointed toward the replication fork, but the strand cannot grow in that direction. Instead, as the replication fork unwinds, it grows away from the fork and forms a stretch of single-stranded DNA of a few hundred to a few thousand nucleotides, depending on the species. Then, as the parental DNA duplex unwinds further, a new daughter strand is initiated with its 5′ end near the replication fork, and this strand is elongated at the 3′ end as usual. The result is that the daughter strand shown at the top in Fig. 12.5 is actually synthesized in short, discontinuous pieces. As the parental double helix unwinds, a new piece is initiated at intervals, and each new piece is elongated at its 3′ end until it reaches the piece in front of it. This daughter strand is called the lagging strand. The short pieces in the lagging strand are sometimes called Okazaki fragments after their discoverer, Japanese molecular biologist Reiji Okazaki.

The presence of leading and lagging strands during DNA replication is a consequence of the antiparallel nature of the two strands in a DNA double helix, and the fact that DNA polymerase can synthesize DNA in only one direction.