discontinuous replication

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discontinuous replication The synthesis of a new strand of a replicating DNA molecule as a series of short fragments that are subsequently joined together. Only one of the new strands, the so-called lagging strand, is synthesized in this way. The other strand (leading strand) is synthesized by continuous addition of nucleotides to the growing end, i.e. continuous replication. The difference arises because of the different orientations of the parent template strands. The template of the leading strand is oriented in the 3′→5′ direction (according to the numbering of atoms in the sugar residues), which means that the leading strand itself is oriented in the opposite 5′→3′ direction, providing an –OH group at the 3′ end for the continual addition of nucleotides by DNA polymerase, which moves forwards as the template strands unwind at the replication fork. However, the template of the lagging strand is oriented in a 5′→3′ direction, so the lagging strand itself is oriented in the 3′→5′ direction, and hence the DNA polymerase complex must move backwards away from the replication fork. Synthesis of the lagging strand proceeds not continuously, as on the leading strand, but discontinuously in a series of repeated steps. Discontinuous replication produces a series of short DNA fragments (Okazaki fragments) complementary to the template strand. These vary in length, being about 100–200 nucleotides in eukaryotes and 1000–2000 nucleotides in prokaryotes. The fragments are then covalently bound together by the enzyme DNA ligase, forming a continuous chain of nucleotides, thus completing replication of the lagging strand. See also DNA replication; primase.