laggingstrand

Lagging strand is the DNA strand that is synthesized discontinuously on the template strand oriented opposite to the direction of fork movement during DNA replication. The two parental strands are separated at replication forks; DNA polymerases can synthesize new DNA only in the 5' to 3' direction, so the lagging strand is produced as a series of short segments called Okazaki fragments.

Each fragment begins with an RNA primer synthesized by primase. DNA polymerase extends from the primer until it reaches the previous fragment. In bacteria, DNA polymerase III carries out synthesis, while DNA polymerase I removes RNA primers and replaces them with DNA; in eukaryotes, primase–pol α lays down initial primers, and DNA polymerase δ extends the fragments, with polymerase ε synthesizing the leading strand. RNA primers are removed (by RNase H and FEN1 in eukaryotes; RNase H and/or DNA polymerase I activity in bacteria), and adjacent fragments are joined by DNA ligase to form a continuous strand.

During replication, the lagging strand is looped so that its polymerase can move in the same physical direction as the fork while synthesizing away from the fork; this “trombone” model enables coordination with leading-strand synthesis.

Proper processing and ligation of Okazaki fragments are essential for high-fidelity replication; defects can lead to gaps, mutations, or genome instability.