Figure 1: Replication fork impediments and processing pathways
A: Graphical representation of a co-transcriptional R-loop, formed by
nascent DNA becomin bound to the template strand behind the polymerase.
B: Replication pass a polymerase blocking lesion: first, the leading
strand polymerase stalls but the replisome continues forming a region of
ssDNA ahead of the fork. The polymerase can be restarted either by
trans-lesion synthesis or by template switching to use the newly
synthesised lagging strand as a template.
C: Initiation of BIR: a stalled replication fork reverses, is resected
then invades the template DNA aided by the homologous recombination
machinery. Replication can then restart in a migrating D-loop with
uncoupled synthesis of the lagging strand, using Pif1 as a helicase to
improve processivity.