Final answer:
The RecA-mediated presynapsis stage in strand exchange involves the formation of a nucleoprotein filament formed by RecA protein binding to single-stranded DNA. The filament scans the other replicating strand for homologous sequences and invades the double-stranded DNA. This allows replication to continue on the leading and lagging strands.
Step-by-step explanation:
During the RecA-mediated presynapsis stage in strand exchange, the RecA protein forms a nucleoprotein filament by binding to single-stranded DNA. This filament scans the other replicating strand for homologous sequences. When homologous sequences are found, the RecA-DNA filament binds to them, causing the filament of new DNA to invade the homologous double-stranded DNA. This separates the template and newly replicated DNA, allowing replication of the leading strand to continue from the invading strand. After strand invasion, replication of the lagging strand resumes at the new replication fork.