Final answer:
In RecA-mediated synapsis, the strand invasion step occurs where a single-stranded DNA, coated with RecA protein, invades a double-stranded DNA to initiate repair or recombination.
Step-by-step explanation:
During the RecA-mediated synapsis stage in strand exchange, choice C) Strand Invasion occurs. RecA protein monomers bind to the single-stranded DNA to form a nucleoprotein filament. Once formed, the filament scans for and binds to homologous sequences in the duplex DNA strand.
This leads to the invasion of the single-strand into the double-strand, displacing one of the original strands and forming a D-loop structure. This process is crucial for the repair of DNA damages, such as single-stranded breaks, by recombination with homologous sequences. It also plays a role in genetic recombination mechanisms like crossing over during meiosis.