The host-vector system is fundamental to rDNA technology as it forms a two-component system essential for gene cloning. The vector provides essential sequences required for replication, while the compatible host supplies various replication functions including enzymes and proteins necessary for the process.

The host organism plays a crucial role in this system. A suitable host should allow easy entry of the recombinant DNA into the cell. Importantly, the host must not recognize the rDNA as foreign DNA and degrade it. Additionally, the host must supply all required enzymes and proteins for smooth replication of the vector DNA along with the insert.

The vector serves as a carrier DNA molecule that carries the target gene or insert. For effective functioning, a vector must be small in size for easy incorporation into the host cell and be able to integrate a large size insert. It must possess an origin of replication (ori) to enable autonomous replication inside the host organism.

Furthermore, the vector needs to have unique restriction sites to avoid fragmentation into several pieces. It also requires at least one selectable marker to screen out transformants, such as resistance to antibiotics like tetracycline and ampicillin.

Together, this compatible host-vector combination ensures successful propagation and expression of the rDNA, making gene cloning achievable for various applications.