Q.When a foreign DNA is introduced into an organism, how is it maintained in the host and how is it transferred to the progeny of the organism?

When foreign DNA is introduced into a host organism, such as a bacterium, it is typically ligated into a cloning vector, most commonly a plasmid. This vector contains a specific DNA sequence known as the origin of replication (ori).

The ori sequence is essential for two main reasons:

• It initiates the replication of the vector DNA, allowing it to multiply independently within the host cell, separate from the host's own chromosomal DNA. This ensures the foreign DNA is stably maintained and amplified in the host.
• It also controls the copy number of the vector within the host cell. …

When a foreign DNA is introduced into an organism, its maintenance and transfer to progeny are fundamental aspects of genetic engineering. The success of this process hinges on ensuring the foreign DNA behaves like the host's own genetic material, at least in terms of replication and inheritance. Let's consider the example of introducing foreign DNA into bacteria, a common and well-understood scenario.

For the foreign DNA to be maintained within the host, it cannot simply exist as a free-floating molecule. It needs a mechanism to ensure its stable presence and replication alongside the host's own genetic material. This is typically achieved by integrating the foreign DNA into a "vector." In bacteria, plasmids are the most commonly used vectors. Plasmids are small, circular, extra-chromosomal DNA molecules that naturally exist in bacteria and can replicate independently of the main bacterial chromosome.

• Ligation into a Vector: The foreign DNA, often a specific gene of interest, is first isolated and cut with specific restriction enzymes. The plasmid vector is also cut with the same restriction enzymes, creating complementary "sticky ends." These sticky ends allow the foreign DNA to be joined with the plasmid DNA using an enzyme called DNA ligase, forming a "recombinant DNA" molecule.
• Transformation: This recombinant plasmid is then introduced into the host bacterial cell through a process called transformation. Bacterial cells are made "competent" (able to take up foreign DNA) by various treatments, such as exposure to calcium chloride followed by heat shock.
• Origin of Replication (ori): The crucial element for maintenance is a specific DNA sequence on the plasmid called the "origin of replication" (ori). This sequence is recognized by the host cell's DNA replication machinery. Once the recombinant plasmid enters the host bacterium, the host cell's enzymes initiate replication at the ori sequence, making multiple copies of the entire plasmid, including the foreign DNA inserted within it. This ensures that as the host cell grows, the foreign DNA is also duplicated.
• Selectable Markers: To identify which host cells have successfully taken up the recombinant DNA, vectors often carry "selectable markers," such as genes conferring resistance to a particular antibiotic (e.g., ampicillin resistance). Only those bacteria that have taken up the plasmid will be able to grow on a medium containing that antibiotic, allowing for the selection of transformed cells.

Once the foreign DNA is stably maintained and replicated within the host cell, its transfer to the progeny becomes a natural consequence of the host organism's reproductive process. …

Method — separate the MAINTENANCE question from the INHERITANCE question

Maintenance (how it survives inside one cell): the foreign DNA is ligated into a vector carrying an origin of replication (ori) — the ori is recognised by the host's own replication machinery, so the vector (and the foreign DNA riding on it) gets copied every time the host DNA is copied. …