(a) Vectors are DNA molecules that can carry a foreign DNA segment into the host cell.
i) Write the significance of 'ori' in this vector.
ii) Give one example each of vectors used for cloning genes in plants and animals.

(b) Write how can an alien DNA be introduced into
i) an animal cell
ii) a plant cell.

Correct Answer:

(a)
i) 'ori' (Origin of Replication): It is a sequence from where replication starts. Any piece of DNA, when linked to this sequence, can be made to replicate within the host cells. This sequence is also responsible for controlling the copy number of the linked DNA.
ii) Vector examples:
- Plants: Agrobacterium tumefaciens (specifically the Ti plasmid)
- Animals: Retrovirus

OR

(b) Methods to introduce alien DNA:
i) Animal cell:
- Microinjection: Recombinant DNA is directly injected into the nucleus of an animal cell.
- Disarmed pathogens: Animal cells are infected with a disarmed retrovirus, which transfers the recombinant DNA into the host cell.
ii) Plant cell:
- Biolistics or Gene gun: Plant cells are bombarded with high-velocity microparticles of gold or tungsten coated with DNA (rDNA).
- Disarmed pathogens: Using Agrobacterium tumefaciens to deliver the desired gene into the plant host cell.

---

Detailed Explanation:

Part (a) Cloning Vectors and their Components

i) Significance of 'ori' (Origin of Replication):
The 'ori' is a specific genetic sequence in a plasmid or vector DNA where the process of replication is initiated. In recombinant DNA technology, if a foreign or alien DNA fragment is integrated next to this sequence, the host cell's replication machinery recognizes it and starts copying the foreign DNA along with the vector DNA. Furthermore, the 'ori' site controls the copy number of the cloned gene. If we want to obtain a high number of copies of our target gene, we must select a vector whose 'ori' supports a high copy number.

ii) Vectors for Plants and Animals:
- Plants: Agrobacterium tumefaciens is a naturally occurring soil bacterium. It contains a Ti (tumor-inducing) plasmid that can transfer a specific DNA segment (T-DNA) into plant cells, integrating it into the plant genome. In genetic engineering, this plasmid is disarmed (rendered non-pathogenic) and used to deliver desired genes into plant genomes.
- Animals: Retroviruses have the natural ability to transform normal animal cells into cancerous cells by integrating their genetic material into the host genome. By removing the harmful genes and replacing them with the gene of interest, disarmed retroviruses serve as excellent vectors for delivering foreign genes into animal cells.

Part (b) Methods of Introducing Alien DNA into Host Cells

Since DNA is a hydrophilic molecule, it cannot pass through cell membranes easily. Therefore, special techniques are utilized to force recombinant DNA into host cells:

i) Introduction into Animal Cells:
- Microinjection: This is a direct physical method where a glass micropipette is used to inject the recombinant DNA directly into the nucleus of the animal host cell under a specialized microscope.
- Disarmed Retroviruses: The host animal cells are allowed to be infected by a modified, harmless retrovirus containing the alien gene. The virus naturally inserts the gene into the host genome during infection.

ii) Introduction into Plant Cells:
- Biolistics / Gene Gun: In this physical method, plant cells are bombarded with microscopic particles of heavy metals (like gold or tungsten) that are coated with the recombinant DNA. These particles are accelerated to high velocities, allowing them to penetrate the plant cell wall and deliver the DNA into the nucleus.
- Disarmed Agrobacterium tumefaciens: Similar to the viral infection method in animals, plant cells are incubated with disarmed Agrobacterium tumefaciens, which naturally transfers the recombinant T-DNA into the plant genome without causing tumor disease.