Q.Nematode specific genes were introduced into the tobacco host plant using a vector (A) pBR 322 (B) Plasmid (C) Bacteriophage (D) Agrobacterium

Concept: Plant genetic engineering using a natural vector.

Reasoning:

1. Tobacco is a plant, and the goal is to introduce foreign genes into its genome.
2. Agrobacterium tumefaciens is a soil bacterium that naturally infects plants and transfers a segment of its Ti (tumor-inducing) plasmid, called T-DNA, into the host plant's genome.
3. This natural gene transfer mechanism has been widely adapted in biotechnology to introduce desired genes, such as nematode-specific genes, into various plant species, including tobacco. …

Concept and Intuition

When we want to introduce a new gene into an organism, we need a way to carry that gene into the host cells and ensure it gets incorporated into the host's genetic material or expressed. This carrier is called a vector. The choice of vector is crucial and depends heavily on the target host organism.

For plants, especially dicotyledonous plants like tobacco, nature provides an excellent tool: the bacterium Agrobacterium tumefaciens. This bacterium is known as "nature's genetic engineer" because it naturally transfers a segment of its plasmid DNA, called T-DNA (Transfer DNA), into plant cells. This T-DNA integrates into the plant's chromosome, causing the plant to develop crown gall tumors. Scientists have cleverly disarmed this bacterium by removing the tumor-inducing genes from its plasmid (the Ti plasmid) and replacing them with the desired foreign genes. This modified Agrobacterium can then be used to deliver the foreign genes into plant cells without causing disease, making it an ideal vector for plant genetic engineering.

Step-by-Step Solution

1. Analyze the problem: The core of the question is about introducing "nematode specific genes" into a "tobacco host plant" using a "vector." The key here is the plant host. We need a vector that can efficiently deliver and integrate foreign DNA into plant cells.

2. Evaluate option (A) pBR322:

   • pBR322 is a widely used bacterial plasmid cloning vector. It was one of the first plasmids engineered for cloning in Escherichia coli.
   • While it's excellent for cloning and amplifying DNA fragments within bacteria, it lacks the mechanisms required to transfer DNA into plant cells, integrate it into the plant genome, or ensure its expression in a plant host. Therefore, pBR322 is not suitable for direct gene transfer into plants.

3. Evaluate option (B) Plasmid (general):

   • A plasmid is a small, circular, extrachromosomal DNA molecule found in bacteria and some other organisms. Many types of plasmids exist, and some are indeed used as vectors.
   • However, this option is too general. While Agrobacterium's Ti plasmid is a type of plasmid, simply stating "Plasmid" doesn't specify the crucial features needed for plant transformation. Not all plasmids can function as vectors for plants. For instance, bacterial plasmids like pBR322 (discussed above) cannot. The specificity of the vector to the host is vital.

4. Evaluate option (C) Bacteriophage:

   • Bacteriophages are viruses that specifically infect bacteria. They inject their genetic material into bacterial cells to replicate.
   • Bacteriophages are used as vectors for cloning large DNA fragments in bacteria (e.g., lambda phage vectors) but are entirely unsuitable for introducing genes into plant cells, as they do not infect plant cells.

5. Evaluate option (D) Agrobacterium: …