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Step-by-Step Explanation
1. Introduction
Genetic engineering involves transferring a desired DNA segment (gene) into a host cell using a carrier molecule called a vector. Among the given agents (A) bacterium, (B) plasmid, (C) plasmodium, and (D) bacteriophage, the correct vectors are plasmid (B) and bacteriophage (D). Below is a clear explanation of why these two are effective vectors.
2. Plasmids (Option B) as Vectors
• A plasmid is an autonomously replicating, circular, extra-chromosomal DNA present in many bacteria.
• Plasmids can be transferred from one bacterial cell to another within a colony. This natural ability is harnessed in genetic engineering to introduce desired genes into a host cell.
• Plasmids often carry genes for antibiotic resistance, which scientists use as markers for selecting cells that successfully receive the plasmid containing the inserted foreign DNA.
3. Bacteriophages (Option D) as Vectors
• Bacteriophages are viruses that infect bacteria. They inject their DNA into host bacterial cells, multiply inside them, and then cause the bacterial cell to lyse, releasing new phage particles.
• Scientists exploit this natural infection process by modifying the phage DNA to carry desired genes. When the phage infects a bacterium, it delivers the engineered DNA into the host, facilitating gene cloning.
4. Why (A) Bacterium and (C) Plasmodium Are Not Suitable Vectors
• Although bacteria can harbor plasmids, the bacterial cell itself is not typically used as the direct DNA-transferring vehicle (vector) in genetic engineering. Rather, specific DNA components like plasmids from bacteria are used.
• Plasmodium is a protozoan parasite (causative agent of malaria) and is not conventionally used for transferring genes. It does not have the ideal properties of a vector such as easy manipulation, predictable replication in the host, or stable insertion of foreign genes.
5. Conclusion
The correct vectors from the given options are plasmids and bacteriophages, hence option “(B) and (D) only” is correct.
