An Overview of Recombinant DNA Technology

Given/Context: The question asks about the historical development of recombinant DNA (rDNA) technology.

Answer:

Recombinant DNA (rDNA) technology was developed through a series of landmark discoveries and experiments:

1. Discovery of Restriction Enzymes (1960s–70s): Werner Arber, Daniel Nathans, and Hamilton Smith discovered restriction endonucleases — enzymes that cut DNA at specific sequences. This provided the molecular 'scissors' needed to cut DNA precisely.

2. Discovery of DNA Ligase: The enzyme DNA ligase was identified, which could join (ligate) two DNA fragments together, acting as molecular 'glue'.

3. Discovery of Plasmids: Joshua Lederberg discovered plasmids — small, circular, self-replicating extrachromosomal DNA molecules in bacteria — which became the first vectors used to carry foreign DNA.

4. First Recombinant DNA Molecule (1972): Paul Berg created the first recombinant DNA molecule in vitro by combining DNA from two different organisms (SV40 virus and bacteriophage lambda DNA) using restriction enzymes and ligase.

5. Cohen and Boyer Experiment (1973): Stanley Cohen and Herbert Boyer successfully cloned a recombinant plasmid into *E. coli*, demonstrating that foreign genes could be expressed in a host organism. This is considered the foundation of modern rDNA technology.

6. Development of Gene Cloning Techniques: Subsequent development of transformation methods, selectable markers, expression vectors, and PCR (by Kary Mullis, 1983) further refined the technology.

Conclusion: Thus, rDNA technology evolved from the convergence of restriction enzymes, vectors (plasmids), DNA ligase, and transformation techniques, enabling scientists to isolate, cut, join, and propagate specific genes of interest in host organisms.

Given/Context: The question asks about two major application areas of rDNA technology.

Applications in Crop Improvement:

1. Bt Crops: Genes from *Bacillus thuringiensis* (Bt) encoding insecticidal proteins (Cry proteins) have been introduced into crops like cotton and maize to make them pest-resistant. *Bt cotton* was commercially released in 1996.

2. Herbicide Tolerance: Crops like Roundup Ready soybean have been engineered to tolerate herbicides, reducing crop loss.

3. Golden Rice: Developed by Ingo Potrykus and Peter Beyer (2000), Golden Rice was engineered to produce $\beta$-carotene (provitamin A) to address vitamin A deficiency.

4. Disease Resistance: Recombinant DNA techniques have been used to develop virus-resistant and fungus-resistant crop varieties.

5. Improved Nutritional Quality: Crops have been engineered for enhanced protein content, improved amino acid profiles, and longer shelf life.

Applications in Therapeutics:

1. Recombinant Insulin: Human insulin (Humulin) was the first recombinant therapeutic protein, produced in *E. coli* by inserting the human insulin gene, replacing animal-derived insulin.

2. Recombinant Vaccines: Vaccines such as Hepatitis B vaccine and Gardasil (against HPV, FDA approved 2006) are produced using rDNA technology.

3. Tissue Plasminogen Activator (tPA) / Urokinase: Used as blood clot-dissolving (thrombolytic) drugs.

4. Erythropoietin (EPO): A recombinant hormone used to treat anaemia.

5. Monoclonal Antibodies: Avastin (anti-VEGF antibody, 2004) is used in cancer treatment.

6. Gene Therapy: rDNA technology forms the basis of gene therapy, where defective genes are corrected or replaced.

7. mRNA Vaccines: Recombinant vaccines against COVID-19 (2020) and mRNA vaccine technology (Nobel Prize 2023) represent the latest therapeutic applications.

Conclusion: rDNA technology has revolutionised both agriculture and medicine by enabling precise genetic modifications for improved crop traits and production of life-saving therapeutic agents.

Correct Option: (c) Joshua Lederberg

Justification: Joshua Lederberg discovered plasmids — small, circular, self-replicating extrachromosomal DNA elements found in bacteria. He coined the term 'plasmid' in 1952. Paul Berg created the first recombinant DNA molecule, Sir Alec Jeffreys developed DNA fingerprinting, and Kary Mullis invented PCR.

Correct Option: (b) Blood clot dissolving drug

Justification: Plasminogen activator (tissue plasminogen activator, tPA) and Urokinase are thrombolytic (fibrinolytic) agents. They convert plasminogen into plasmin, which dissolves fibrin clots. They are used clinically to treat conditions such as myocardial infarction (heart attack) and stroke caused by blood clots. Both are produced using recombinant DNA technology.

Correct Option: (b) Both assertion and reason are true but the reason is not the correct explanation of the assertion.

Justification:

- Assertion is TRUE: Restriction endonucleases do cut DNA at specific recognition sequences, and they are indeed isolated mostly from bacteria (e.g., *EcoRI* from *Escherichia coli*, *HindIII* from *Haemophilus influenzae*). Bacteria use them as a defence mechanism against foreign (bacteriophage) DNA.

- Reason is TRUE: Restriction endonucleases are indeed a type of nuclease (specifically, endonucleases that cleave phosphodiester bonds within a DNA strand).

- However, the reason does NOT correctly explain the assertion. The fact that restriction endonucleases are a type of nuclease does not explain why they cut DNA at specific sites or why they are found mostly in bacteria. The correct explanation would involve the bacterial restriction-modification system used for defence against foreign DNA.

Hence, option (b) is correct.

Correct Option: (a) Both assertion and reason are true and the reason is the correct explanation of the assertion.

Justification:

- Assertion is TRUE: *E. coli* has a generation time (cell division time) of approximately 20 minutes under optimal conditions, yet its chromosome takes approximately 40–60 minutes to fully replicate.

- Reason is TRUE: *E. coli* employs a multifork (or multi-fork) replication mechanism. In this mechanism, a new round of DNA replication begins at the origin of replication (*oriC*) before the previous round is completed. This means multiple replication forks are active simultaneously on the same chromosome.

- The reason correctly explains the assertion: Because of multifork replication, *E. coli* can divide every 20 minutes even though a single complete replication of its chromosome would take ~60 minutes. By initiating new rounds of replication before the previous one finishes, the bacterium ensures that daughter cells receive complete chromosomes at each division.

Hence, option (a) is correct.