Chapter 9: Biotechnology : Principles and Processes Case Study Questions | biology Class 12 CBSE

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Chapter 9: Biotechnology : Principles and Processes Case Study Questions | biology Class 12 CBSE | ByteNotes.in

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A research student working in a molecular biology lab was assigned the task of constructing a recombinant DNA molecule. She isolated genomic DNA from a bacterial source and chose a plasmid as her vector. Using EcoRI restriction enzyme, she digested both the source DNA and the plasmid vector. After gel electrophoresis to confirm digestion, she mixed the fragments with the plasmid and added DNA ligase. The resultant recombinant plasmid was then introduced into E. coli cells. She observed that some bacterial colonies survived on ampicillin-containing agar plates, indicating successful transformation. The experiment demonstrated the fundamental steps of recombinant DNA technology, including the roles of restriction enzymes, vectors, ligase, and selectable markers in building and identifying recombinant organisms.

Question 1: What is the role of DNA ligase in the construction of recombinant DNA?

DNA ligase joins the cut ends of the vector DNA and foreign DNA fragment together.
It seals the nicks between the sticky ends to form a covalently closed circular recombinant DNA molecule.

Question 2: Why was EcoRI used for both source DNA and plasmid digestion in the experiment?

Cutting both DNAs with the same restriction enzyme produces compatible sticky ends on both fragments.
Only compatible sticky ends can anneal and be joined by DNA ligase to form a recombinant molecule.

Question 3: Explain why only some bacterial colonies survived on ampicillin plates and how you would further distinguish recombinants from non-recombinants.

Only bacteria that took up the plasmid containing the ampicillin resistance gene (transformants) could survive on ampicillin plates; untransformed cells died.
To distinguish recombinants from non-recombinants, the transformants are replica-plated onto a medium containing a second antibiotic (e.g., tetracycline) in whose resistance gene the insert was ligated.
Recombinants show insertional inactivation of the second antibiotic resistance gene and will NOT grow on that medium, while non-recombinants will grow on both.

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During a forensic investigation, a DNA sample recovered from a crime scene was extremely small and partially degraded. The forensic lab scientist decided to amplify a specific region of the DNA for further analysis. She designed two short oligonucleotide sequences complementary to the flanking regions of the target DNA. The reaction mixture was prepared with deoxynucleotides, a thermostable enzyme, and the DNA template, and subjected to repeated cycles of high-temperature denaturation, low-temperature annealing, and moderate-temperature extension. After 30 cycles, the target region was amplified more than a billion times, providing sufficient material for gel electrophoresis and sequencing. The technique used is now routinely applied in medical diagnosis, paternity testing, and evolutionary studies.

Question 1: Name the technique described in the passage and identify the thermostable enzyme used.

The technique is Polymerase Chain Reaction (PCR).
The thermostable enzyme used is Taq DNA polymerase, isolated from Thermus aquaticus.

Question 2: Why is a thermostable DNA polymerase essential for PCR?

During PCR, the double-stranded DNA is denatured at high temperatures (around 94°C), which would denature normal DNA polymerases.
Thermostable Taq polymerase remains active at high temperatures and can extend primers after each denaturation step without needing to be replaced.

Question 3: Describe the three steps of each PCR cycle and explain how DNA is amplified exponentially.

Step 1 – Denaturation: The reaction mixture is heated to ~94°C, which breaks the hydrogen bonds between the two strands of template DNA, separating them into single strands.
Step 2 – Primer annealing: The temperature is lowered to ~50–60°C so that the short oligonucleotide primers bind to their complementary sequences on the single-stranded template DNA.
Step 3 – Extension: The temperature is raised to ~72°C; Taq polymerase extends the primers by adding complementary nucleotides, synthesising a new strand. Because each newly synthesised strand also acts as template in subsequent cycles, the number of copies doubles each cycle, leading to approximately 2^n copies after n cycles (exponential amplification).

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Case Set 3

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A biotechnology company wanted to produce a human therapeutic protein on a large scale. The gene encoding this protein was cloned into a suitable expression vector and introduced into bacterial host cells. Initially, the protein was produced in small shake flasks, but the yields were insufficient for commercial purposes. The production team decided to scale up the process using bioreactors that could handle volumes of 100–1000 litres. These stainless-steel vessels were equipped with agitators, oxygen delivery systems, pH and temperature control systems, foam control, and sampling ports. The stirred-tank bioreactor ensured uniform mixing and maintained optimal growth conditions for the recombinant bacteria throughout the culture period, resulting in high yields of the desired protein.

Question 1: Define a bioreactor and state one advantage it has over shake flasks.

A bioreactor is a vessel in which raw materials are biologically converted into specific products using microbial, plant, animal, or human cells under optimally controlled conditions.
Unlike shake flasks, bioreactors can process large volumes (100–1000 litres), enabling large-scale production of the desired product.

Question 2: List any two systems present in a stirred-tank bioreactor and explain the role of the agitator.

Two systems: oxygen delivery system and pH control system (also acceptable: temperature control system, foam control system, sampling ports).
The agitator (stirrer) facilitates even mixing of the culture medium and ensures uniform oxygen availability throughout the bioreactor, maintaining consistent growth conditions.

Question 3: What is downstream processing? Describe the key steps involved after biosynthesis is complete.

Downstream processing refers to the series of processes applied to the biosynthetic product after it is produced by the host cells, before it is ready for marketing.
Key steps include: (i) separation and purification of the desired product from the culture medium and cellular debris using techniques like chromatography or centrifugation; (ii) formulation of the product with suitable preservatives; (iii) thorough clinical trials (in the case of drugs) and strict quality control testing to ensure safety and efficacy.

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Case Set 4

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A plant biotechnologist was studying crown gall disease in dicot plants caused by the soil bacterium Agrobacterium tumefaciens. She observed that the pathogen transfers a segment of its plasmid DNA, called T-DNA, into the plant's nuclear genome, causing uncontrolled cell proliferation (tumour) and forcing the plant cells to produce chemicals beneficial for the bacterium. Fascinated by this natural gene delivery mechanism, the researcher decided to exploit the Ti plasmid as a cloning vector. She disarmed the plasmid by removing the tumour-inducing genes while retaining the infection and integration machinery. The modified Ti plasmid was then loaded with a gene of interest and used to successfully deliver the gene into plant cells, opening doors for crop improvement.

Question 1: What is T-DNA and what role does it play in natural crown gall disease?

T-DNA (Tumour-inducing DNA) is a segment of the Ti plasmid of Agrobacterium tumefaciens that is transferred into the plant cell's nuclear genome.
It causes the plant cells to form a tumour (crown gall) and directs them to produce chemicals (opines) required by the pathogen.

Question 2: How was the Ti plasmid modified to make it suitable as a cloning vector for plants?

The tumour-inducing genes responsible for causing crown gall disease were removed from the Ti plasmid, making it non-pathogenic (disarmed).
The infection and DNA integration machinery was retained so that the modified plasmid could still deliver the gene of interest into plant cells.

Question 3: Compare the use of Agrobacterium-mediated transformation with biolistics for introducing foreign DNA into plant cells. State one limitation of the Agrobacterium method.

Agrobacterium-mediated transformation uses a disarmed Ti plasmid as a biological vector; the bacterium infects the plant cell and stably integrates the foreign DNA into the plant genome.
Biolistics (gene gun) involves bombarding plant cells with high-velocity microparticles (gold or tungsten) coated with the foreign DNA — a purely physical method that does not require a biological vector.
One limitation of the Agrobacterium method: it is largely restricted to dicot plants, as many monocot crops (e.g., wheat, rice) are not natural hosts of Agrobacterium, making transformation less efficient.

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Case Set 5

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In a microbiology practical, students were shown how bacterial cells are made competent to take up foreign plasmid DNA. The instructor treated E. coli cells with calcium chloride solution and then placed the cells mixed with recombinant plasmid DNA on ice. The tube was then briefly shifted to 42°C (heat shock) and immediately returned to ice. Some students asked why the cells needed this special treatment. The instructor explained that the negatively charged DNA cannot cross the hydrophobic bacterial cell membrane on its own. The divalent calcium ions reduce the repulsion between the DNA and cell surface, allowing DNA to pass through the pores in the cell wall. The brief heat shock transiently increases membrane permeability, further facilitating DNA uptake.

Question 1: Why are bacterial cells treated with calcium chloride to make them competent?

DNA is a hydrophilic, negatively charged molecule and cannot pass through the hydrophobic bacterial cell membrane on its own.
Calcium ions (divalent cations) from CaCl₂ reduce the electrostatic repulsion between the negatively charged DNA and the cell surface, increasing the efficiency of DNA entry through pores in the cell wall.

Question 2: What is the significance of the heat shock step in bacterial transformation?

The brief shift to 42°C (heat shock) transiently increases the permeability of the bacterial cell membrane.
This enables the recombinant DNA to enter the cells; the immediate return to ice arrests further membrane changes and reduces DNA degradation.

Question 3: Name THREE methods (other than chemical competence) used to introduce alien DNA into host cells and mention the type of host for each method.

Micro-injection: Recombinant DNA is directly injected into the nucleus of an animal cell using a fine needle — used for animal cells.
Biolistics/Gene gun: Plant or animal cells are bombarded with high-velocity micro-particles of gold or tungsten coated with DNA — mainly used for plant cells.
Disarmed pathogen vectors: Recombinant DNA is carried by disarmed viral or bacterial vectors (e.g., retroviruses, modified Ti plasmid) that infect the host cell — used for plant and animal cells.

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Chapter 9: Biotechnology : Principles and Processes | Case Study Questions

Passage

Question 1: What is the role of DNA ligase in the construction of recombinant DNA?

Question 2: Why was EcoRI used for both source DNA and plasmid digestion in the experiment?

Question 3: Explain why only some bacterial colonies survived on ampicillin plates and how you would further distinguish recombinants from non-recombinants.

Passage

Question 1: Name the technique described in the passage and identify the thermostable enzyme used.

Question 2: Why is a thermostable DNA polymerase essential for PCR?

Question 3: Describe the three steps of each PCR cycle and explain how DNA is amplified exponentially.

Passage

Question 1: Define a bioreactor and state one advantage it has over shake flasks.

Question 2: List any two systems present in a stirred-tank bioreactor and explain the role of the agitator.

Question 3: What is downstream processing? Describe the key steps involved after biosynthesis is complete.

Passage

Question 1: What is T-DNA and what role does it play in natural crown gall disease?

Question 2: How was the Ti plasmid modified to make it suitable as a cloning vector for plants?

Question 3: Compare the use of Agrobacterium-mediated transformation with biolistics for introducing foreign DNA into plant cells. State one limitation of the Agrobacterium method.

Passage

Question 1: Why are bacterial cells treated with calcium chloride to make them competent?

Question 2: What is the significance of the heat shock step in bacterial transformation?

Question 3: Name THREE methods (other than chemical competence) used to introduce alien DNA into host cells and mention the type of host for each method.

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