(a) i) Identify and name the structures 'A' and 'B' marked in the image given below :

ii) State their importance in various biotechnology experiments.

or

(b) Explain the process by which a bacterial cell can be made divalent cations and temperature treatment.

Correct Answer:

(a)
i) Structure 'A' is Circular DNA/Plasmid, and Structure 'B' is Bacteriophage.
ii)

• Plasmid (A): Can carry a foreign gene into the host cell (acts as a cloning vector), possesses selectable markers, replicates independently of the host chromosomal DNA control, and maintains a high copy number.
• Bacteriophage (B): Serves as a cloning vector that naturally infects and replicates within bacterial cells, independent of chromosomal DNA control, achieving a very high copy number per cell.

or
(b) Bacterial cells are made competent by treating them with a specific concentration of a divalent cation like calcium. This increases the efficiency with which DNA enters the bacteria through cell wall pores. The recombinant DNA is forced into these cells by incubating them with the DNA on ice, followed by a brief heat shock at
$42\°C$
and then placing them back on ice.

Step-by-Step Educational Explanation:

Part (a) i): Identification of the Structures from the Image
By analyzing the provided illustration:

• Structure 'A' shows a circular, double-stranded helical molecule. This is characteristic of a plasmid (extrachromosomal circular DNA found naturally in many bacterial species).
• Structure 'B' shows a classic virus structure consisting of an icosahedral head capsid containing genetic material, a sheath (tail), and tail fibers. This is a virus that infects bacteria, known as a bacteriophage.

Part (a) ii): Importance in Biotechnology Experiments
Both plasmids and bacteriophages are widely utilized in recombinant DNA technology as cloning vectors:

• Plasmids:
  • They act as vehicles to deliver foreign genes into host organisms (e.g., Escherichia coli).
  • They contain an origin of replication (ori), allowing them to replicate autonomously and maintain high copy numbers (ranging from 15 to 100 or more copies per cell) independent of the host chromosome.
  • They carry selectable markers (such as antibiotic resistance genes like ampicillin or tetracycline resistance), which allow researchers to easily distinguish between transformant and non-transformant cells.
• Bacteriophages:
  • They naturally infect bacterial cells with high efficiency, injecting their genetic material.
  • Because of their aggressive replication cycles, they can achieve a very high copy number per cell, resulting in high yields of the cloned gene/DNA segment.

Part (b): Making Host Cells Competent via Divalent Cations and Heat Shock
DNA is a hydrophilic molecule and cannot easily pass through the hydrophobic lipid bilayer of cell membranes. To force bacterial cells to take up foreign plasmid DNA, they must be made "competent" using the following chemical and thermal treatments:

1. Divalent Cation Treatment: The bacterial cells are treated with a specific concentration of divalent cations, most commonly calcium
$\left({\mathrm{Ca}}^{2+}\right)$
ions. The positively charged calcium ions neutralize the negative charges on both the phosphate backbone of the DNA and the lipopolysaccharides in the bacterial outer membrane. This charge neutralization brings the DNA in close proximity to the cell wall pores.

2. Incubation on Ice: The host cells and recombinant DNA are incubated together on ice, allowing the DNA to adsorb stably onto the cell surface.

3. Heat Shock Treatment: The mixture is briefly exposed to a temperature of
$42\°C$
for about 30 to 90 seconds. This sudden increase in temperature alters the permeability of the bacterial cell membrane, creating transient thermal pores that allow the DNA to enter the cytosol.

4. Recovery on Ice: The cells are quickly returned to ice to restore membrane integrity, trapping the internalized recombinant DNA inside the cell.