(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 'competent' to take up foreign DNA from its surroundings, using divalent cations and temperature treatment.

Part (a)

i) Identification of structures 'A' and 'B':
Structure 'A' represents the circular DNA or Plasmid.
Structure 'B' represents the Bacteriophage (a virus that infects bacteria).

ii) Importance in Biotechnology Experiments:
Plasmid (Structure 'A'): Plasmids act as excellent cloning vectors in biotechnology. They are extrachromosomal circular DNA molecules that replicate independently of the host chromosomal DNA. They can carry a foreign gene (gene of interest) into a host cell, possess selectable markers for easy selection of transformants, and can maintain a high copy number within the host cell.
Bacteriophage (Structure 'B'): Bacteriophages also serve as cloning vectors. They have a natural ability to infect bacterial cells and replicate their genome inside them, independent of host chromosomal control. This allows them to clone foreign DNA with a very high copy number per cell.

OR

Part (b) Making a bacterial cell 'competent' to take up foreign DNA:

Since DNA is a hydrophilic molecule, it cannot pass through cell membranes easily. Bacterial cells must be made "competent" to take up external DNA through the following process:

1. Divalent Cation Treatment: The bacterial cells are treated with a specific concentration of a divalent cation, such as calcium (Ca²⁺) ions. This treatment increases the efficiency with which DNA enters the bacterium through pores in its cell wall by neutralizing the negative charges on both the DNA and the bacterial cell membrane.
2. Heat Shock Treatment: The recombinant DNA is incubated with the treated bacterial cells on ice. The mixture is then subjected to a brief thermal shock by placing it at 42 °C. Finally, the cells are immediately put back on ice. This sudden change in temperature (heat shock) forces the recombinant DNA into the competent bacterial cells.