Question

DNA being hydrophilic cannot pass through the cell membrane of a host cell. Explain how the host cell is made 'competent' to take up the recombinant DNA ?

Hint:

Other methods include Micro-injection (for animal cells) and Biolistics/Gene gun (for plant cells).

Answer 1

Step 1: Understanding the Concept:
Because DNA is negatively charged and hydrophilic, it cannot cross lipid-rich cell membranes naturally. Host cells must be specifically treated to force them to take up recombinant DNA.
Step 2: Detailed Explanation:
1. Chemical Treatment: The bacterial cells are treated with a specific concentration of a divalent cation, such as calcium ($Ca^{2+}$). This increases the efficiency with which DNA enters the bacterium through pores in its cell wall.
2. Incubation: Recombinant DNA is incubated with the host cells on ice.
3. Heat Shock: The cells are then placed briefly at $42^\circ C$ (heat shock) and then put back on ice.
4. Result: This series of steps allows the bacteria to take up the recombinant DNA.
Step 3: Final Answer:
Host cells are made competent by treating them with $Ca^{2+}$ ions and applying a heat-shock treatment, which alters membrane permeability.