Question

Choose the correct sequence of steps involved in the process of making bacterial cells competent before transformation:
I. Heat shock at ${42}^o$C
II. Place on ice
III. Treat bacterial cells with a compound containing divalent cation
IV. Incubate bacterial cells with rDNA

• A. IV → I → III → II → I
• B. III → II → IV → I → II
• C. III → IV → II → I → II
• D. IV → II → III → I → II

Answer 1

The correct option is C III → IV → II → I → II

The ability of a cell to take up foreign (extracellular) DNA from its surrounding environment is called cell competence. The process by which exogenous DNA is transferred into host cells is termed as transformation, which brings about a genetic alteration in the cell.
DNA is a hydrophilic molecule with a negative charge and cannot pass through cell membranes. Therefore, bacterial cells have to be made competent to force uptake of DNA. The following steps are performed to make bacterial cells competent:
1. Bacterial cells are first treated with a specific concentration of a compound containing divalent cation, such as calcium $CaC{l}_2$. It increases the efficiency with which DNA enters the bacterium through pores in its cell wall.
2. Such bacterial cells along with recombinant DNA are then incubated on ice.
3. They are then placed briefly at ${42}^o$C (heat shock).
4. They are then placed back on ice.
This enables the bacteria to take up the recombinant DNA.