Question

The correct sequence of steps for making a cell competent is:

I. Treatment with divalent cations
II. Heat shock ( $42°$C)
III. Placing on ice
IV. Incubation of cells with rDNA on ice

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

Answer 1

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

Competent cell preparation is the process of making cells receptive for taking up foreign DNA.

During the process of transformation, the cells need to be made competent by chemical processes. The process of competent cell formation and transformation is described below:

- Treatment with divalent cations: ${\mathrm{Mg}}^{2+}$or ${\mathrm{Ca}}^{2+}$bind to the rDNA and help it to adsorb on the bacterial host cells thereby eliminating the risk of detachment of the rDNA

- Incubation of cells with rDNA on ice: After the treatment of the host cells with divalent cations, the host cells are now incubated with rDNA.

- Providing heat shock to the cells followed by placing them on ice: Treating the cells with alteranate high and low temperatures results in destabilisation of the membrane and its permeability to DNA increases. It is then again placed on ice to restore the stability of the membranes and pores.

Hence, the correct sequence of events which helps the cells to become competent and take up the rDNA is I → IV → II → III.