Which of the following is used to make the bacterial cell as 'Competent cell' to take up DNA?

Calcium ion

The correct answer is Option (2) -Calcium ion

Due to its hydrophilic nature, DNA molecules are unable to pass through cell membranes. To introduce plasmid DNA into bacterial cells, it is necessary to first make the bacterial cells "competent" for DNA uptake. This process involves treating the cells with a specific concentration of a divalent cation, such as calcium. This treatment enhances the efficiency with which DNA can enter the bacterium through pores present in its cell wall.

Once the bacterial cells have been rendered competent, recombinant DNA can be introduced into them using a series of steps. First, the cells are incubated with the recombinant DNA on ice. Afterward, they are briefly exposed to a temperature of 42^°C (a process known as heat shock). Finally, the cells are returned to an ice-cold environment. This carefully controlled process allows the bacterial cells to take up the recombinant DNA effectively.

In summary, making bacterial cells "competent" by treating them with divalent cations like calcium enables them to take up recombinant DNA. This uptake is facilitated by a series of steps involving incubation on ice, a brief exposure to heat shock, and subsequent cooling, which collectively promote the successful introduction of the recombinant DNA into the bacterial cells.