Which temperature is used in the heat shock step to force bacterial cells to take up recombinant DNA?

42^°C

The correct answer is Option (2)- 42°C

During the heat shock step, bacterial cells are briefly exposed to a temperature of 42°C. This sudden increase in temperature helps to create temporary pores or openings in the cell membrane, allowing the recombinant DNA to enter the bacterial cells more efficiently. After the heat shock, the cells are placed back on ice, which further aids in the uptake of the DNA. This process is known as bacterial transformation and is a common method used to introduce recombinant DNA into bacterial cells for further manipulation and cloning.